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PRIVATE PILOT
AND
COMMERCIAL PILOT
METEOROLOGY
2.06 COMPOSITION OF THE ATMOSPHERE
The atmosphere is primarily composed of the following:
NITROGEN
:
________
OXYGEN
:
________
OTHER GASES
:
________
(These comprise of Argon, Carbon Dioxide, Helium,
Hydrogen, ozone and water vapour)
INTERNATIONAL STANDARD ATMOSPHERE ISA The international standard atmosphere was conceived to act as a measuring stick to compare with actual conditions in the atmosphere.
The standard atmosphere has the following values:
·
PRESSURE
______ hectopascals(reducing at 1hPa / ___ft ascent)
·
TEMPERATURE
______
(reducing at __ oC / 1000ft ascent)
·
AIR DENSITY
1225 grams / cubic meter
DIVISIONS OF THE ATMOSPHERE
The atmosphere is divided into layers according to certain properties.
The most useful method of division is by temperature changes with increase in altitude
THE TROPOSPHERE
In the troposphere there is a general decrease in temperature with height gain.
In the ISA atmosphere the temperature drops steadily from +15o C at Mean Sea Level (MSL) to ______ at 36 000ft.
This equates too roughly ____ C per 1000ft.
However, this rate of decrease is not constant, and at times the temperature may even increase with height (this is called an inversion and will be discussed in depth later).
Almost all clouds and weather occur in the troposphere.
The top of the troposphere is called the tropopause.
The height of the tropopause varies over the earth, being the highest over the equator at approx. 55 000ft, and the lowest over the poles at approx. 25 000ft.
It also varies on a daily and seasonal basis.
Because the height of the tropopause is greater over the equator, the lowest temperatures in the atmosphere occur in the vicinity of the equatorial tropopause.
Above the tropopause, the temperature stops decreasing with height and now remains constant, then actually increases slightly toward the top of the next level of the atmosphere called the Stratosphere.
It is important to note that the tropopause is not a continuous layer around the earth, but rather a series of layers at various altitudes dependent on __________ .
The tropopause actually overlaps in the mid – latitude regions.
Short wave radiation from the sun enters the troposphere, warming the earth.
The earth radiates long wave radiation, which is partly absorbed by the troposphere, causing atmospheric warming.
Tops of cumulus clouds often reach the tropopause, and may go higher if the updraughts are strong enough.
THE STRATOSPHERE
This region of the atmosphere is immediately above the troposphere, and is characterised by a steady temperature in the lower levels and a slow temperature rise in the upper levels.
There is little vertical movement of the air in the stratosphere, and very little moisture or cloud.
The _________ layer is located in the stratosphere, which absorbs the UV radiation.
This explains why the temperature will rise in the upper part of the stratosphere.
Like the tropopause, the layer at the top of the stratosphere is called the stratopause.
It is located approximately 48km (30 miles) above the earth.
At this level the temperature has risen to –2.5oC.
THE MESOSPHERE
The mesosphere is a turbulent layer in which the temperature drops rapidly with height.
At the mesosphere the temperature is approx. –92.5oC.
Its height is about 80km (50 miles) above the earth’s surface.
THE IONOSPHERE
An important property of solar radiation is that it can ionize air.
That is, electrons are dissociated from neutrally charged air molecules and atoms, producing positively charged ions and free electrons.
This ionisation occurs mainly in the mesosphere and above.
This region is called the ionosphere and extends from about 55 to 500 km (35 to300 miles).
The ionosphere contains vast numbers of freely moving charged particles
The main interest to pilots is the ability of the ionosphere to refract, reflect and absorb radio waves.
This property makes long distance communication possible by “bouncing” radio waves off the ionosphere.
Because the ionosphere is produced by solar radiation, its characteristics vary markedly with the passage of day and night.
THE THERMOSPHERE
The thermosphere is a layer of increasing temperature with height gain.
The high temperatures are partly due to the absorption of ultra – violet radiation by oxygen molecules.
The Ionosphere is part of the thermosphere.
DIVISIONS OF THE ATMOSPHERE
TRANSFER OF HEAT ENERGY
The following are four processes through which heat is transmitted from one body to another or is distributed within a body.
RADIATION
All bodies with a temperature above absolute zero (-273oC) emit energy in the form of electromagnetic radiation.
The wavelength of this radiation varies with temperature.
The higher the temperature the shorter the wavelength.
CONDUCTION
Conduction is transfer of energy by contact.
Heat energy can be conducted through a single body or from one body to another.
Metals such as iron are good conductors of heat.
Wood and air are not goo conductors.
CONVECTION
Convection is the vertical movement of air.
It occurs when air heated from below expands and becomes less dense.
This causes it to rise.
ADVECTION
Advection is the horizontal transport of air.
NATURE OF THE SURFACE
The heating of a surface is affected by the following:
·
SPECIFIC HEAT
This is the amount of energy required to raise the
of Temperature of a 1 gram sample by 1oC.
Water
requires a great deal more energy to than land
does.
This leads to the land being hotter than the
water during the day and colder than the water at
night.
The reason for this is due to the land losing
the heat quicker than the water.
Coastal regions
often have moderate temperature changes
throughout the day, whereas desert inland regions
often have large changes in temperature between
day and night.
·
REFLECTIVITY
Light surfaces such as snow or water reflect a lot of
energy and this reduces energy absorption.
Dark
surfaces like ploughed earth absorb a lot of heat
energy and are heated quickly.
·
CONDUCTIVITY
How easily heat is conducted through a surface is
important.
Soil or sand are poor conductors and
hence only a thin layer is heated.
This means it will
cool quickly.
Water is a good conductor so it warms
to a greater depth and retains this energy much
better.
FACTOR AFFECTING SOLAR RADIATION
SEASONAL VARIATION
The earth orbits the sun once a year.
The earth’s spin axis is tilted to 23.5o.
This leads to variation in the intensity and length of sunlight over the earth as the year progresses i.e. the four seasons.
EFFECT OF CLOUD COVER
During the day cloud cover reduces the amount of radiation to reach the earth’s surface.
This leads to lower surface temperatures.
At night cloud cover acts like a blanket and prevents the loss of terrestrial radiation.
This leads to warmer temperatures.
DIURNAL VARIATION
During the day the sun shines causing a net energy gain – therefore a maximum temperature is generally reached about 3PM
At night there is a net loss of energy – this causes a lowest temperature to be around sunrise
ATMOSPHERIC STABILITY
WATER VAPOUR AND HUMIDITY
Water is present in some degree everywhere in the atmosphere.
It usually occurs in the form of water vapour, which is the invisible gaseous state of water.
The proportion of water vapour in the atmosphere varies widely from place to place and over time.
The highest values of water vapour occur in the tropics near the ocean surface.
The driest air is usually found in central Antarctica during the winter months where the air is very cold.
Most of the water vapour in the atmosphere is found in the lower levels.
As a general rule, the atmosphere tends to be drier as altitude increases.
FOG, MIST and most CLOUDS are composed of tiny _______
droplets held in suspension in the atmosphere.
At high altitudes where the temperature is cold enough some clouds are composed of ___
crystals.
Water can exist in three states in the atmosphere: _________, __________ and ___________ (vapour).
Water may change from one state to another either directly or indirectly.
The process by which it changes state is listed in the diagram below.
Whenever a change of state occurs, LATENT ENERGY (or HEAT as it is often called) is either drawn from the atmosphere or released into the atmosphere.
For a change to a higher state eg. A liquid to a gas, this latent energy is drawn from the atmosphere thus causing cooling of the air.
For the opposite, latent energy is released into the atmosphere.
RELATIVE HUMIDITY
The amount of water vapour in the air is often expressed as relative humidity.
Relative humidity can be defined as a ratio of water vapour actually in the air compared with the amount of water vapour that would cause saturation of that air, and is expressed as a percentage (definition).
For example: A sample of air is at 70% humidity at 20oC
This means that the air contains 70% of water that would saturate it at 20oC.
It is important to note that air is never entirely dry.
The maximum proportion of water that air can hold is depends mainly on the TEMPERATURE.
Warmer air at a given pressure (temperature isn’t the only condition for humidity) can hold more water vapour than cooler air, at the same pressure.
Thus if our sample of air at 70% humidity, at 20oC, was heated to 30oC, without adding moisture, the relative humidity will decrease.
If however we cool the sample the relative humidity will continue to increase until, at a certain temperature it will become saturated (this means that 100% relative humidity has been reached.
The temperature at which 100% relative humidity has been reached is called ____________________________.
If we now continue to cool our sample of saturated air (after it has reached 100% relative humidity), condensation will occur and in the atmosphere this leads to the formation of clouds, fog etc.
HOW IS THE HUMIDITY OF THE AIR MEASURED
To understand how humidity is calculated you must first understand where the readings are obtained.
Temperature recording, for aerodromes is done by using a device called a Stevenson Screen.
Basically this is a small white, ventilated box containing 2 thermometers.
One gives the actual temperature (or dry bulb temperature) and the other gives the dew point temperature (or wet bulb temperature). The dry bulb is an ordinary, mercury – in – glass thermometer and measures the air temperature.
A muslin bag (that is kept moist) covers the bulb of the wet – bulb thermometer.
It gives a lower reading than the dry bulb thermometer because the cooling effect of water, evaporating from the muslin bag.
Here are a few definitions that will help you to understand HUMIDITY.
ABSOLUTE HUMIDITY:
The mass of water vapour in a unit volume of air.
It is a measure of the actual water vapour content in the air.
DEW POINT:
The temperature to which air must be cooled (at a constant pressure and constant water vapour content) for saturation to occur.
WET BULB TEMPERATURE: In simple terms, this is the lowest temperature to which air can be cooled by evaporating water into it.
RELATIVE HUMIDITY:
The ratio of the actual amount of water vapour in the air to the amount it could hold when saturated (expressed as a percentage) OR the ratio of the actual vapour pressure to the saturation vapour pressure expressed as a percentage.
You will never be asked to calculate relative humidity, however the formula for its calculation is listed below.
Relative humidity = The actual amount of water vapour in the air x 100%
The amount of water vapour required to saturate air at that temperature
HUMIDITY AND DENSITY
Humid air is slightly less dense than dry air of the same temperature. This is because water molecules actually weigh less than the molecular mass of air.
At 15o C, saturated air is about 1% less dense than dry air, but at 39o C the difference is 4%.
This is because warmer air can hold more water vapour than cold air.
As a result, humidity has more effect on performance at high temperatures, such as found in tropical regions.
RELATIVE HUMIDITY
From a standpoint of cloud probability we are concerned not so much with absolute humidity (the vapour content on a weight per volume basis) but more with the degree to which an air mass is saturated (or the relative humidity).
Air of _____ relative humidity requires little cooling in order to give cloud and the dew point temperature is consequently not very much different from that of the free air.
