Turbulence Reporting Criteria Table
Intensity Aircraft Reaction Reaction Inside Aircraft Reporting Term-Definition
Light Turbulence that momentarily causes
slight, erratic changes in altitude and/or
attitude (pitch, roll, yaw). Report as
Light Turbulence; 1
or
Turbulence that causes slight, rapid and
somewhat rhythmic bumpiness without
appreciable changes in altitude or
attitude. Report as Light Chop.
Occupants may feel a slight strain
against seat belts or shoulder straps.
Unsecured objects may be displaced
slightly. Food service may be con-
ducted and little or no difficulty is
encountered in walking.
Occasional-Less than 1
/3 of the time.
Intermittent-1
/3 to 2
/3.
Continuous-More than 2
/3.
Moderate Turbulence that is similar to Light
Turbulence but of greater intensity.
Changes in altitude and/or attitude occur
but the aircraft remains in positive
control at all times. It usually causes
variations in indicated airspeed. Report
as Moderate Turbulence; 1
or
Turbulence that is similar to Light Chop
but of greater intensity. It causes rapid
bumps or jolts without appreciable
changes in aircraft altitude or attitude.
Report as Moderate Chop.1
Occupants feel definite strains against
seat belts or shoulder straps. Unse-
cured objects are dislodged. Food
service and walking are difficult.
NOTE
1. Pilots should report location(s),
time (UTC), intensity, whether in or
near clouds, altitude, type of aircraft
and, when applicable, duration of
turbulence.
2. Duration may be based on time
between two locations or over a single
location. All locations should be
readily identifiable.
Severe Turbulence that causes large, abrupt
changes in altitude and/or attitude. It
usually causes large variations in
indicated airspeed. Aircraft may be
momentarily out of control. Report as
Severe Turbulence. 1
Occupants are forced violently against
seat belts or shoulder straps. Unse-
cured objects are tossed about. Food
Service and walking are impossible.
EXAMPLES:
a. Over Omaha. 1232Z, Moderate
Turbulence, in cloud, Flight
Level_310, B707.
Extreme Turbulence in which the aircraft is
violently tossed about and is practically
impossible to control. It may cause
structural damage. Report as Extreme
Turbulence. 1
b. From 50 miles south of Albuquer-
que to 30 miles north of Phoenix,
1210Z to 1250Z, occasional Moderate
Chop, Flight Level 330, DC8.
1
High level turbulence (normally above 15,000 feet ASL) not associated with cumuliform cloudiness, including thunderstorms,
should be reported as CAT (clear air turbulence) preceded by the appropriate intensity, or light or moderate chop.
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Meteorology
7-1-24. Wind Shear PIREPs
a. Because unexpected changes in wind speed and
direction can be hazardous to aircraft operations at
low altitudes on approach to and departing from
airports, pilots are urged to promptly volunteer
reports to controllers of wind shear conditions they
encounter. An advance warning of this information
will assist other pilots in avoiding or coping with a
wind shear on approach or departure.
b. When describing conditions, use of the terms
“negative” or “positive” wind shear should be
avoided. PIREPs of “negative wind shear on final,”
intended to describe loss of airspeed and lift, have
been interpreted to mean that no wind shear was
encountered. The recommended method for wind
shear reporting is to state the loss or gain of airspeed
and the altitudes at which it was encountered.
EXAMPLE1. Denver Tower, Cessna 1234 encountered wind shear,
loss of 20 knots at 400.
2. Tulsa Tower, American 721 encountered wind shear on
final, gained 25 knots between 600 and 400 feet followed
by loss of 40 knots between 400 feet and surface.
1. Pilots who are not able to report wind shear in
these specific terms are encouraged to make reports
in terms of the effect upon their aircraft.
EXAMPLE-
Miami Tower, Gulfstream 403 Charlie encountered an
abrupt wind shear at 800 feet on final, max thrust required.
2. Pilots using Inertial Navigation Systems
(INSs) should report the wind and altitude both above
and below the shear level.
7-1-25. Clear Air Turbulence (CAT) PIREPs
CAT has become a very serious operational factor to
flight operations at all levels and especially to jet
traffic flying in excess of 15,000 feet. The best
available information on this phenomenon must
come from pilots via the PIREP reporting procedures.
All pilots encountering CAT conditions are urgently
requested to report time, location, and intensity (light,
moderate, severe, or extreme) of the element to the
FAA facility with which they are maintaining radio
contact. If time and conditions permit, elements
should be reported according to the standards for
other PIREPs and position reports.
REFERENCE-
AIM, PIREPs Relating to Turbulence, Paragraph 7-1-23.
7-1-26. Microbursts
a. Relatively recent meteorological studies have
confirmed the existence of microburst phenomenon.
Microbursts are small scale intense downdrafts
which, on reaching the surface, spread outward in all
directions from the downdraft center. This causes the
presence of both vertical and horizontal wind shears
that can be extremely hazardous to all types and
categories of aircraft, especially at low altitudes. Due
to their small size, short life span, and the fact that
they can occur over areas without surface precipita-
tion, microbursts are not easily detectable using
conventional weather radar or wind shear alert
systems.
b. Parent clouds producing microburst activity
can be any of the low or middle layer convective
cloud types. Note, however, that microbursts
commonly occur within the heavy rain portion of
thunderstorms, and in much weaker, benign
appearing convective cells that have little or no
precipitation reaching the ground.
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FIG 7-1-13
Evolution of a Microburst
T-5 Min T-2 Min T T 5 Min T 10 Min
HEIGHT (feet)
10,000
5,000
WIND SPEED
10-20 knots
> 20 knots
SCALE (miles)
0 1 2 3
Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence at
the surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984,
Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAA
Report No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp.
c. The life cycle of a microburst as it descends in
a convective rain shaft is seen in FIG 7-1-13. An
important consideration for pilots is the fact that the
microburst intensifies for about 5 minutes after it
strikes the ground.
d. Characteristics of microbursts include:
1. Size. The microburst downdraft is typically
less than 1 mile in diameter as it descends from the
cloud base to about 1,000-3,000 feet above the
ground. In the transition zone near the ground, the
downdraft changes to a horizontal outflow that can
extend to approximately 2 1
/2 miles in diameter.
2. Intensity. The downdrafts can be as strong
as 6,000 feet per minute. Horizontal winds near the
surface can be as strong as 45 knots resulting in a
90_knot shear (headwind to tailwind change for a
traversing aircraft) across the microburst. These
strong horizontal winds occur within a few hundred
feet of the ground.
3. Visual Signs. Microbursts can be found
almost anywhere that there is convective activity.
They may be embedded in heavy rain associated with
a thunderstorm or in light rain in benign appearing
virga. When there is little or no precipitation at the
surface accompanying the microburst, a ring of
blowing dust may be the only visual clue of its
existence.
4. Duration. An individual microburst will
seldom last longer than 15 minutes from the time it
strikes the ground until dissipation. The horizontal
winds continue to increase during the first 5 minutes
with the maximum intensity winds lasting approxi-
mately 2-4 minutes. Sometimes microbursts are
concentrated into a line structure, and under these
conditions, activity may continue for as long as an
hour. Once microburst activity starts, multiple
microbursts in the same general area are not
uncommon and should be expected.
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Meteorology
FIG 7-1-14
Microburst Encounter During Takeoff
A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing
performance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft
(3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane.
Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWG
Associates, Inc., Tullahoma, Tennessee.
e. Microburst wind shear may create a severe
hazard for aircraft within 1,000 feet of the ground,
particularly during the approach to landing and
landing and take-off phases. The impact of a
microburst on aircraft which have the unfortunate
experience of penetrating one is characterized in
FIG 7-1-14. The aircraft may encounter a headwind
(performance increasing) followed by a downdraft
and tailwind (both performance decreasing), possibly
resulting in terrain impact.
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FIG 7-1-15
NAS Wind Shear Product Systems
f. Detection of Microbursts, Wind Shear and
Gust Fronts.
1. FAA's Integrated Wind Shear Detection
Plan.
(a) The FAA currently employs an integrated
plan for wind shear detection that will significantly
improve both the safety and capacity of the majority
of the airports currently served by the air carriers.
This plan integrates several programs, such as the
Integrated Terminal Weather System (ITWS),
Terminal Doppler Weather Radar (TDWR), Weather
System Processor (WSP), and Low Level Wind Shear
Alert Systems (LLWAS) into a single strategic
concept that significantly improves the aviation
weather information in the terminal area. (See
FIG 7-1-15.)
(b) The wind shear/microburst information
and warnings are displayed on the ribbon display
terminals (RBDT) located in the tower cabs. They are
identical (and standardized) in the LLWAS, TDWR
and WSP systems, and so designed that the controller
does not need to interpret the data, but simply read the
displayed information to the pilot. The RBDTs are
constantly monitored by the controller to ensure the
rapid and timely dissemination of any hazardous
event(s) to the pilot.
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Meteorology
FIG 7-1-16
LLWAS Siting Criteria
(c) The early detection of a wind shear/
micro-burst event, and the subsequent warning(s)
issued to an aircraft on approach or departure, will
alert the pilot/crew to the potential of, and to be
prepared for, a situation that could become very
dangerous! Without these warnings, the aircraft may
NOT be able to climb out of, or safely transition, the
event, resulting in a catastrophe. The air carriers,
working with the FAA, have developed specialized
training programs using their simulators to train and
prepare their pilots on the demanding aircraft
procedures required to escape these very dangerous
wind shear and/or microburst encounters.
2. Low Level Wind Shear Alert System
(LLWAS).
(a) The LLWAS provides wind data and
software processes to detect the presence of
hazardous wind shear and microbursts in the vicinity
of an airport. Wind sensors, mounted on poles
sometimes as high as 150 feet, are (ideally) located
2,000 - 3,500 feet, but not more than 5,000 feet, from
the centerline of the runway. (See FIG 7-1-16.)
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FIG 7-1-17
Warning Boxes
(b) LLWAS was fielded in 1988 at 110_air-
ports across the nation. Many of these systems have
been replaced by new TDWR and WSP technology.
Eventually all LLWAS systems will be phased out;
however, 39 airports will be upgraded to the
LLWAS-NE (Network Expansion) system, which
employs the very latest software and sensor
technology. The new LLWAS-NE systems will not
only provide the controller with wind shear warnings
and alerts, including wind shear/microburst detection
at the airport wind sensor location, but will also
provide the location of the hazards relative to the
airport runway(s). It will also have the flexibility and
capability to grow with the airport as new runways are
built. As many as 32 sensors, strategically located
around the airport and in relationship to its runway
configuration, can be accommodated by the
LLWAS-NE network.
3. Terminal Doppler Weather Radar
(TDWR).
(a) TDWRs are being deployed at 45_loca-
tions across the U.S. Optimum locations for TDWRs
are 8 to 12 miles off of the airport proper, and
designed to look at the airspace around and over the
airport to detect microbursts, gust fronts, wind shifts
and precipitation intensities. TDWR products advise
the controller of wind shear and microburst events
impacting all runways and the areas 1
/2 mile on either
side of the extended centerline of the runways out to
3 miles on final approach and 2 miles out on
departure.