Air of ______ relative humidity will require a good deal of cooling before cloud will form.
In this case a wide gap exists between the free air and the dew point temperatures.
You will notice, when listening to the Avalon AWS, that it will give temperature and dew point.
The difference is often an indication of how close or far away cloud formation is.
There is obviously other factors that have to be taken into account, condensation nuclei etc. but it is a good indication.
We have seen that RH is calculated by the above formula, but sometimes there will be a chart like this given.
Relative humidity depends upon both the amount of moisture present and the temperature; with no change in moisture content, the relative humidity falls with a rise in temperature and rises with a fall in temperature.
Owing to its small capacity for water vapour, cold air is in general more saturated than warm air.
The exception to this rule is the equatorial belt where, owing to an abundance of moisture, the relative humidity is high, in spite of the high temperature.
From the fact that rising temperature, within the air, means falling relative humidity, it follows automatically the lowest values occur in air which has been subject to prolonged “dry heating”, while saturation point is nearly approached when the air has suffered cooling.
Saturation
Given a quantity of dry air at a certain temperature there is a definite maximum amount of water vapour, which it can be made to hold.
When the air contains this maximum amount it is said to be saturated.
The air’s capacity for holding water vapour depends upon its temperature.
Suppose a cubic foot of air contains a maximum amount of water vapour at a temperature at T1.
If the temperature is increased to T2 the same cubic foot of air, which has now expanded, is now able to contain an additional quantity of vapour.
Should the temperature now be reduced to the original T1 value, the additional water vapour will be ejected, or condensed out, in a visible mist or cloud.
This is the physical process by which all cloud or mist is formed.
If any sample of air were cooled to the requisite degree it will begin to make cloud, no matter how minute the water content.
The density of the resulting visible vapour, will needless to say, is greater for heavy rain than for lightly laden air.
ATMOSPHERIC STABILITY
The stability of the atmosphere largely determines the amount of vertical motion that occurs in the atmosphere at any particular time.
As this vertical motion is often associated with the formation of clouds and turbulence it is important for pilots to understand Atmospheric Stability.
A stable atmosphere will generally resist vertical movement whilst an unstable atmosphere will tend to have a lot of vertical motion.
A neutrally stable atmosphere will occur when conditions neither resist nor encourage vertical motion.
ADIABATIC PROCESSES
Before looking deeper at the stability of the atmosphere it is necessary to understand the process of adiabatic cooling and heating.
In simple terms an adiabatic process is when the temperature of a sample is changed due to a change in the pressure on the sample.
In other words, the temperature of a sample is changed without heat being added or taken out.
·
In the atmosphere, as a parcel of air rises, the pressure will steadily decrease and as a result of this the temperature will also drop.
·
If a parcel of air sinks through the atmosphere the pressure will increase and as a result of this the temperature will rise.
DRY ADIABATIC LAPSE RATE (DALR)
The DALR is the rate at which a parcel of air, which is unsaturated, will change temperature as it ascends or descends through the atmosphere.
THE DRY ADIABATIC LAPSE RATE IS APPROXIMATELY 3OC / 1000 FEET
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SATURATED ADIABATIC LAPSE RATE (SALR)
The SALR is the rate at which a parcel of saturated air will change temperature as it ascends or descends through the atmosphere.
THE SATURATED ADIABATIC LAPSE RATE IS APPROXIMATELY 1.5OC / 1000 FEET
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The SALR is less than the DALR due to the release of latent heat as water vapour condenses to form cloud droplets as the parcel of air rises and cools.
The latent heat is released to the parcel of air and this reduces the rate of cooling.
Conversely as a parcel of saturated air sinks in the atmosphere latent heat is absorbed from the air as the liquid water droplets evaporate.
This reduces the rate of warming of the saturated air.
ENVIRONMENTAL LAPSE RATE
The ELR is the actual change of temperature with height gained in the atmosphere at a particular place and time.
This information is obtained by using radio – sonde equipment taken aloft by weather balloons.
Of course the ELR will change markedly from place to place and day to day.
It is these changes that determine atmospheric stability and ultimately cloud types.
Below I have created graphs comparing what temperature rates will give stable, unstable and conditionally stable environments.
| If the environmental temperature distribution is such that a parcel of air rising through it remains warmer than the environmental air, it will continue to rise and conditions are said to be UNSTABLE
ELR>DALR>SALR
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UNSTABLE
 | If the environmental temperature distribution is such that a parcel of air rising through it remains cooler than the environmental air, the parcel will tend to sink back down and conditions are said to be STABLE
ELR
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STABLE
| Conditional stability is said to exist if a lifted parcel remains cooler than the environmental air as it cools at the DALR but becomes warmer than the environmental air if it becomes saturated and then cools at the SALR.
Thus conditions will remain stable as long as the air remains unsaturated but becomes unstable if saturation occurs.
SALR
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CONDITIONAL STABILITY
[size=100pt] [size=100pt]
[size=100pt]CLOUDS CLOUDS
Clouds are made up of minute particles of liquid, water or ice.
Clouds are formed by the cooling of moist air to below it’s dew point.
This causes condensation to occur and cloud droplets are formed.
Tiny particles such as dust, salt etc are necessary in this process as they give the water something to condense onto.
These tiny particles are known as condensation nuclei or hygroscopic nuclei.
CLOUD FORMS
When clouds are produced they will be in one of two forms – cumuliform or strataform.
·
Cumuliform type clouds will be produced in an unstable atmosphere by rising convective air cooled below dewpoint.
Cumuliform clouds are heaped or towering in appearance.
·
Strataform type clouds will generally form in stable atmosphere and are generally flat and layered in appearance.
CLOUD CHARACTERISTICSCLOUD TYPE | DESCRIPTION | ICING | PRECIPITATION | Cirrus (high) Ci | White tufts or filaments composed of ice crystals indicating upper level jet streams, wind shear and turbulence | Nil | Nil | Cirrocumulus (high) Cc | Small puffy or rippled elements composed of ice crystals width less than 1o (1 finger) | Usually nil | Nil | Cirrostratus (high) Cs | Nearly transparent sheet or veil like cloud covering.
Halo effect around sun is often observed.
Composed of ice crystals | Usually nil | Nil | Altocumulus (middle) Ac | White to grey, rippled elements 1-5 (3 fingers) | Light rime possible and light turbulence | Light showers of rain possible if castellanus | Altostratus (middle) As | Grey sheet or layer.
Sun appears through ground glass | Moderate Rime possible and light turbulence | Light to moderate rain or snow | Cumulus (low) Cu | Small to large cauliflower shaped cloud, flat base needs unstable or conditionally instable atmosphere | Clear ice in towering Cu (TCU) above freezing level | Showers of Rain or Snow | Stratocumulus (low) Sc | White to grey layered cloud | Occasional Rime ice | Drizzle | Stratus (low) St | Thin greyish sheet or layer associated with bad weather | Usually nil | Drizzle | Cumulonimbus (low and middle) Cb | Towering cauliflower shape, anvil top, lightning, thunder | Sever clear ice | Heavy showers of rain, hail or snow | Nimbostratus (middle) Ns | Thick grey cloud layer.
Often dark | Moderate rime ice, clear ice probable | Continuous rain or snow |
CLOUD DECODE
Visibility
The prevailing visibility encountered when flying is extremely important to the safe conduct of flight, particularly to the VFR (Visual Flight Rules) pilot who must maintain ‘visual’ conditions at all time.
To help in deciding if conditions are suitable for flying, many meteorological reports and forecasts include an observed or forecast visibility level.
METEOROLOGICAL VISIBILITY
Meteorological visibility is given in aerodrome weather reports and forecasts and refers to the greatest horizontal distance which an object can be seen and recognised with normal eyesight.
Meteorological visibility is intended primarily to be a measure of atmospheric transparency.
It does not depend upon how bright the lighting conditions are, so the nighttime visibility should, unless conditions change, be the same as in daylight.
On occasion the visibility may be found to vary in different directions from the point of observation.
In this case the visibility reported will be will be the greatest visibility over half or more of the horizon taken at eye level.
If a significant reduction in visibility is observed in one direction it will be separately reported in plain language.
What is
visibility?
____________________________
Controlled Airspace – Class C
| TYPE OF AIRCRAFT
| HEIGHT
| FLIGHT VISIBILITY
| DISTANCE FROM CLOUDS HORIZONTAL / VERTICAL
| ADDITIONAL CONDITIONS
| | Aeroplanes, Helicopters and Balloons
| At or above 10,000FT AMSL
| 8KM
| 1,500M Horizontal
1,000FT Vertical
|
| | Below 10,000FT AMSL
| 5,000M
| 1,500M Horizontal
1,000FT Vertical
| ATC may permit operations in weather conditions that do not meet this criteria (Special VFR)
|
Controlled Airspace – Class D
| TYPE OF AIRCRAFT
| HEIGHT
| FLIGHT VISIBILITY
| DISTANCE FROM CLOUDS HORIZONTAL / VERTICAL
| ADDITIONAL CONDITIONS
| | Aeroplanes, Helicopters and Balloons
| Within Class D CTR
| 5,000M
| 1,500M Horizontal
1,000FT Vertical
| ATC may permit operations in weather conditions that do not meet this criteria (Special VFR)
|
Controlled Airspace – Class E
| TYPE OF AIRCRAFT
| HEIGHT
| FLIGHT VISIBILITY
| DISTANCE FROM CLOUDS HORIZONTAL / VERTICAL
| ADDITIONAL CONDITIONS
| | Aeroplanes, Helicopters and Balloons
| At or above 10,000FT AMSL
| 8KM
| 1,500M Horizontal
1,000FT Vertical
|
| | Below 10,000FT AMSL
| 5,000M
| 1,500M Horizontal
1,000FT Vertical
|
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GAAP Control Zone
| TYPE OF AIRCRAFT
| HEIGHT
| FLIGHT VISIBILITY
| DISTANCE FROM CLOUDS HORIZONTAL / VERTICAL
| ADDITIONAL CONDITIONS
| | Aeroplanes, Helicopters and Balloons
| Within GAAP CTR
| 5,000M
| Clear of Cloud
| ATC may permit operations in weather conditions that do not meet this criteria (Special VFR)
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Non - Controlled Airspace – Class G
| TYPE OF AIRCRAFT
| HEIGHT
| FLIGHT VISIBILITY
| DISTANCE FROM CLOUDS HORIZONTAL / VERTICAL
| ADDITIONAL CONDITIONS
| | Aeroplanes, Helicopters and Balloons
| At or above 10,000FT AMSL
| 8KM
| 1,500M Horizontal
1,000FT Vertical
|
| | Below 10,000FT AMSL
| 5,000M
| 1,500M Horizontal
1,000FT Vertical
| | At or below 3,000FT AMSL or 1,000FT AGL whichever is the higher
| 5,000M
| Clear of Cloud and in sight of ground or water
| Carriage and use of radio is required when operating to these conditions for communications on the MBZ frequency or CTAF when within the prescribed distance of an aerodrome, or on the area VHF whilst en route.