(FIG 7-1-17 is a theoretical view of the warning
boxes, including the runway, that the software uses in
determining the location(s) of wind shear or
microbursts). These warnings are displayed (as
depicted in the examples in subparagraph 5) on the
RBDT.
(b) It is very important to understand what
TDWR does NOT DO:
(1) It_DOES NOT warn of wind shear
outside of the alert boxes (on the arrival and departure
ends of the runways);
(2) It_DOES NOT detect wind shear that is
NOT a microburst or a gust front;
(3) It_DOES NOT detect gusty or cross
wind conditions; and
(4) It DOES NOT detect turbulence.
However, research and development is continuing on
these systems. Future improvements may include
such areas as storm motion (movement), improved
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Meteorology
gust front detection, storm growth and decay,
microburst prediction, and turbulence detection.
(c) TDWR also provides a geographical
situation display (GSD) for supervisors and traffic
management specialists for planning purposes. The
GSD displays (in color) 6 levels of weather
(precipitation), gust fronts and predicted storm
movement(s). This data is used by the tower
supervisor(s), traffic management specialists and
controllers to plan for runway changes and
arrival/departure route changes in order to both
reduce aircraft delays and increase airport capacity.
4. Weather System Processor (WSP).
(a) The WSP provides the controller, supervi-
sor, traffic management specialist, and ultimately the
pilot, with the same products as the terminal doppler
weather radar (TDWR) at a fraction of the cost of a
TDWR. This is accomplished by utilizing new
technologies to access the weather channel capabili-
ties of the existing ASR-9 radar located on or near the
airport, thus eliminating the requirements for a
separate radar location, land acquisition, support
facilities and the associated communication landlines
and expenses.
(b) The WSP utilizes the same RBDT display
as the TDWR and LLWAS, and, just like TDWR, also
has a GSD for planning purposes by supervisors,
traffic management specialists and controllers. The
WSP GSD emulates the TDWR display, i.e., it also
depicts 6 levels of precipitation, gust fronts and
predicted storm movement, and like the TDWR GSD,
is used to plan for runway changes and arrival/depar-
ture route changes in order to reduce aircraft delays
and to increase airport capacity.
(c) This system is currently under develop-
ment and is operating in a developmental test status
at the Albuquerque, New Mexico, airport. When
fielded, the WSP is expected to be installed at
34_airports across the nation, substantially increasing
the safety of the American flying public.
5. Operational aspects of LLWAS, TDWR
and WSP.
To demonstrate how this data is used by both the
controller and the pilot, 3 ribbon display examples
and their explanations are presented:
(a) MICROBURST ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A MBA 35K- 2MF 250 20
NOTE(See FIG 7-1-18 to see how the TDWR/WSP determines
the microburst location).
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
RUNWAY 27 ARRIVAL, MICROBURST ALERT, 35 KT
LOSS 2 MILE FINAL, THRESHOLD WIND 250 AT 20.
In plain language, the controller is telling the pilot
that on approach to runway 27, there is a microburst
alert on the approach lane to the runway, and to
anticipate or expect a 35 knot loss of airspeed at
approximately 2 miles out on final approach (where
it will first encounter the phenomena). With that
information, the aircrew is forewarned, and should be
prepared to apply wind shear/microburst escape
procedures should they decide to continue the
approach. Additionally, the surface winds at the
airport for landing runway 27 are reported as
250_degrees at 20 knots.
NOTE-
Threshold wind is at pilot's request or as deemed
appropriate by the controller.
REFERENCE-
FAA Order JO 7110.65, Air Traffic Control, Low Level Wind
Shear/Microburst Advisories, Paragraph 3-1-8b2(a).
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FIG 7-1-18
Microburst Alert
(b) WIND SHEAR ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A WSA 20K- 3MF 200 15
NOTE(See FIG 7-1-19 to see how the TDWR/WSP determines
the wind shear location).
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KT
LOSS 3 MILE FINAL, THRESHOLD WIND 200 AT 15.
In plain language, the controller is advising the
aircraft arriving on runway 27 that at about 3 miles
out they can expect to encounter a wind shear
condition that will decrease their airspeed by 20 knots
and possibly encounter turbulence. Additionally, the
airport surface winds for landing runway 27 are
reported as 200 degrees at 15 knots.
NOTE-
Threshold wind is at pilot's request or as deemed
appropriate by the controller.
REFERENCE-
FAA Order JO 7110.65, Air Traffic Control, Low Level Wind
Shear/Microburst Advisories, Paragraph 3-1-8b2(a).
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Meteorology
FIG 7-1-19
Weak Microburst Alert
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FIG 7-1-20
Gust Front Alert
(c) MULTIPLE WIND SHEAR ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A WSA 20K+ RWY 250 20
27D WSA 20K+ RWY 250 20
NOTE(See FIG 7-1-20 to see how the TDWR/WSP determines
the gust front/wind shear location.)
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
MULTIPLE WIND SHEAR ALERTS. RUNWAY 27
ARRIVAL, WIND SHEAR ALERT, 20 KT GAIN ON
RUNWAY; RUNWAY 27 DEPARTURE, WIND SHEAR
ALERT, 20 KT GAIN ON RUNWAY, WIND 250 AT 20.
EXAMPLE-
In this example, the controller is advising arriving and
departing aircraft that they could encounter a wind shear
condition right on the runway due to a gust front
(significant change of wind direction) with the possibility
of a 20 knot gain in airspeed associated with the gust front.
Additionally, the airport surface winds (for the runway in
use) are reported as 250 degrees at 20 knots.
REFERENCE-
FAA Order JO 7110.65, Air Traffic Control, Low Level Wind
Shear/Microburst Advisories, Paragraph 3-1-8b2(d).
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Meteorology
6. The Terminal Weather Information for
Pilots System (TWIP).
(a) With the increase in the quantity and
quality of terminal weather information available
through TDWR, the next step is to provide this
information directly to pilots rather than relying on
voice communications from ATC. The National
Airspace System has long been in need of a means of
delivering terminal weather information to the
cockpit more efficiently in terms of both speed and
accuracy to enhance pilot awareness of weather
hazards and reduce air traffic controller workload.
With the TWIP capability, terminal weather
information, both alphanumerically and graphically,
is now available directly to the cockpit on a test basis
at 9 locations.
(b) TWIP products are generated using
weather data from the TDWR or the Integrated
Terminal Weather System (ITWS) testbed. TWIP
products are generated and stored in the form of text
and character graphic messages. Software has been
developed to allow TDWR or ITWS to format the
data and send the TWIP products to a database
resident at Aeronautical Radio, Inc. (ARINC). These
products can then be accessed by pilots using the
ARINC Aircraft Communications Addressing and
Reporting System (ACARS) data link services.
Airline dispatchers can also access this database and
send messages to specific aircraft whenever wind
shear activity begins or ends at an airport.
(c) TWIP products include descriptions and
character graphics of microburst alerts, wind shear
alerts, significant precipitation, convective activity
within 30 NM surrounding the terminal area, and
expected weather that will impact airport operations.
During inclement weather, i.e., whenever a predeter-
mined level of precipitation or wind shear is detected
within 15 miles of the terminal area, TWIP products
are updated once each minute for text messages and
once every five minutes for character graphic
messages. During good weather (below the predeter-
mined precipitation or wind shear parameters) each
message is updated every 10 minutes. These products
are intended to improve the situational awareness of
the pilot/flight crew, and to aid in flight planning prior
to arriving or departing the terminal area. It is
important to understand that, in the context of TWIP,
the predetermined levels for inclement versus good
weather has nothing to do with the criteria for
VFR/MVFR/IFR/LIFR; it only deals with precipita-
tion, wind shears and microbursts.
7-1-27. PIREPs Relating to Volcanic Ash
Activity
a. Volcanic eruptions which send ash into the
upper atmosphere occur somewhere around the world
several times each year. Flying into a volcanic ash
cloud can be extremely dangerous. At least two
B747s have lost all power in all four engines after
such an encounter. Regardless of the type aircraft,
some damage is almost certain to ensue after an
encounter with a volcanic ash cloud.
b. While some volcanoes in the U.S. are
monitored, many in remote areas are not. These
unmonitored volcanoes may erupt without prior
warning to the aviation community. A pilot observing
a volcanic eruption who has not had previous
notification of it may be the only witness to the
eruption. Pilots are strongly encouraged to transmit a
PIREP regarding volcanic eruptions and any
observed volcanic ash clouds.
c. Pilots should submit PIREPs regarding volcanic
activity using the Volcanic Activity Reporting (VAR)
form as illustrated in Appendix 2. If a VAR form is
not immediately available, relay enough information
to identify the position and type of volcanic activity.
d. Pilots should verbally transmit the data required
in items 1 through 8 of the VAR as soon as possible.
The data required in items 9 through 16 of the VAR
should be relayed after landing if possible.
7-1-28. Thunderstorms
a. Turbulence, hail, rain, snow, lightning, sus-
tained updrafts and downdrafts, icing conditions-all
are present in thunderstorms. While there is some
evidence that maximum turbulence exists at the
middle level of a thunderstorm, recent studies show
little variation of turbulence intensity with altitude.
b. There is no useful correlation between the
external visual appearance of thunderstorms and the
severity or amount of turbulence or hail within them.
The visible thunderstorm cloud is only a portion of a
turbulent system whose updrafts and downdrafts
often extend far beyond the visible storm cloud.
Severe turbulence can be expected up to 20 miles
from severe thunderstorms. This distance decreases
to about 10 miles in less severe storms.
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c. Weather radar, airborne or ground based, will
normally reflect the areas of moderate to heavy
precipitation (radar does not detect turbulence). The
frequency and severity of turbulence generally
increases with the radar reflectivity which is closely
associated with the areas of highest liquid water
content of the storm. NO FLIGHT PATH THROUGH
AN AREA OF STRONG OR VERY STRONG
RADAR ECHOES SEPARATED BY 20-30 MILES
OR LESS MAY BE CONSIDERED FREE OF
SEVERE TURBULENCE.
d. Turbulence beneath a thunderstorm should not
be minimized. This is especially true when the
relative humidity is low in any layer between the
surface and 15,000 feet. Then the lower altitudes may
be characterized by strong out flowing winds and
severe turbulence.
e. The probability of lightning strikes occurring to
aircraft is greatest when operating at altitudes where
temperatures are between minus 5 degrees Celsius
and plus 5 degrees Celsius. Lightning can strike
aircraft flying in the clear in the vicinity of a
thunderstorm.
f. METAR reports do not include a descriptor for
severe thunderstorms. However, by understanding
severe thunderstorm criteria, i.e., 50 knot winds or
3
/4_inch hail, the information is available in the report
to know that one is occurring.
g. Current weather radar systems are able to
objectively determine precipitation intensity. These
precipitation intensity areas are described as “light,”
“moderate,” “heavy,” and “extreme.”
REFERENCE-
Pilot/Controller Glossary, Precipitation Radar Weather Descriptions.
EXAMPLE1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between
ten o'clock and two o'clock, one five miles. Precipitation
area is two five miles in diameter.