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AIR TO GROUND VISIBILITY
The view of an aerodrome as seen by the pilot when flying overhead an aerodrome from what they see on final approach.
The main cause of this, is a shallow layer of fog on the aerodrome.
It is easy, when overhead to see through the shallow layer but on final the runway is unable to be seen.
This situation can be particularly dangerous at night where the pilot may not realise there is fog until he descends into it.
Factors affecting visibility
·
Fog or Mist
______________________
·
Precipitation
______________________
·
Haze (oils)
______________________
·
Smoke
______________________
·
Dust and Sand
______________________
FOG Definitions
FOG – is a reduction in visibility caused by suspended water droplets where visibility is less than 1000m and the relative humidity is 95% or greater.
In sub zero temperatures fog may be composed of ice-crystals and is then known as ice fog.
MIST – is a reduction in visibility caused by suspended water droplets where visibility is 1000m or greater and relative humidity is 90% or greater.
Radiation Fog
Radiation fog is formed when moist air is cooled below its dewpoint.
This cooling occurs due to the air’s contact with a surface that has lost heat energy due to radiation.
The conditions necessary for the formation of radiation fog are;
·
CLEAR SKIES – A cloudless night allows heat loss of the ground by terrestrial radiation which in turn cools the air in contact with it.
·
HIGH HUMIDITY – Moist air requires little cooling before reaching dewpoint temperature.
At and below dewpoint, condensation of water occurs onto the condensation nucleus to form visible water droplets.
·
LIGHT WINDS – A light wind of 5 – 7 knots allows mixing of the air and deepens the fog layer.
In completely calm conditions dew or frost or a very shallow layer of fog / mist will form rather than fog.
·
STABLE CONDITIONS – Radiation fogs will usually form in an area of high pressure and stable conditions.
·
DISPERSAL – Fog will generally be broken down by warming of the air or increasing wind speed.
Warming of the air causes evaporation whereas strong winds will mix in warmer and drier air and cause fog to break up.
ADVECTION FOG
Advection fog forms when warm moist air moves horizontally over a cold surface.
Advection fog can form at any time of day if the conditions are right.
Light or moderate winds cause greater mixing and a deeper fog to form.
A common type of advection fog is sea fog where warm moist air over the sea moves over cold land or a cool area of ocean.
FRONTAL FOG
Frontal fog occurs between the boundary of two airmasses and is described as either extensive cloud down to ground level or is formed by precipitation where saturation of the air occurs.
Frontal fog can form very rapidly and cover extensive areas.
Frontal fogs are usually associated with warm fronts or slow moving cold fronts and are therefore not very common.
UPSLOPE FOG
Upslope fog forms when moist air is pushed up sloping terrain.
As it rises it cools adiabatically and if cooling below dewpoint occurs a fog will form.
DUST STORMS
Dust storms can be a hazard to aviation, typically reducing visibility to below 1000 meters.
There are three requirements for their formation;
·
A source region of dust.
This could be an arid inland region ( which is common in droughts
·
Moderate to strong surface winds, to lift the dust into the atmosphere; and
·
An unstable atmosphere which will keep the dust in the air – usually ahead of rapidly moving fronts or lows.
DUST DEVILS
Dust devils are small-scale swirling clouds of dust, usually occurring in arid inland areas in the hotter months.
This is not to say dust devils only occur in these regions.
In 1998 a dust devil over turned 3 of our aircraft at Point Cook.
The greatest risk occurs not only at the bottom but near the top of the dust devil where the swirling winds can affect the aircraft’s lift and control responses.
WIND Changes in Direction
VEERING:
A wind is said to be veering if its direction changes in a clockwise manner i.e. from 360o / 20 to 090o / 20
BACKING:
A wind is said to be backing if its direction changes in an anti clockwise manner i.e. from 360o / 20 to 270o / 20
CHANGES IN SPEED
GUSTS:
A gust is a brief increase in wind speed above the mean wind speed
generally lasting for only a few seconds (shorter than a squall).
Gusts
will be applied on forecasts when the wind increases (gusts) 10 knots
above the forecast mean wind speed.
On a forecast Gusts are written
as 36020G35 KT.
Gustiness is indicative of instability and turbulence in
the friction layer.
SQUALLS:
A squall is a strong wind that rises suddenly, last for some time and
then rapidly dies away.
To be classified as a squall the increase in
wind speed must be at least 16 knots and the new speed must be
greater than 22 knots or more.
The increase must also last for more
than 1 minute.
The use of the term MAX indicates that the wind has no set direction it is expected to be a maximum.
IMPORTANT POINT
All meteorological forecasts and observations give wind direction in degrees TRUE
ATIS and listening to AWS are the only time wind direction is given in degrees MAGNETIC
WHAT IS WIND
Wind is air in natural motion.
It is usually referred as the broad flow of air either near the earth’s surface or in the free atmosphere.
Air is seldom smooth.
Particularly near the friction layer, irregular changes in both speed and direction of the wind are common.
Wind is a vector quantity, having both speed and direction.
Direction is referenced to a compass direction where the wind is coming from.
Wind speed is measured in knots, which is approx. 1.8 kms/hr.
The ways which we usually measure wind is via a windsock or an anemometer.
Wind is affect by many different things.
Terrain, night or day, location etc. but how is wind created?
The answer to this lies with differences in pressure.
As we already know a force will be created when we have an area of high pressure near an area of low pressure (see diagram below).
This force is known as the Pressure Gradient Force (PGF) however air rarely ever moves in this direction.
The reason for this is due to the rotation of the earth. This is know as Coriolis force and has varies depending on a few factors (which will be discussed later)
Air moving from High pressure to Low pressure (Pressure Gradient Force)
CORIOLIS FORCE
Coriolis force is an apparent force due to the rotation of the earth.
Because the earth is spheroid in shape, looking at the earth from the side view the equator could be said to be rotating faster than the poles (see diagram below, I have used unrealistic speed for the rotation of the earth)
Imagine now that you are at 60o S latitude with a loaded cannon.
If you fire this cannon toward the equator because the speed of the cannon ball has the speed of that particular latitude, the cannon ball will appear to turn to the left in the Southern Hemisphere.
If now you move to 30o S latitude, with the cannon loaded, and you fire it toward the equator the displacement compared to the first cannon ball will be less.
(See diagram below)
Some important points about Coriolis Force
·
Coriolis Force depends upon latitude it is zero at the equator and its effect grows in strength toward the poles
·
The speed of the wind also affects the strength of the Coriolis force, increase wind speed = increased Coriolis force
·
Decrease in wind speed means decrease in Coriolis Force
·
Coriolis force acts perpendicular to the motion of air, regardless of the direction of motion – deflecting motion to the left in the Southern Hemisphere and to the right in the Northern Hemisphere.
GEOSTROPHIC WIND
Lets consider a parcel of air moving horizontally at a constant speed in a region where the friction force is negligible.
The horizontal forces acting on it are the pressure gradient force and Coriolis force.
If the magnitudes of these two forces are exactly equal, the pressure gradient force will prevent the Coriolis force from causing a deflection either to the left in the Southern Hemisphere or Right in the Northern Hemisphere.
Motion of this type is known as Geostrophic Flow, see example below.
Because geostrophic flow is horizontal, the air actually flows along a great circle.
When the motion is projected onto a flat surface of a synoptic chart, it appears to be a straight line.
For a given latitude there is a certain wind speed that coriolis force just balances the existing pressure gradient force.
This is called geostrophic wind.
Its value may be determined at any point on the synoptic chart where the distance between the isobars is known.
Geostrophic wind is not common between latitudes 15o N and 15o S due to Coriolis force being zero.
Wind at the equator is affected by local effects and will move from a high pressure to a low pressure.
BUYS BALLOTS LAW
Buys Ballots Law states:
“If you stand with your back to the wind the lower pressure will be on your right (in the southern hemisphere)”
THE GRADIENT WIND
As we all would have seen a chart like on the previous page, this shows that the lines or Isobars (lines joining places or equal pressure) do not flow in a straight line but follow a curved path around the various high and low pressures.
The wind that blows parallel to straight isobars is known as geostrophic wind.
The wind that follows the curved path of the isobars is known as gradient wind.
This is the wind that you would expect to find at approximately 3000’ (above the friction layer)
In summary the lines on a synoptic chat indicate which way the wind is flowing.
Later when we talk about synoptic meteorology we will be able to identify which direction the wind is coming from at a specific location.
SURFACE WIND FLOW
Between ground level and approx. 2000 – 3000 feet above ground level (AGL) lies the friction layer.
This is the area where friction from the ground slows the wind down; the closer to the ground the more it is slowed.
Remember back to aerodynamics, and how air was slowed down the closer to the wing you got.
The main effect of this apart from the lower surface wind speeds than at altitude are as follows:
·
As the wind speed decrease, due to friction, Coriolis force will decrease.
This will tend to change the direction of the wind in a veering manner.
Even though we are not aware which ways a high pressure or low pressure will turn the effects of friction slowing down the wind and causing it to veer.
EFFECTS OF FRICTION ON WIND
Over land surface, friction slows the wind speed by about 2 / 3 and veers the wind by 30o
Over the water there is less friction so the wind speed drops by 1 / 3 and direction veers by 10o
NOTE:
These figures are good theoretical estimates of the effect of friction however varying surfaces and conditions can produces widely different effects.
DIURNAL VARIATION OF SURFACE WIND
During the day the earth is heated, vertical movement of the air occurs as convective thermals.
This causes mixing of the air at various levels and brings down a stronger gradient wind to the lower levels.
This results in stronger winds at the surface during the day.
At night convective activity ceases and this allows friction to once again reduce the surface wind speed whilst gradient wind continues to blow above the friction layer.
In summary: The wind during the day is usually stronger than at night.
At night the wind direction on the surface will veer more at night due to the reduced coriolis.
LOCAL WINDS
SEA BREEZES
Sea breezes are usually caused by the differential heating rates of the land and the sea during the day.
The surface of the land warms rapidly when exposed to solar radiation.
The sea however warms comparatively little because of the deep mixing of the water and its high specific heat.
Air over the land is in turn heated and begins to rise (convection).
This causes a pressure gradient to develop, over the land a relatively low pressure will be present and the cool air over the sea a relatively high pressure.
The air will now begin to flow from the sea to the land and the sea breeze begins.
A sea breeze will generally begin in the late morning to early afternoon.
The strength of the sea breeze, and its inland penetration will depend upon:
·
How high the heated air from over the land rises
Generally the higher the convection the stronger the sea
See diagram on next page for Sea Breeze
If there is a gradient wind blowing before the onset of the sea breeze the effect will be to modify that wind.
If the prevailing wind is blowing in the opposite direction it may delay or prevent the breeze from reaching the land.
In the tropics there is a marked difference between land and sea temperatures.
This means that the sea breezes tend to be stronger in these regions and can cause the development of thunderstorms if the air over the land is unstable.
LAND BREEZES
Land breezes often develop in coastal regions during the night.
The land surface cools quickly and causes air above to cool and subside.
The sea remains relatively warm and convection occurs.
This sets up a weak pressure gradient and hence a flow of air from the land to the sea.
In general land breezes will not be as strong as sea breezes and will seldom penetrate more than a few miles off shore.
See previous page for Land Breeze
KATABATIC WINDS
At night the ground cools due to terrestrial radiation.
The cool ground then cools the air above it by conduction.
As it is cooled it becomes denser than the surrounding air and if it lies on a slope the force of gravity will cause it to slide down the slope to the lower levels.
Generally katabatic winds are light, seldom exceeding 10 kts in strength.
In some cases however they may be much stronger if the slope is steep, long and smooth.
In the Antarctic these winds can reach upward of 100 kts.
ANABATIC WINDS
An anabatic wind works in the reverse to the katabatic wind.
During the day the sun heats the hill slopes and in turn heats the air above it.
This air is now warmer than the free air at the same level and therefore rises up the slope.
As the air rises it tends to cool adiabatically.
This would tend to stop the upward flow however the air is continually warmed by contact with the slope.
Anabatic winds are generally rather weak due to the air rising against gravity.
FöHN WINDS
In Europe a warm dry southerly wind often flows down the northern slopes of the Alps.
This wind is known as a Föhn Wind.
The process that causes this warm dry wind also occurs in other parts of the world and are also named Föhn winds by meteorologists.
The process producing Föhn winds is as follows:
Air meeting a mountain barrier is forced upward and cools adiabatically at the DALR.
If moisture content is sufficient cloud will form on the windward and the air will continue to rise at the SALR.
If precipitation occurs from the forming clouds moisture will be removed from the air which will then become drier.
The dewpoint temperature of the air will now be less and hence as the air travels down the leeward side of the mountain the air will become unsaturated at a higher level i.e. the cloud base will be higher on
the lee side.
The air will now warm at the DALR as it moves down the mountain and will therefore become a warm dry wind flowing down the mountain.
The significant features of the Föhn effect for aviation are:
a)
Low cloud base and precipitation on the windward slopes
b)
Higher cloud base on the lee slopes
c)
Higher temperatures and hence lower density at low levels in the lee of the mountain
LOW LEVEL JET
The low level jet is a phenomenon that occurs on the eastern side of an anticyclone as it approaches a mountain barrier obstructing its progress eastward.
(see diagram below)
The process by which the low level jet is forms is as follows :
·
The high-pressure system produces subsiding air and stable conditions.
·
On clear nights a strong surface inversion is created
·
This radiation inversion prevents the air aloft from being slowed by the frictional affects of the ground surface and hence the wind speed increases.
·
At higher altitude a subsidence inversion forms which impedes the passage over the mountain barrier.
·
The low level jet is invariably a southerly wind
·
It is at its strongest between 0300 and 0600 LMT when the surface inversion is the strongest and dissipates quickly after sunrise.
·
Strong turbulence caused by viscous drag is often encountered at the boundary of the surface inversion and the strong winds above it
·
Peak wind speeds in the low level jet can reach upwards of 70 kts (50 kts is common)
The low-level jet is normally confined to within 3000’ of the surface.
The strongest winds will be found at around 1500’ above the ground.
The jet will disappear during the morning as surface heating destroys the surface inversion.
| MOUNTAIN RANGE
(I.E. GREAT DIVIDING RANGE)
|
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______________________________
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AIRFLOW PARALLEL TO THE MOUNTAIN RANGE
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AIRMASSES AND FRONTS
When a large body of air remains over one place for a period of time it will tend to acquire the characteristics of the surface beneath it.
For example a mass of air lying over arid central Australia will tend to become warm and dry.
Similarly a mass of air over the southern ocean would, over time become cooler as well as pick up moisture from evaporation.
An air mass is a large body of air that possesses uniform characteristics throughout.
For an airmass to become uniform throughout it must remain over a particular region with a uniform surface (such as all ocean, or all landmass).
Areas of the earth such as this are known as source regions.
AIRMASS CLASSIFICATION
Airmasses are classified according to their source region that they have last occupied.
There are four main source regions:
·
Tropical Maritime
·
Tropical Continental
·
Polar maritime
·
Polar Continental
MARITIME airmasses are formed over oceans and such generally carry
significant
moisture.
CONTINENTAL airmasses originate over land masses and are usually much
drier than maritime airmasses.
TROPICAL airmasses will be formed in tropical regions and such will generally
be warmer throughout.
POLAR airmasses on the other hand will consist of cold air.
MODIFICATION OF AIRMASSES
As an airmass moves over surfaces with different characteristics to itself it will be modified by that surface.
For example as the polar maritime airmass moves toward the equator it passes into warmer regions which causes heating in the lower levels and instability will develop.
Southern Australia experiences this type of situation with the cool southwesterly flows and cold changes that occur regularly, particularly in winter.
An airmass that moves over a colder surface will lose heat in the lower levels and become very stable.
An example of this would be the tropical continental airmass moving into southern Australia.
This usually brings dry warm air and stable conditions to this region.
OCCLUDED FRONTS
A cold or warm front occlusion is just a fancy name for either overtaking the other.
Because cold fronts generally move faster than warm fronts a situation where cold air over takes warm air can occur.
This situation is called an occluded front.
The cold air of the cold front undercuts and pushes up the warm air behind the warm front.
Cloud and weather associated with an occluded front will depend upon the cloud pre-existing in the cold and warm fronts.
There are two main types of occluded front – the cold front occlusion and the warm front occlusion.
Below are examples of what an idealised cold and warm front looks like and then a diagram of a cold and warm front occlusion.
COLD FRONT OCCLUSION
The cold front occlusion is formed when the cold front air is colder than the air ahead of the warm front.
When this occurs the cold front undercuts the cold air and pushes it aloft.
WARM FRONT OCCLUSION
The warm front occlusion occurs if the air ahead of the warm front is colder than the cold front air.
This causes the cold front to slide up over the colder air in the front.
On a synoptic chart the occluded front looks something like the picture below
QUASI-STATIONARY FRONT
A front that is not moving or is tending to move along its own length is called a quasi-stationary front.
These fronts align themselves with the isobars and roughly lie east west.
The polar front that will be discussed later, is a typical example of a quasi-stationary front.
Quasi-stationary fronts are not associated with a trough, and usually have an area of high pressure on both sides.
Weather associated with the front is likely to be nil, winds to the north are generally from the west in the warmer air while the south winds are cooler easterlies.
COLD FRONTS
Fronts are classified by their motion.
A front that moves so that cold air moves to replace warm air at ground level is known as a cold front.
The leading edge of a cold front is usually fairly steep, a typical gradient being 1 / 50.
Because of this steep slope, rapidly moving cold fronts quickly push up the warm air ahead of the front producing clouds of considerable vertical development.
This in turn leads to frequent showers and thunderstorms before and with the passage of the front.
PASSAGE OF A COLD FRONT
·
Pressure > The air pressure will _______ steadily with the approach of a front.
At the cold front boundary the pressure bottoms out and then starts to rise.
·
Temperature >
There is a sudden _______ in air temperature with the passage of a cold front.
·
Wind >
The wind direction changes from north westerly to south westerly with the passage of a cold front i.e. the wind
______
·
Cloud >
large Cu and Cb in warm air, Heavy showers, gusty winds and possible thunderstorms confined to a narrow region for steep sloping front.
·
Tropopause >
Cold southerly airstreams lead to a ________
tropopause.
WIND BACKS
WARM FRONTS
A warm front is the leading edge of an advancing mass of warm air.
The slope of a warm front is generally much flatter than a cold front.
The warm air rises over the cold airmass much more gradually than a cold front.
This leads to the development of widespread strataform cloud and precipitation.
Rain falling out of warm air into cold air below may saturate the air and lead to widespread stratus or fog developing.
PASSAGE OF A WARM FRONT
The passage of a warm front can be recognised by the following:
·
Pressure >
The air pressure _________ with the approach of the front.
With the passage of the front the pressure drops slowly.
·
Temperature >
The air temperature _______
with the passage of a warm front.
·
Wind >
The wind direction _______
with the passage of a warm front.
PRESSURE SYSTEMS
By now we know that there are high and low pressure systems.
But what determines that a system is a high pressure of low pressure.
A high pressure usually means that conditions are stable i.e. air is subsiding.
A low pressure usually means that conditions are unstable i.e. air is rising.
From our previous discussion about the forces acting on a horizontal moving parcel of air we know that Coriolis force acts to turn air to the left in the Southern Hemisphere and right in the Northern Hemisphere.
This will help us understand what way a high pressure and low pressure turn.
In the Southern Hemisphere when there is a low pressure it will tend to flow in a clockwise motion.
This is primarily due to Coriolis.
A high pressure system will tend to flow in an anti-clockwise motion.
Consider what we already know about Coriolis:
·
It acts at right angles to the direction of the motion of air (the wind) so it alters the direction of the wind but not it’s speed
·
It increases with latitude and with wind speed.
It is zero at the equator, and for the same wind speed, increases as you move further away from the equator.
We must also remember that there is the overall force of a high pressure wanting to replace, or move to, a low pressure.
If we now look at more detail at the individual pressure systems we can see why a low pressure will rotate to the clockwise in the Southern Hemisphere and a high pressure will rotate anti-clockwise in the Southern Hemisphere.
As shown from the picture below keeping in mind that Coriolis want to turn the wind to the left and acts 90o to the wind direction, it is fairly self-explanatory.
Now that we understand how the pressure systems flow we can begin to understand and interpret Australian synoptic charts.
Before we do this however we will look further a field to world flows and the cycle of motion throughout the earth.
WORLD FLOWS
From looking at the distribution of the sun over the earth, we know where the sun is at its greatest intensity.
This would be at the equator naturally, as it is always approximately exposed to the suns greatest intensity.
We must however consider the fact that the earth is tilted at 23.5o and one hemisphere will experience more sun than the other at different times of the year (hence we have the seasons).
Recalling the heat distribution over the earth during the different seasons we can see on this picture that this would be the northern summer and southern winter.
If we now look at the bigger scale flows air, we can see that due to the greater intensity of heat at the equator causes mass uplift or convection.
This causes the pressure around the equator to be a low pressure (called the equatorial trough).
At the poles however there are extremely cold conditions which create large scale descent of air.
This causes the pressure around the poles to be high.
As we know high pressure must flow to fill the low pressure so we have a circulation created.