2. Alert provided by an AFSS/FSS:
(aircraft identification) EXTREME precipitation two zero
miles west of Atlanta V-O-R, two five miles wide, moving
east at two zero knots, tops flight level three niner zero.
7-1-29. Thunderstorm Flying
a. Above all, remember this: never regard any
thunderstorm “lightly” even when radar observers
report the echoes are of light intensity. Avoiding
thunderstorms is the best policy. Following are some
Do's and Don'ts of thunderstorm avoidance:
1. Don't land or takeoff in the face of an
approaching thunderstorm. A sudden gust front of
low level turbulence could cause loss of control.
2. Don't attempt to fly under a thunderstorm
even if you can see through to the other side.
Turbulence and wind shear under the storm could be
disastrous.
3. Don't fly without airborne radar into a cloud
mass containing scattered embedded thunderstorms.
Scattered thunderstorms not embedded usually can
be visually circumnavigated.
4. Don't trust the visual appearance to be a
reliable indicator of the turbulence inside a
thunderstorm.
5. Do avoid by at least 20 miles any
thunderstorm identified as severe or giving an intense
radar echo. This is especially true under the anvil of
a large cumulonimbus.
6. Do clear the top of a known or suspected
severe thunderstorm by at least 1,000 feet altitude for
each 10 knots of wind speed at the cloud top. This
should exceed the altitude capability of most aircraft.
7. Do circumnavigate the entire area if the area
has 6
/10 thunderstorm coverage.
8. Do remember that vivid and frequent
lightning indicates the probability of a strong
thunderstorm.
9. Do regard as extremely hazardous any
thunderstorm with tops 35,000 feet or higher whether
the top is visually sighted or determined by radar.
b. If you cannot avoid penetrating a thunderstorm,
following are some Do's before entering the storm:
1. Tighten your safety belt, put on your shoulder
harness if you have one and secure all loose objects.
2. Plan and hold your course to take you through
the storm in a minimum time.
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Meteorology
3. To avoid the most critical icing, establish a
penetration altitude below the freezing level or above
the level of minus 15 degrees Celsius.
4. Verify that pitot heat is on and turn on
carburetor heat or jet engine anti-ice. Icing can be
rapid at any altitude and cause almost instantaneous
power failure and/or loss of airspeed indication.
5. Establish power settings for turbulence
penetration airspeed recommended in your aircraft
manual.
6. Turn up cockpit lights to highest intensity to
lessen temporary blindness from lightning.
7. If using automatic pilot, disengage altitude
hold mode and speed hold mode. The automatic
altitude and speed controls will increase maneuvers
of the aircraft thus increasing structural stress.
8. If using airborne radar, tilt the antenna up and
down occasionally. This will permit you to detect
other thunderstorm activity at altitudes other than the
one being flown.
c. Following are some Do's and Don'ts during the
thunderstorm penetration:
1. Do keep your eyes on your instruments.
Looking outside the cockpit can increase danger of
temporary blindness from lightning.
2. Don't change power settings; maintain
settings for the recommended turbulence penetration
airspeed.
3. Don't attempt to maintain constant altitude;
let the aircraft “ride the waves.”
4. Don't turn back once you are in the
thunderstorm. A straight course through the storm
most likely will get you out of the hazards most
quickly. In addition, turning maneuvers increase
stress on the aircraft.
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7-1-30. Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR)
FIG 7-1-21
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Front)
U.S. Department
of Transportation
Federal Aviation
Administration
KEY to AERODROME FORECAST (TAF) and
AVIATION ROUTINE WEATHER REPORT
(METAR) (FRONT)
TAF KPIT 091730Z 091818 15005KT 5SM HZ FEW020 WS010/31022KT
FM 1930 30015G25KT 3SM SHRA OVC015 TEMPO 2022 1/2SM +TSRA
OVC008CB
FM0100 27008KT 5SM SHRA BKN020 OVC040 PROB40 0407 1SM -RA BR
FM1015 18005KT 6SM -SHRA OVC020 BECMG 1315 P6SM NSW SKC
METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB
18/16 A2992 RMK SLP045 T01820159
FORECAST EXPLANATION REPORT
TAF Message type : TAF-routine or TAF AMD-amended forecast,
METAR-hourly, SPECI-special or TESTM-non-commissioned
ASOS report
METAR
KPIT ICAO location indicator KPIT
091730Z Issuance time: ALL times in UTC “Z”, 2-digit date, 4-digit time 091955z
091818 Valid period: 2-digit date, 2-digit beginning, 2-digit ending times
In U.S. METAR: CORrected of; or AUTOmated ob for automated
report with no human intervention; omitted when observer logs on
COR
15005KT Wind: 3 digit true-north direction , nearest 10 degrees (or VaRiaBle);
next 2-3 digits for speed and unit, KT (KMH or MPS); as needed, Gust
and maximum speed; 00000KT for calm; for METAR, if direction varies
60 degrees or more, Variability appended, e.g., 180V260
22015G25KT
5SM Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in
TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as required,
lowest value with direction)
3/4SM
Runway Visual Range: R; 2-digit runway designator Left, Center, or
Right as needed; “/”, Minus or Plus in U.S., 4-digit value, FeeT in U.S.,
(usually meters elsewhere); 4-digit value Variability 4-digit value (and
tendency Down, Up or No change)
R28L/2600FT
HZ Significant present, forecast and recent weather: see table (on back) TSRA
FEW020 Cloud amount, height and type: Sky Clear 0/8, FEW >0/8-2/8,
SCaTtered 3/8-4/8, BroKeN 5/8-7/8, OVerCast 8/8; 3-digit height in
hundreds of ft; Towering Cumulus or CumulonimBus in METAR; in
TAF, only CB. Vertical Visibility for obscured sky and height “VV004”.