In both hemispheres the typical layout of pressure is:
·
The equatorial trough, being around 0o (the equator) _______ pressure
·
The sub tropical high, being around 30o latitude and a _______ pressure
·
The sub polar low, being around 60o latitude _______ pressure
·
The polar high, being around 90o (the poles) _______ pressure
If we were to look at the earth from the side view the circulation would look like the diagram below.
As we now understand what effect coriolis has on the wind, and appreciation of what is the effect of global airflows we can be properly understand mass circulation.
SOUTHEAST TRADE WINDS
Just below the equatorial trough we encounter the southeast trade winds.
These winds can be best explained if we look back at the different speeds at which the earth rotates at various latitudes
(see diagram below).
If we have air that is descending and spreading out at a particular latitude (i.e. sub tropical high) this air will flow towards the equator.
It does this because it has the speed of the earth at that latitude it will deflect to the left causing wind from the southeast, hence the southeast trade winds.
PREVAILING WESTERLIES
As the sub tropical high produces airflow toward the equator it will also produce airflow toward the poles.
This air having the speed of the earth at that latitude will accelerate toward the poles and due to those latitudes being slower the air will deflect to the right, causing westerlies.
The wind flow that occurs between the polar high and the sub polar low is attributed to the convergence of the air from the sub tropical high and the polar high.
This causes mass lifting of the air and hence the sub polar low.
The diagram on the following page illustrates the flow of air throughout the earth
FLIGHT CONSIDERATIONS THUNDERSTORMS
Knowledge of the types, causes and hazards associated with thunderstorms is extremely important for a pilot.
Thunderstorms contain many hazards that must be avoided – hail, lightning, icing, sever turbulence and even tornadoes.
CONDITIONS NECESSARY FOR FORMATION
There are three main conditions necessary before a thunderstorm can develop.
They are:
·
INSTABILITY- A deep layer of unstable or at least conditionally stable air is necessary to allow large vertical development.
·
ABUNDANT MOISTURE - There must a considerable depth of moisture in the atmosphere.
·
TRIGGER - A trigger mechanism is required to start the upward motion of the air.
Common triggers of thunderstorms are the same triggers that create cloud being:
·
Convection
·
Frontal activity
·
Orographic uplift
LIFE CYCLE OF A THUNDERSTORM
Each thunderstorm goes through three stages in its life cycle:
1.
Cumulus or growing stage
2.
Mature stage
3.
Dissipating stage
CUMULUS STAGE
The first stage in the formation of a thunderstorm is the cumulus or growing stage.
This period lasts for up to 30 minutes.
As the moist air rises it cools until dew point temperature is reached.
Condensation then occurs and the cloud forms.
Latent heat is released keeping the air warm and adding to the instability.
Strong up draughts predominate in this stage and the vertical speed may reach upwards of 5000’ / minute.
Water droplets are carried higher and higher by these up draughts and if carried above the freezing level may either freeze or remain as super cooled water droplets.
Collision of the droplets causes them to become larger and heavier.
This process is known as coalescence (see next page).
The cloud grows to cumulonimbus stage.
MATURE STAGE
The mature stage begins when the up draughts can no longer support the heavy raindrops and ice crystals and they begin to fall through the cloud.
As they fall coalescence occurs and air and raindrops are dragged downwards.
A strong downdraught and develops and the first rain will start to reach the ground.
Lightning will also commence at this time.
These two factors herald the onset of the mature stage and the greatest intensity of the thunderstorm.
The cloud has now grown to the characteristic cumulonimbus shape and a cirrus anvil top forms.
The cloud may reach the tropopause which can be up to 55 000’ in the tropics or higher.
The combination of strong up draughts and down draughts lead to extreme turbulence and windshear.
The cold downdraughts falling out of the bottom of the cloud flow outward causing low-level windshear.
DISSIPATING STAGE
The downdraught has spread throughout the cloud and cut off the up draught.
As there is no longer warm moist air rising through the cloud, no further condensation takes place and the storm gradually dies.
The dissipating stage is characterised by downdraughts and precipitation and lasts for around 30 minutes.
SEVERE THUNDERSTORMS
If significant horizontal windshear exists through the atmosphere, severe and longer lasting thunderstorms can be produced.
This is because the windshear tilts the up draughts and downdraughts and keeps them well separated.
This means the downdraught will not choke the thunderstorm and can continue to let it grow in intensity to form a super cell.
HAZARDS
Thunderstorms contain many dangerous conditions and so must be avoided by aircraft at all costs.
These hazards include:
·
TURBULENCE AND WINDSHEAR - This can cause violent changes in aircraft attitude and altitude, severe ‘g’ loading may also be encountered.
·
LOW LEVEL WINDSHEAR – The cold downdraught falling from the bottom of the cloud spreads outward at low level and creates a gust front.
This “FIRST GUST” emanating from the cloud can be hazardous to aircraft up to 10 kilometers away, as the guts may cause a sudden change in wind speed and direction.
·
ICING – Severe icing particular clear ice is likely just above the freezing level.
Clear ice is usually encountered in temperatures from 0oC to -15oC (and down to
– 20oC in some cases)
·
HAIL -
Hail occurs in most thunderstorms and can cause serious damage to aircraft.
It is frequently encountered between 10000 – 30000’ and often occurs in clear air underneath the cirrus anvil (i.e downwind of the cell)
Subtropical and tropical thunderstorms generally have less hail than mid-latitude storms because of a higher freezing level and melting of the hail before it reaches ground level.
·
LIGHTNING - The main danger of lightning is that it can cause damage to radios and electronic equipment.
Damage to aircraft skin is usually minimal.
A lightning strike on an aircraft looks like a pinhole.
·
VISIBILITY - Reduced visibility when flying in heavy precipitation is a hazard to VFR aircraft.
TYPES OF THUNDERSTORMS
Types of thunderstorms are usually classified into two main types – Airmass and Frontal
FRONTAL, Shear and Squall line thunderstorms all form along the boundary of two airmasses or lines of significant weather, such as a cold front.
They are most dangerous of the two types because a line of thunderstorms may develop.
These storms are often fast moving and have lower cloud bases than airmass thunderstorms.
The stronger downdraughts near the ground make passage around very difficult.
AIRMASS thunderstorms tend to be more widely spaced or isolated cells with higher cloud bases than frontal thunderstorms.
Airmass thunderstorms can be divided into 4 types
·
HEAT and CONVERGENCE - Surface-heating leading to convective activity is a common factor causing thunderstorms that usually form in the afternoon or evening.
Convergence type thunderstorms are most likely to occur when two airmasses going different directions collide and create up draughts.
·
OROGRAPHIC - Significant terrain lifting air in an unstable atmosphere can lead to the development of thunderstorms
·
COLD STREAM - These thunderstorms develop when cold air flows over warmer seas, causing warming of the lower layers.
Moisture will be picked up and in an unstable atmosphere may cause a thunderstorm to develop.
These thunderstorms are most likely to occur in a southwesterly airflow behind a cold front
·
NOCTURNAL EQUATORIAL - As the name suggest this thunderstorm usually occur at night (nocturnal) near the equator (equatorial).
They usually develop near the equator over the ocean, a considerable distance from land, and approximately 100 – 200 nm from land.
The ocean temperatures remain warm at night (around 28oC) but the tops of cumulus clouds are cooled due to radiation.
This leads to instability and thunderstorms can develop around sunrise.
VERY IMPORTANT POINT
Within the thunderstorm the most severe turbulence is near the freezing level particularly in the middle part of the storm.
VISUAL INDICATIONS OF WINDSHEAR AND DOWN BURSTS
Severe windshear and downdraughts are most commonly associated with:
·
Thick precipitation from a CB or CU, particularly with curling outflow
·
Blowing dust, particularly in a circular pattern
·
Virga
·
Roll clouds or ragged stratus clouds near the gust fronts indicating rapid air rotation
The most sever turbulence from downdraughts occurs shortly after the onset of the mature stage of thunderstorms
VIRGA
Virga is defined as rain that evaporates before hitting the ground.
This can be an indication of down burst / micro burst.
As the rain is being evaporated before it hits the ground latent heat is being absorbed from the atmosphere, reducing the overall temperature of the air.
The colder the air get the denser it gets.
If the air gets too dense to be held it will fall as a downburst / microburst.
TURBULENCE Turbulence can be produced in a number of ways.
Generally however we consider three basic mechanisms for the formation of turbulence:
·
Convection
·
Mechanical
·
Windshear
CONVECTIVE TURBULENCE
Convective or thermal turbulence is caused by rising air currents.
It forms by the sun heating up the ground and then the ground heats up the air above it.
This causes parcels of air to rise into the atmosphere.
If the air is moist, cumulus or cumulonimbus cloud may form.
Convective turbulence may be found underneath and within the cloud.
If the air is dry cloud will not form but convective turbulence will still be experienced.
Convective turbulence will be strongest on hot summer days.
MECHANICAL TURBULENCE
Mechanical or friction layer turbulence is created by the interruption of smooth wind flow into turbulent eddies caused by buildings, rough terrain, trees etc.
The wind strength and nature of the obstruction will determine the size of turbulent eddies formed.
WINDSHEAR TURBULENCE
There are two main types of atmospherically created wind shear.
Frontal and Inversion turbulence.
FRONTAL TURBULENCE
Frontal turbulence is caused mainly by the horizontal windshear along the frontal zone (the boundary of two airmasses, with wind flowing in opposite directions).
Winds of different speed and direction meet at the frontal line and can cause severe turbulence.
INVERSION TURBULENCE
Turbulence caused by windshear can often be encountered when flying through an inversion.
The wind shear is caused by markedly different winds either side of the inversion.
There are many other types of turbulence, Mountain or standing waves and Clear Air Turbulence (CAT).
For PPL and CPL you should have a good understanding of how mountain wave turbulence is formed and what hazards are associated.
CAT is covered in more detail in ATPL.
MOUNTAIN OR STANDING WAVES
Mountain waves can cause considerable problems to pilots so knowledge of the way in which they form and the dangers they represent is important.
Mountain waves are basically a wave type motion of the wind flow after passing over a mountain.
Mountain waves form when a strong wind flow blows at right angles across a mountain range.
The required conditions for these waves are:
·
A mountain range barrier at right angles to the wind flow.
·
A wind speed of at least 25 knots at the top of the mountain and increasing with height.
·
An upper inversion or stable layer sandwiched between an unstable layer at the surface and weakly stable or unstable layer above.
·
If sufficient moisture is present a lens or almond shape cloud called a lenticular cloud can form in the crest of the waves.
The picture below indicates what the conditions would look like:
CHARACTERISTICS OF MOUNTAIN WAVES
·
Wavelength can vary from 5 – 50 km.
·
Lenticular clouds in crests will form if sufficient moisture is present.
·
Vertical currents of up to 4000’/min.
·
Severe downdraughts on the lee side of the mountain
·
Rotor zones can be formed under the wave crests – turbulence can be very severe.
Rotor cloud will form in these zones if moisture is present
·
A lee trough forms near the lee side of the mountain
FLYING ASPECTS
·
When flying in valley always keep to the downwind side where rising air is likely.
·
Fly upwind of lenticular or rotor clouds to stay in rising air
·
On the lee side a lee trough can form which can cause altimeters to over-read
INVERSIONS In the troposphere the temperature of the air in the environment usually decreases with height.
This is known as a positive lapse rate.
Sometimes however temperature increases with height gain (negative lapse rate).
This increase in temperature with height is known as an inversion.
Inversions usually occur in stable conditions and resist the upward movement of air.
Any cloud produced below an inversion will tend to be strataform as the cloud spreads out below the inversion.
In the same way that smoke and pollution can be trapped beneath an inversion layer and cause reduced visibility below the layer.
There are 3 main types of inversion:
RADIATION INVERSION
This type of inversion forms on clear, light wind nights and are strongest around sunrise.
The cooling of the ground cools the layer of air in contact with it through a shallow layer.
A light wind may mix the air somewhat and cause a deepening of the inversion.
A strong radiation inversion is usually not more than a few hundred feet thick.
A radiation inversion often causes a decoupling of surface winds from winds aloft.
This may cause windshear turbulence when flying through the inversion layer
SUBSIDENCE INVERSION
Subsidence inversions occur in high-pressure systems and are formed by the subsidence or sinking of air from high level.
As this air sinks it is warmed adiabatically.
The top part of the subsiding air usually subsides more than the bottom portion.
This means that the top of the layer is warmed more than the bottom, and an inversion is formed.
Subsidence inversions generally occur at about 4000’ and can trap smoke and haze thus causing reduced visibility below the inversion.
Above the inversion the visibility is usually extremely good.
FRONTAL OR SEA BREEZE INVERSION
Frontal or sea breeze inversions form when a wedge of cold air undercuts warmer air as a cold front or sea breeze advances.
This leads to cool or cold air with a warm layer above it.
ICING
Airframe icing can be broken down into three main types.
Two of them rime ice and clear ice can occur when the outside air temperature (OAT) is 0o or below and visible moisture is present.
The third type, hoarfrost, can occur in clear air when the temperature of the aircraft surface falls below 0oC and the relative humidity is high.
Icing rarely occurs below –40oC because the clouds contain ice crystals only.
The three conditions necessary for the formation of ice are:
·
Visible moisture
·
Temperature must be at or below freezing 0oC
·
The airframe must be less than 0oC
RIME ICE
Rime ice is a white opaque and brittle deposit of ice that is formed by the rapid freezing of small supercooled water droplets on contact with the sub-zero surface of an aircraft.
Because the freezing process occurs almost instantly air is trapped between the ice particles.
This leads to the brittle nature and opaque appearance of rime ice.
The ideal temperature for rime ice is from 0oC to –40oC but it is most commonly encountered in the range –10oC to –20oC.
Rime ice is usually encountered in strataform cloud, however it must be said that cumuliform clouds can produce rime ice at temperatures below –10oC.
Hazards associated with rime ice:
·
Distortion of the airflow and consequent reduction in aerodynamic efficiency of the wings
·
Build up of ice on the air intakes for the engines, as well as blocking of pitot heads, static sources and venturis.
Removal
With all ice that builds up on aircraft, rime ices can be removed by conventional de-icing methods, but it should be realised that rime ice can occur with clear ice in which case removal of the ice may be more difficult.
The other way to remove ice would be to descend to a lower level where the temperature is warmer.
CLEAR ICE
Clear ice is a transparent smooth or rippled sheet of ice that is formed by the relatively slow freezing of large supercooled water droplets.
The drops tend to flow back over the aircraft’s surface as it freezes thus a solid glaze of ice will be formed that adheres very quickly and is very difficult to remove.
This makes clear ice the most dangerous form of icing encountered by aircraft.
Clear ice is usually encountered in cumuliform clouds (convective), as the updraughts can support large water droplets.
The usual temperature range for clear ice is from 0oC to –15oC.
Clear ice can also occur in thick nimbostratus.
Severe ice is possible in thick Ns and Cb.
Hazards associated with rime ice:
·
One of the most sever forms of clear ice occurs when supercooled rain droplets or drizzle are encountered by aircraft.
This is known as freezing rain or freezing drizzle, it may result in the aircraft being enshrouded in a clear ice layer within a matter of seconds.
Such conditions may occur ahead of a warm front or behind a cold front.
·
Freezing drizzle around 0oC may result in a combination of clear and rime ice.
Removal:
As with rime ice, clear ice can be removed by conventional by de-ice devices or by descending to a lower altitude where the temperature is warmer
Main Hazards to Aircraft
·
Ice accumulations disrupt the flow around aerofoils, resulting in a loss of lift and increase in drag.
·
It will also cause the aircraft to stall at a higher airspeed than normal.
·
Ice will also add to the weight of the aircraft, which is crucial when we are talking about how much lift and thrust are lost.
·
It has been tested that 1 cm of ice on the leading edge of an aerofoil can reduce the lift and increase the drag by about 50%.
Remember that 1 cm of ice can accumulate in a minute of two in some cases.
·
Ice on a propeller hub and blades will drastically reduce the overall efficiency of the propeller.
·
Propellers with low RPM are more susceptible to icing than propellers with high RPM.
·
Wing tip tanks are good surfaces for ice.
The greatest effect of icing on these surfaces is to increase the drag on the aircraft.
·
Pitot tube and static pressure ports is dangerous because it causes inaccurate airspeed and altimeter readings.
When ice is seen to be accumulating, the pilot should expect that the static ports are accumulating ice as fast if not faster than other areas of the aircraft.
·
Radio antennae are very likely to accumulate ice and cause loss of radio communication.
This can be dangerous, as the pilot will loose radio communication when a change in heading or altitude is needed the most.
Ice loading on the antenna may cause it to break and flap against the fuselage.
·
Formation of ice on windscreens is not very dramatic to a pilot who is flying IFR at altitude, but can cause a problem upon breaking visual on exiting the cloud.
·
In a normally aspirated engine ice can cause a huge problem.
Especially in the carburettor with the risk of carburettor ice.
It is important to note that carburettor ice can for when temperatures reach as high as 20oC.
Flying at low power settings, with a cool temperature and high humidity is the time at which you are at most risk of carburettor ice.
HOAR FROST
Hoarfrost is a light crystalline deposit of ice that forms on the aircraft skin surface.
Hoarfrost can be likened to frost deposits that occur on your car on mornings where the temperature is 0oC or less.
It develops in clear conditions with high relative humidity.
The surface of the aircraft cools to below 0oC and this in turn cools the air closest to it.
The water vapour changes to ice and is deposited on the aircraft skin.
Hoar ice generally occurs in two situations – on the ground (as mentioned like frost on your car) and airborne.
On the ground hoarfrost usually forms on a calm, clear winter night when the aircraft is cooled below the freezing point.
The deposits of frost must be cleared prior to flight as the airflow over the wings and tail surfaces is changed and can lead to reduced aerodynamic efficiency (i.e. a much higher stalling speed and a lower stalling angle).
Visibility through the windows can also be affected.
In flight, hoarfrost may develop when the aircraft passes from a region of sub zero temperature into a region of warm moist air.
When the skin temperature of the aircraft remains below freezing for a short time and the aircraft descends into this moist air a coating of frost can quickly cover the aircraft.
This is not generally dangerous except for the loss of visibility, and will melt in flight fairly quickly as the aircraft remains in the warmer air.
SYNOPTIC METEOROLOGY
Decode of Synoptic Charts
High Pressure (anti-cyclone)
In a high-pressure system, stability of the air is usually greatest near the center of the high.
If the air is dry, cloudless conditions may be associated with light and variable winds.
Dew or frost may form at night.
If the air is moist, early morning mist or fog may occur in winter.
During the day an inversion may develop leading to overcast stratus or stratocumulus.
Light drizzle may occur.
In cities this inversion may cause pollution to be capped causing reduced visibility.
As you get further away from the center of the high pressure the pressure will decrease and stability will reduce and wind will increase.
It is important to note that the actual weather depends largely upon the nature of the underlying surface over which the air is moving.
This affects the stability and moisture content of the air in the lower levels.
Subsidence aloft may, however, still limit cloud development.
Ridges
Air in ridges is also relatively stable, owing to the effects of subsidence.
The general weather characteristics associated with high pressure apply to ridges.
Low pressures (depressions)
Falling pressures are associated with rising air.
Weather associated with various parts of the depression depends on the various types of wind flows but is ultimately affected by the ascent of air that occurs in the depression.
Some non-frontal inland depressions are formed as a result of inland heating.
These are called heat lows.
These depressions are often a little more than wind circulation’s, because of the low moisture content of the air, making it impossible for extensive cloud formation.
Troughs
Troughs are often associated with cold fronts, however very often troughs appear without a front.
These non-frontal troughs are however, regions of relatively low pressure.
Surface convergence and rising air therefore often lead to the development of cloudy conditions and bad weather.
Col
A col is an area of very light winds.
The weather depends upon the characteristic of the particular airmass present.
Diurnal effects often exert a strong influence on the conditions experienced.
OTHER TYPES OF WEATHER ANALYSIS
There are many other type of weather analysis.
The weather bureau will use many different pieces of information to construct a fairly correct picture of current conditions and give a pretty good guess as to what is expected.
As far as charts go there are two main charts being the isotach and isotherm chart (both shown below).
Also shown below are some common symbols used on these charts (in particular the isotach chart).
An Example of a MSL Synoptic chart
TROPICAL METEOROLOGY
The tropic refer to the region between 23.5o North (tropic of Cancer) and 23.5o South (tropic of Capricorn).
This is where the sun will be directly overhead at sometime during the year.
As we have already looked at why the winds flow from and towards the equator we can understand why there are southeast trade winds in the Southern hemisphere between the equatorial trough and the subtropical high.
We can also understand why there are the prevailing westerlies between the subtropical high and the polar low.
The area located in the tropics is called the equatorial trough.
The mean position of this trough isn’t actually the equator but approximately 5o north.
The reason for this is due to there being a greater mass of land in the Northern Hemisphere and therefore greater heating effect.
Due to Coriolis force near the equator being 0, winds are strongly affected by the local effects of friction.
As a result, the airflow is no longer parallel to the isobars but tends to move from areas of high pressure to areas of low pressure.
TRADE WINDS
In the Southern Hemisphere, air moving towards the equator is deflected left by the Coriolis force creating SE trade winds.
Although local effects and pressure gradients cause wind directions to vary, the trade winds are noted for their persistence and steadiness over the ocean (no surface friction) and are typically associated with Cumulus (Cu) clouds between 3000’ – 7000’.
The good weather and restricted cloud development is due to a trade wind inversion, which weakens closer to the equatorial trough.
Cloud development increases as instability increases, with frequent and heavy rain near the equatorial trough.