More than 1 layer may be reported or forecast. In automated METAR
reports only, CLeaR for “clear below 12,000 feet”
OVC 010CB
Temperature: degrees Celsius; first 2 digits, temperature “/” last 2
digits, dew-point temperature; Minus for below zero, e.g., M06
18/16
Altimeter setting: indicator and 4 digits; in U.S., A-inches and
hundredths; (Q-hectoPascals, e.g., Q1013)
A2992
AIM 2/14/08
7-1-59
Meteorology
FIG 7-1-22
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Back)
U.S. Department
of Transportation
Federal Aviation
Administration
KEY to AERODROME FORECAST (TAF) and
AVIATION ROUTINE WEATHER REPORT
(METAR) (BACK)
FORECAST EXPLANATION REPORT
WS010/31022KT In U.S. TAF, non-convective low-level (2,000 ft) Wind Shear;
3-digit height (hundreds of ft); “/”; 3-digit wind direction and 2-3
digit wind speed above the indicated height, and unit, KT
In METAR, ReMarK indicator & remarks. For example: Sea-
Level Pressure in hectoPascals & tenths, as shown: 1004.5 hPa;
Temp/dew-point in tenths _C, as shown: temp. 18.2_C, dew-point
15.9_C
RMK
SLP045
T01820159
FM1930 FroM and 2-digit hour and 2-digit minute beginning time:
indicates significant change. Each FM starts on a new line,
indented 5 spaces
TEMPO 2022 TEMPOrary: changes expected for <1 hour and in total, < half of
2-digit hour beginning and 2-digit hour ending time period
PROB40 0407 PROBability and 2-digit percent (30 or 40): probable condition
during 2-digit hour beginning and 2-digit hour ending time
period
BECMG 1315 BECoMinG: change expected during 2-digit hour beginning
and 2-digit hour ending time period
Table of Significant Present, Forecast and Recent Weather- Grouped in categories and used in the
order listed below; or as needed in TAF, No Significant Weather.
QUALIFIER
INTENSITY OR PROXIMITY
`-'
Light
“no sign” Moderate `+' Heavy
VC Vicinity: but not at aerodrome; in U.S. METAR, between 5 and 10SM of the point(s) of
observation; in U.S. TAF, 5 to 10SM from center of runway complex (elsewhere within 8000m)
DESCRIPTOR
MI Shallow BC Patches PR Partial TS Thunderstorm
BL Blowing SH Showers DR Drifting FZ Freezing
WEATHER PHENOMENA
PRECIPITATION
DZ Drizzle RA Rain SN Snow SG Snow grains
IC Ice Crystals PL Ice Pellets GR Hail GS Small hail/snow
UP Unknown precipitation in automated observations pellets
OBSCURATION
BR Mist (5/8SM) FG Fog (<5/8SM) FU Smoke VA Volcanic ash
SA Sand HZ Haze PY Spray DU Widespread dust
OTHER
SQ Squall SS Sandstorm DU Duststorm PO Well developed
FC Funnel cloud +FC tornado/waterspout dust/sand whirls
-Explanations in parentheses “( )” indicate different worldwide practices.
- Ceiling is not specified; defined as the lowest broken or overcast layer, or the vertical visibility.
- NWS TAFs exclude turbulence, icing & temperature forecasts; NWS METARs exclude trend forecasts
January 1999 Department of Transportation
Aviation Weather Directorate FEDERAL AVIATION ADMINISTRATION
AIM 2/14/08
7-1-60 Meteorology
7-1-31. International Civil Aviation
Organization (ICAO) Weather Formats
The U.S. uses the ICAO world standard for aviation
weather reporting and forecasting. The utilization of
terminal forecasts affirms our commitment to a single
global format for aviation weather. The World
Meteorological Organization's (WMO) publication
No. 782 “Aerodrome Reports and Forecasts”
contains the base METAR and TAF code as adopted
by the WMO member countries.
a. Although the METAR code is adopted
worldwide, each country is allowed to make
modifications or exceptions to the code for use in
their particular country, e.g., the U.S. will continue to
use statute miles for visibility, feet for RVR values,
knots for wind speed, and inches of mercury for
altimetry. However, temperature and dew point will
be reported in degrees Celsius. The U.S. will continue
reporting prevailing visibility rather than lowest
sector visibility. Most of the current U.S. observing
procedures and policies will continue after the
METAR conversion date, with the information
disseminated in the METAR code and format. The
elements in the body of a METAR report are
separated with a space. The only exceptions are RVR,
temperature and dew point, which are separated with
a solidus (/). When an element does not occur, or
cannot be observed, the preceding space and that
element are omitted from that particular report. A
METAR report contains the following sequence of
elements in the following order:
1. Type of report.
2. ICAO Station Identifier.
3. Date and time of report.
4. Modifier (as required).
5. Wind.
6. Visibility.
7. Runway Visual Range (RVR).
8. Weather phenomena.
9. Sky conditions.
10. Temperature/dew point group.
11. Altimeter.
12. Remarks (RMK).
b. The following paragraphs describe the ele-
ments in a METAR report.
1. Type of report. There are two types of
report:
(a) Aviation Routine Weather Report
(METAR); and
(b) Nonroutine (Special) Aviation Weather
Report (SPECI).
The type of report (METAR or SPECI) will always
appear as the lead element of the report.