It is important to note that the wind does not follow the isobars near the equator due to the low Coriolis force.
MONSOON WINDS
Land and sea surfaces affect the wind systems.
Landmasses warm quickly in summer and cool, while oceans remain relatively constant in temperature.
In summer, air flows from oceans towards the hot continents, then Coriolis and friction forces apply.
As the moist air crosses the coast it is heated by the land and becomes unstable, rising air to great heights.
Conditions expected to come from monsoon winds are:
·
Frequent heavy showers
·
Thunderstorms
·
Violent squalls
Due to Australia being so close to the equatorial trough the northern parts of Australia experience different weather than the southern parts of Australia for the same season.
The reason for this is due to the movement of the equatorial trough and the subtropical high.
As we know the equator is notorious for bad weather.
Due to the movement of the equatorial trough with the changing of the seasons the northern part of Australia will have rain and generally bad weather in summer, whilst the southern part of Australia will have predominantly finer weather.
In northern Australia, winter is the dry season, with se trade winds and generally fine weather.
Near the end of the year (summer) the wet season starts, with isolated thunderstorms.
Later in the ‘wet’ the NW monsoons begin bringing persistent heavy rains.
When the equatorial trough is established over the land, thunderstorms violent squalls and heavy rain can be expected.
INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
In some regions the trade winds from each hemisphere meet in a narrow zone know as the inter-tropical convergence zone.
The ITCZ produces extremely bad weather over a wide region.
Cloud tops reach the higher tropical tropopause, with very low bases.
The ITCZ has the following features:
·
It may be several hundred kilometers wide
·
It is associated with heavy rain
·
Frequent storms
·
Violent wind squalls
DIURNAL EFFECTS
A sea breeze may force moist and unstable air over land, resulting in afternoon thunderstorms.
Similarly, a land breeze may produce early morning off shore thunderstorms around dawn.
The easiest way to pick whether or not a synoptic chart is a summer or winter chart is to look at the position of the high pressures and the equatorial trough.
If the equatorial trough is over the land then it is most likely to be a summer chart, If the equatorial trough is closer to the equator then it is most likely going to be a winter chart (high pressures over the middle / top of Australia).
TYPICAL SEASONAL SYNOPTIC CHARTS
The sun moves between the Tropic of Capricorn (Dec 22) and the Tropic of Cancer (Jun 21), passing over the equator twice a year.
The thermal equator follows the sun, thus moving north and south of the geographic equator.
Therefore, weather pressure systems not only move from west to east but north and south according to the season.
The diagrams of the synoptic charts below show typical charts for the different seasons.
The summer chart shows the tropical lows over the northern end of Australia, with the sub tropical highs moving across the southern section of the continent.
The southern oceanic low and associated troughs are well south of Australia, resulting in fairly dry conditions.
The autumn chart shows bands of highs and lows moving northward.
The frontal effects and associated rain are felt along the southern area of the mainland of Tasmania.
The winter chart shows the highs and lows at their northern most point.
The effects of the troughs and fronts are more significant over the mainland.
The spring chart shows the systems moving southward and having less effect.
However, the eastern coast has an inflow of moist air from over the Pacific Ocean.
See below for a 4 day progression of synoptic systems for both summer (northern wet season) and winter (northern dry season).
Day 1
Day 2
Day 3
Day 4
Day 1
Day 2
Day 3
Day 4
What season is this!
TROPICAL CYCLONES Tropical cyclones are intense tropical depressions that develop over most tropical oceans between 5o latitude and 15o latitude.
Sea surface temperatures need to be above 27oC for a tropical cyclone to form as the storm derives most of its energy from the heat stored in these warm waters.
The life span of a tropical cyclone is around 6 days but can sometimes last up to 2 weeks before entering sub tropical regions or crossing land.
The life cycle of a tropical cyclone is usually divided into 4 stages however the development of the cyclone may terminate at any stage that the conditions do not remain favorable.
The four stages are:
1.
FORMATIVE STAGE
A tropical cyclone always forms from an existing disturbance.
The pressure drops below 1000 hPa.
This stage could take a matter of hours or even a few days.
The eye of the cyclone forms and winds reach gale force (34 kts or more) although usually only in one quadrant.
2.
IMMATURE STAGE
The pressure in the eye is reduced below 1000 hPa and continues to drop.
The wind has now reached hurricane force (64 kts or more) but is limited to a relatively small area of perhaps 20 – 30 nm radius.
The characteristic spiral bands form at this stage.
3.
MATURE STAGE
The storm expands in an area with hurricane winds extending up to 100 nm or more from the center.
The air pressure stabilises.
The most intense weather is found in the front left quadrant in the direction of motion of the cyclone.
4.
DECAYING STAGE
The pressure begins to rise and the affected area contracts as the tropical cyclone enters the decaying stage.
This stage often occurs as the cyclone moves over the land or south of 15o S latitude.
As dissipation occurs a rain depression or mid latitude depression of considerable intensity often results which can cause wide spread rains for some days.
Cyclone Season goes from November to March.
More Frequent between January through March.
As you can see this cyclone ‘SOSE’ actually occurred in April.
WEATHER FORECAST INTERPRETATION The key to safe flying is being able to know what the weather is doing at your departure and arrival aerodromes and what conditions you can expect enroute.
I have prepared some notes on what the weather bureau is trying to indicate when they write certain things on Area forecasts (ARFOR) and Terminal Area Forecasts (TAFS).
The first step in weather interpretation is actually being able obtain the forecasts then being able to interpret them.
AREA FORECASTS (ARFOR)
We will firstly look at Area Forecasts.
Below is an Area forecast for Area 30.
All of the areas used for area forecasts are shown on the Planning Chart of Australia (PCA).
When you obtain the weather you should have with you the PCA and any relevant maps that are to be used for your proposed flight.
Remember that abbreviations for most weather jargon can be fond in the AIP (Amendment 27 20 APR 2000) GEN 2.2 – 25 or GEN 3.5 – 32 or the ERSA (23 MARCH 2000) MET – 1.
Also remember if you are having trouble finding a location given on your weather:
1.
Look on the back of the PCA
2.
Look in the ERSA
3.
Look on your WAC / VNC / VTC or finally,
4.
Give the weather bureau a call on 03 9669 4850 and ask them.
AREA 30
Area QNH 01 / 04
AREA 30 / 32 SE OF FLIKI / COM 1015
W OF WEBS /YSWH 1024 REST 1019
As shown on the PCA below, this gives us an indication of what the area QNH will be for those relevant areas between 0100 Z and 0400 Z.
This can be useful when flying in a particular area where radio contact with ATS may not be available.
The rest of the information is for other already expired time like
AREA 30
AREA QNH 22 / 01
AREA 30 / 32 SE FLIKI /YCOM 1014
W OF WEBS / YSWH 1024 REST 1019
Are the area QNH areas for time 2200 Z to 0100 Z.
AREA 30
AMEND AREA FORECAST 302200 TO 311100 AREAS 30 / 32
AMD OVERVIEW:
S / SW AIRFLOW. ISOL FG NEAR RANGES TILL
24Z.
SCT SHRA S OF BORDERTOWN / MANGALORE /
CORRYONG.
SHSN ABV 3500FT.
LOW CLOUD PATCHES IN
PRECIPITATION, PARTICULARLY WINDWARD SLOPES
AND RANGES.
AMEND AREA FORECAST means that this forecast has been amended and is now valid from 302200 UTC 30th of the month at 2200 UTC (converted to EST this equals 0800 hrs on the 31st) to 311100 UTC 31st of the month at 1100 UTC (converted to EST this equals 2200 hrs on the 31st).
AMD OVERVIEW means that below is given an amended overview of the conditions in area 30.
This will also indicate where possible fronts, troughs, lows, ridges or other weather phenomena will be at specific times.
S / SW AIRFLOW indicates that the airflow throughout area 30 is South to Southwesterly.
ISOL FG NEAR RANGES TILL 24Z.
As I mentioned earlier the abbreviations are available to you in many different location (AIP, ERSA etc) so accurate interpretation is important.
The above reads Isolated Fog near the ranges until 2400 UTC, meaning exactly that.
Near the Great Dividing Range (also called the Ranges) there will be fog until 2400 UTC or 1000 hrs EST.
SCT SHRA S OF BORDERTOWN / MANGALORE /
CORRYONG.
This statement means that South of a line joining Bordertown, Mangalore, Corryong you can expect Scattered showers of rain.
SHSN ABV 3500FT.
It is expected that above 3500 feet above mean sea level (AMSL) there will be showers of snow.
This information is useful to pilots as it will indicate altitudes where if you were flying through a shower of what you though was rain it may be snow and therefore a risk of ice forming is likely.
LOW CLOUD PATCHES IN PRECIPITATION means that if you are flying in precipitation low cloud could form and therefore be a danger.
These low cloud patches are likely are PARTICULARLY likely WINDWARD SLOPES AND RANGES, meaning the side that the wind is blowing onto the mountain will experience low cloud patches in precipitation in particular
WIND:
2000
5000
7000
10000
14000
18500
200 / 25
200 / 30
190 / 30
180 / 30 MS07
180 / 35 MS14
180 / 40 MS23
The above is the wind information for Area 30
At 2000 feet AMSL the wind is forecast to be 200oT at 25 knots
At 5000 feet AMSL the wind is forecast to be 200 oT at 30 knots
At 7000 feet AMSL the wind is forecast to be 190 oT at 30 knots
At 10000 feet AMSL the wind is forecast to be 180 oT at 30 knots and the Temperature is minus 7 oC (-7 oC)
At 14000 feet (or Flight Level FL140) the wind is forecast to be 180 oT at 35 knots and the temperature is minus 14oC (-14 oC)
At 18500 feet (or flight level FL185) the wind is forecast to be 180 oT at 40 knots with a temperature of minus 23oC (-23 oC)
The temperature could also be written PS which means Plus.
IT IS VERY IMPORTANT TO REMEMBER THAT WIND ON ANY WRITTEN FORECAST IS GIVEN IN DEGREES TRUE AND MUST BE CONVERTED TO MAGNETIC BY ADDING OR SUBTRACTING VARIATION.
THE ONLY MAGNETIC INFORMATION IS GIVEN FROM THE ATIS OR AUTOMATIC WEATHER STATION (When you are listening to them both)
CLOUD:
SCT ST 1000 / 2500 IN SHRA, BKN WINDWARD SLOPES OF RANGES AND
IN SHSN.
AREAS OF BKN CU / SC 3000 / 8000, SCT SEA BETWEEN KING
ISLAND AND FLINDERS ISLAND, ALSO SCT N OF NHILL / BENDIGO /
COROWA.
ISOL CU TOPS 15000 IN SE.
SCT ACAS ABV 10000 IN E.
This gives the reader a general overview of the cloud to be expected in area 30.
In decoding this you must have a thorough understanding of the abbreviations used for cloud and the abbreviations for the amount of cloud. SCT ST 1000 / 2500 IN SHRA means that in Showers of Rain the cloud you would expect is Scattered Stratus with a base of 1000’ AMSL and a top of 2500’ AMSL.
This scattered stratus will be broken on the windward slopes of the ranges and also broken when encountering showers of snow (BKN WINDWARD SLOPES OF RANGES AND IN SHSN).
You will encounter areas of Broken cumulus and stratocumulus with a base of 3000’ and tops of 8000’ in area 30 however they will be scattered over the sea between King Island and Flinders Island.
The cumulus and also stratocumulus will also be scattered north of a line Nhill, Bendigo, and Corowa (AREAS OF BKN CU / SC 3000 / 8000, SCT SEA BETWEEN KING ISLAND AND FLINDERS ISLAND, ALSO SCT N OF NHILL / BENDIGO /
COROWA).
There will be isolated cumulus tops that will reach 15000’ AMSL in the Southeast (ISOL CU TOPS 15000 IN SE).
There will be scattered Altocumulus and Altostratus above 10000’ AMSL in the east.
AMD WEATHER:
FG NEAR RANGES TILL 24Z, SHSN ABOVE 3500FT, SHRA
This is just informing the reader that there will be weather such as fog near the ranges until 2400 UTC (or 1000 hrs EST) (FG NEAR RANGES TILL 24Z), there will be Showers of Snow above 3500’ AMSL (SHSN ABOVE 3500FT) and also Showers of Rain (SHRA).
VISIBILITY
500M FG / SN, 600M SHRA
This indicates that the visibility is expected to be 500 meters in Fog and Snow (500M FG / SN),
600 meters in showers of rain (600M SHRA), and something that is not written on a forecast but is implied visibility otherwise will be 10 kilometers or better, if you are not flying in any of the above conditions (Fog, Snow or Showers of Rain).
FREEZING LEVEL
4000 IN E RISING TO 5000 IN W
This indicates that the freezing level east of the center of the area (in area 30’s case it would be a line going longitudinally through Melbourne) is 4000’ however it will be rising to the 5000’ in the west of the area (west of the line through Melbourne).
This is not so important to VFR pilots however it is very useful for IFR pilots wanting to know when to expect ice and where to avoid.
ICING
MOD CU / SC / ACAS
This means that you can expect moderate icing in Cumulus, Stratocumulus and Altocumulus Altostratus.
TURBULENCE
MOD BLW 7000FT NEAR AND LEE OF RANGES.
MOD CU / AC
Another important point for the safety of the flight would be turbulence.
The forecaster expects that there will be moderate turbulence below 7000’ near and lee of the ranges (MOD BLW 7000FT NEAR AND LEE OF RANGES).
This is probably because there is the possibility that a mountain wave pattern could develop.
Moderate turbulence is also expected in unstable type clouds such as Cumulus and Altocumulus (MOD CU / AC).
CRITICAL LOCATIONS: [HEIGHTS ABOVE MSL]
KILMORE GAP:
3000 –SHRA BKN ST 1500 BKN CU / SC 2500
FM02 9999 –SHRA SCT 1800 BKN CU / SC 3000
INTER 0211 5000 SHRA BKN ST 1500
This is information pertaining to the Kilmore Gap.
This location is important to VFR pilots because at this point the terrain begins to rise as well as there being a mountain on the left and right and sides.
(see diagram below)
3000 –SHRA BKN ST 1500 BKN CU / SC 2500.
This should be broken down into individual parts. 3000 is the visibility expected at the Kilmore Gap. –SHRA is light showers of rain (- indicates light + indicates heavy). BKN ST 1500 indicates that the cloud expected is going to be broken (5 – 7 oktas) of stratus with a base of 1500’ AMSL.
There will also be broken cumulus and stratocumulus with a base of 2500’ AMSL (BKN CU / SC 2500).
However there is going to be a definite change at time 0200 UTC, visibility will increase to 10 kilometers or better FM02 9999,
the showers will still be light showers of rain –SHRA, the cloud will probably still be stratus (although this is not indicated) with a base of 1800’ AMSL but this time it will be Scattered (2 – 3 oktas) SCT 1800 there will be however Broken cumulus and stratocumulus with a base of 3000’ AMSL BKN CU / SC 3000.
There is a period however where the conditions are going to change for periods up to 30 minutes where the conditions are expected to deteriorate.
This is an INTER period and it is between 0200 UTC to 1100 UTC.
The conditions are going to deteriorate to visibility of 5000 meters Showers of Rain and broken stratus cloud with a base of 1500’ AMSL.
This is generally how the weather is written out.
You should always try and understand why a particular weather occurrence occurs.
Always use all the facilities you have available to you and if in doubt call the weather bureau.
There are certain things that must be considered as far as weather is considered.
We have already seen the interpretation of Area Forecasts and how this can help in our planning.
Now let us look at little close at TAFs, TTFs, METARs, and SPECIs.
TAF
Examining the TAF we can see that there is more to it than just decoding letters.
A TAF is a statement of meteorological conditions expected for a specified period in the airspace within a 5 nm radius of the center of the aerodrome or runway complex.
The degree of service provided depends upon the category of the aerodrome.
The category is determined by the type and amount of traffic using the aerodrome.
Category | Aerodrome Type | TAF Service | 1 | International
| Issued 6 hourly, valid 24 hours, continuous met watch and amendment service.
| 2 | Domestic with more than 500 movements per month
| Issued 6 hourly, valid for 12 hours, MET watch and amendment service during validity period
| 3 | Other selected domestic and selected remote aerodromes
| Issued as required, valid for 12 hours, MET watch and amendment service during validity period
| 4 | Remainder
| No TAF
|
Examples of a category 1 aerodrome would be Melbourne Airport, an example of a category 2 aerodrome would be Essendon Airport,an example of a category 3 aerodrome would be Mangalore Airport, an example of a category 4 aerodrome would be Baccaush
Marsh aerodrome.
·
Information given on a TAF includes the use of abbreviations such as:
SKC
=
Sky Clear
=
0 oktas
FEW
=
Few
=
1 to 2 oktas
SCT
=
Scattered
=
3 to 4 oktas
BKN
=
Broken
=
5 to 7 oktas
OVC
=
Overcast
=
8 oktas
·
The only cloud types that are give on TAFs are:
TCU
=
Towering Cumulus
CB
=
Cumulonimbus
·
For Area Forecasts the amount of CB will be indicated as follows:
ISOL
=
Isolated – for individual or well separated CBs
OCNL
=
Occasional – for well separated CBs
FREQ
=
Frequent – for CBs with little or no separation
·
The uses of the abbreviations INTER and TEMPO indicates variations to the mean conditions of an INTERmittent or TEMPOrary nature.
INTER is used to indicate changes expected to occur frequently for periods of less than 30 minutes duration, the conditions fluctuating almost constantly, between the times specified on the forecast
TEMPO is used to indicate change in prevailing conditions expected to last for periods of less than 1 hour in each instance.
FM (or From) is used in forecasts to indicate changes which are significantly different from preceding information in one or more of the following elements:
·
Wind direction and / or speed
·
Visibility
·
Weather
·
Cloud
The changes relate to improvements as well as deterioration’s.
‘From’ periods will continue until the end of the TAF validity period or until replaced by another significant change.
Some other useful terms that are used in TAF interpretation are:
CAVOK is used when conditions reach a certain maximum (commonly associated with good weather).
Although it is not written CAVOK stands for Ceiling And Visibility are OK.
For CAVOK to be used on a TAF the following conditions must be met
·
Visibility 10 km or better
·
No cloud below 5000 ft or below the highest minimum sector altitude (MSA) [MSA these are used in IFR], whichever is greater, AND, no cumulonimbus;
·
No precipitation, thunderstorms, shallow fog, low drifting snow or dust devils.
PROB is abbreviated for probability relating to the chance of an expected occurrence.
NO SIG WX, SKC, WX NIL are all representative of the same thing which is that there is no significant change in the weather expected or occurring at that time.
NO SIG WX means no significant weather, SKC means sky is clear and WX NIL actually means there is nil weather at this time.
PROV when the term PROV gets used in weather it usually means that the authority that is producing the weather information is deficient in its accuracy.
As an example the Melbourne weather bureau is on strike and Sydney is writing the weather for Melbourne.
N.B.
Other abbreviations that could be used
are given in the AIP and ERSA.
Now that were are aware of some of the abbreviations that are used in the presentation of a TAF we can now begin to interpret them.
Below is all of the weather information for EN (Essendon) followed by the interpretation.
Note that there is 2 TAFs given the top TAF being the most current.
For aerodromes like EN and ML the ATIS will also be given.
YMEN
YMEN is the ICAO identifier for the aerodrome, in this case it is Essendon.
These can be found in the ERSA.
TAF YMEN decoded is Terminal Aerodrome forecast for Essendon
310019Z
this is the time at which the TAF was written 31st of the month at 0019 UTC
0214 denotes the validity time valid from 0200 UTC to 1400 UTC
21012KT
This is the wind velocity (direction and speed).
In this situation the wind is 200oT at a strength of 12 knots
9999 is the visibility in kilometers / meters.
This is to be read as 10 km or better.
LIGHT SHOWERS OF RAIN is the weather that is currently at the aerodrome and is easy to understand as it is written in plain english.
SCT015 This is a cloud amount and height decoded as Scattered (3 – 4 oktas) at 1500’ Above Ground Level (AGL).
Essendon is 282’ AMSL so therefore the actual height of the cloud base is 1782’ AMSL.
BKN030 Broken (5 – 7 oktas) at 3000’ AGL
INTER 0210
This means that changes, given after this, are expected to occur frequently for periods of less than 30 minutes duration, between 0200 UTC and 1000 UTC.
The changes that are expected to occur are
6000 Visibility reducing to 6000 meters
SHOWERS OF RAIN the weather expected.
This may be the cause of the visibility reduction.
BKN012 the cloud will reduce to 1200’ AGL and increase to a broken amount (5 – 7 oktas).
TEMPO 1014 This means that between 1000 UTC and 1400 UTC there will be periods where the weather deteriorates to the conditions stated after this for periods up to 1 hour.
The conditions expected include:
4000
Visibility reducing to 4000 meters
DRIZZLE weather expected will be drizzle.
Probably the reason why the visibility will be down to 4000 meters.
BKN006
the cloud will lower to 600’ AGL and remain broken (5 – 7 oktas)
T 13 13 11 10 Temperature is given at three hourly intervals, for a maximum of 9 hours, from the time of commencement of the validity of the forecast:
e.g. Forecast is valid from 0214 The temperature’s are 13 13 11 10
The times that equate to these temperatures are:
Temperature
13
13
11
10
Time
0200
0500
0800
1100
The reason that the last temperature is not |
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发表于 2008-12-23 21:19:09
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