2. ICAO Station Identifier. The METAR
code uses ICAO 4-letter station identifiers. In the
contiguous 48 States, the 3-letter domestic station
identifier is prefixed with a “K;” i.e., the domestic
identifier for Seattle is SEA while the ICAO identifier
is KSEA. Elsewhere, the first two letters of the ICAO
identifier indicate what region of the world and
country (or state) the station is in. For Alaska, all
station identifiers start with “PA;” for Hawaii, all
station identifiers start with “PH.” Canadian station
identifiers start with “CU,” “CW,” “CY,” and “CZ.”
Mexican station identifiers start with “MM.” The
identifier for the western Caribbean is “M” followed
by the individual country's letter; i.e., Cuba is “MU;”
Dominican Republic “MD;” the Bahamas “MY.” The
identifier for the eastern Caribbean is “T” followed
by the individual country's letter; i.e., Puerto Rico is
“TJ.” For a complete worldwide listing see ICAO
Document 7910, Location Indicators.
3. Date and Time of Report. The date and
time the observation is taken are transmitted as a
six-digit date/time group appended with Z to denote
Coordinated Universal Time (UTC). The first two
digits are the date followed with two digits for hour
and two digits for minutes.
EXAMPLE172345Z (the 17th day of the month at 2345Z)
4. Modifier (As Required). “AUTO” identi-
fies a METAR/SPECI report as an automated weather
report with no human intervention. If “AUTO” is
shown in the body of the report, the type of sensor
equipment used at the station will be encoded in the
remarks section of the report. The absence of
“AUTO” indicates that a report was made manually
by an observer or that an automated report had human
augmentation/backup. The modifier “COR” indi-
cates a corrected report that is sent out to replace an
earlier report with an error.
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Meteorology
NOTE-
There are two types of automated stations, AO1 for
automated weather reporting stations without a precipita-
tion discriminator, and AO2 for automated stations with a
precipitation discriminator. (A precipitation discriminator
can determine the difference between liquid and
frozen/freezing precipitation). This information appears in
the remarks section of an automated report.
5. Wind. The wind is reported as a five digit
group (six digits if speed is over 99 knots). The first
three digits are the direction the wind is blowing
from, in tens of degrees referenced to true north, or
“VRB” if the direction is variable. The next two digits
is the wind speed in knots, or if over 99 knots, the next
three digits. If the wind is gusty, it is reported as a “G”
after the speed followed by the highest gust reported.
The abbreviation “KT” is appended to denote the use
of knots for wind speed.
EXAMPLE13008KT - wind from 130 degrees at 8 knots
08032G45KT - wind from 080 degrees at 32 knots with
gusts to 45 knots
VRB04KT - wind variable in direction at 4 knots
00000KT - wind calm
210103G130KT - wind from 210 degrees at 103 knots with
gusts to 130 knots
If the wind direction is variable by 60 degrees or more and
the speed is greater than 6 knots, a variable group
consisting of the extremes of the wind direction separated
by a “v” will follow the prevailing wind group.
32012G22KT 280V350
(a) Peak Wind. Whenever the peak wind
exceeds 25 knots “PK WND” will be included in
Remarks, e.g., PK WND 28045/1955 “Peak wind two
eight zero at four five occurred at one niner five five.”
If the hour can be inferred from the report time, only
the minutes will be appended, e.g., PK WND
34050/38 “Peak wind three four zero at five zero
occurred at three eight past the hour.”
(b) Wind shift. Whenever a wind shift
occurs, “WSHFT” will be included in remarks
followed by the time the wind shift began, e.g.,
WSHFT 30 FROPA “Wind shift at three zero due to
frontal passage.”
6. Visibility. Prevailing visibility is reported in
statute miles with “SM” appended to it.
EXAMPLE7SM - seven statute miles
15SM - fifteen statute miles
1
/2SM - one-half statute mile
(a) Tower/surface visibility. If either visi-
bility (tower or surface) is below four statute miles,
the lesser of the two will be reported in the body of the
report; the greater will be reported in remarks.
(b) Automated visibility. ASOS visibility
stations will show visibility ten or greater than ten
miles as “10SM.” AWOS visibility stations will show
visibility less than 1
/4 statute mile as “M 1
/4SM” and
visibility ten or greater than ten miles as “10SM.”
(c) Variable visibility. Variable visibility is
shown in remarks (when rapid increase or decrease
by 1
/2 statute mile or more and the average prevailing
visibility is less than three miles) e.g., VIS 1V2
“visibility variable between one and two.”
(d) Sector visibility. Sector visibility is
shown in remarks when it differs from the prevailing
visibility, and either the prevailing or sector visibility
is less than three miles.
EXAMPLE-
VIS N2 - visibility north two
7. Runway Visual Range (When Reported).
“R” identifies the group followed by the runway
heading (and parallel runway designator, if needed)
“/” and the visual range in feet (meters in other
countries) followed with “FT” (feet is not spoken).
(a) Variability Values. When RVR varies
(by more than on reportable value), the lowest and
highest values are shown with “V” between them.
(b) Maximum/Minimum Range. “P” indi-
cates an observed RVR is above the maximum value
for this system (spoken as “more than”). “M”
indicates an observed RVR is below the minimum
value which can be determined by the system (spoken
as “less than”).
EXAMPLE-
R32L/1200FT - runway three two left R-V-R one thousand
two hundred.
R27R/M1000V4000FT - runway two seven right R-V-R
variable from less than one thousand to four thousand.
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7-1-62 Meteorology
8. Weather Phenomena. The weather as
reported in the METAR code represents a significant
change in the way weather is currently reported. In
METAR, weather is reported in the format:
Intensity/Proximity/Descriptor/Precipitation/
Obstruction to visibility/Other
NOTE-
The “/” above and in the following descriptions (except as
the separator between the temperature and dew point) are
for separation purposes in this publication and do not
appear in the actual METARs.
(a) Intensity applies only to the first type of
precipitation reported. A “-” denotes light, no symbol
denotes moderate, and a “+” denotes heavy.
(b) Proximity applies to and reported only
for weather occurring in the vicinity of the airport
(between 5 and 10 miles of the point(s) of
observation). It is denoted by the letters “VC.”
(Intensity and “VC” will not appear together in the
weather group).
(c) Descriptor. These eight descriptors ap-
ply to the precipitation or obstructions to visibility:
TS thunderstorm . . . . . . . . . . .
DR low drifting . . . . . . . . . . .
SH showers . . . . . . . . . . .
MI shallow . . . . . . . . . . .
FZ freezing . . . . . . . . . . .
BC patches . . . . . . . . . . .
BL blowing . . . . . . . . . . .
PR partial . . . . . . . . . . .
NOTE-
Although “TS” and “SH” are used with precipitation and
may be preceded with an intensity symbol, the intensity still
applies to the precipitation, not the descriptor.
(d) Precipitation. There are nine types of
precipitation in the METAR code:
RA rain . . . . . . . . . .
DZ drizzle . . . . . . . . . .
SN snow . . . . . . . . . .
GR hail (1
/4” or greater) . . . . . . . . . .
GS small hail/snow pellets . . . . . . . . . .
PL ice pellets . . . . . . . . . .
SG snow grains . . . . . . . . . .
IC ice crystals (diamond dust) . . . . . . . . . . .
UP unknown precipitation . . . . . . . . . .
(automated stations only)
(e) Obstructions to visibility. There are
eight types of obscuration phenomena in the METAR
code (obscurations are any phenomena in the
atmosphere, other than precipitation, that reduce
horizontal visibility):
FG fog (vsby less than 5
/8 mile) . . . . . . . . . .
HZ haze . . . . . . . . . .
FU smoke . . . . . . . . . .
PY spray . . . . . . . . . .
BR mist (vsby 5
/8 - 6 miles) . . . . . . . . . .
SA sand . . . . . . . . . .
DU dust . . . . . . . . . .
VA volcanic ash . . . . . . . . . .
NOTE-
Fog (FG) is observed or forecast only when the visibility is
less than five-eighths of mile, otherwise mist (BR) is
observed or forecast.
(f) Other. There are five categories of other
weather phenomena which are reported when they
occur:
SQ squall . . . . . . . . . . .
SS sandstorm . . . . . . . . . . .
DS duststorm . . . . . . . . . . .
PO dust/sand whirls . . . . . . . . . .
FC funnel cloud . . . . . . . . . . .
+FC tornado/waterspout . . . . . . . . .
Examples:
TSRA thunderstorm with moderate . . . . . . . . .
rain
+SN heavy snow . . . . . . . . . .
-RA FG light rain and fog . . . . . . .
BRHZ mist and haze . . . . . . . .
(visibility 5
/8_mile or greater)
FZDZ freezing drizzle . . . . . . . . .
VCSH rain shower in the vicinity . . . . . . . .
+SHRASNPL heavy rain showers, snow, . .
ice pellets (intensity
indicator refers to the
predominant rain)
9. Sky Condition. The sky condition as
reported in METAR represents a significant change
from the way sky condition is currently reported. In
METAR, sky condition is reported in the format:
Amount/Height/(Type) or Indefinite Ceiling/Height
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Meteorology
(a) Amount. The amount of sky cover is
reported in eighths of sky cover, using the
contractions:
SKC clear (no clouds) . . . . . . . . .
FEW >0 to 2
/8 . . . . . . . .
SCT scattered (3
/8s to 4
/8s of . . . . . . . . .
clouds)
BKN broken (5
/8s to 7
/8s of clouds) . . . . . . . . .
OVC overcast (8
/8s clouds) . . . . . . . . .
CB Cumulonimbus when present . . . . . . . . . .
TCU Towering cumulus when . . . . . . . . .
present
NOTE1. “SKC” will be reported at manual stations. “CLR” will
be used at automated stations when no clouds below
12,000 feet are reported.
2. A ceiling layer is not designated in the METAR code.
For aviation purposes, the ceiling is the lowest broken or
overcast layer, or vertical visibility into an obscuration.
Also there is no provision for reporting thin layers in the
METAR code. When clouds are thin, that layer shall be
reported as if it were opaque.
(b) Height. Cloud bases are reported with
three digits in hundreds of feet. (Clouds above
12,000_feet cannot be reported by an automated
station).
(c) (Type). If Towering Cumulus Clouds
(TCU) or Cumulonimbus Clouds (CB) are present,
they are reported after the height which represents
their base.
EXAMPLE(Reported as) SCT025TCU BKN080 BKN250 (spoken as)
“TWO THOUSAND FIVE HUNDRED SCATTERED
TOWERING CUMULUS, CEILING EIGHT THOUSAND
BROKEN, TWO FIVE THOUSAND BROKEN.”
(Reported as) SCT008 OVC012CB (spoken as) “EIGHT
HUNDRED SCATTERED CEILING ONE THOUSAND
TWO HUNDRED OVERCAST CUMULONIMBUS
CLOUDS.”
(d) Vertical Visibility (indefinite ceiling
height). The height into an indefinite ceiling is
preceded by “VV” and followed by three digits
indicating the vertical visibility in hundreds of feet.
This layer indicates total obscuration.
EXAMPLE1 /8 SM FG VV006 - visibility one eighth, fog, indefinite
ceiling six hundred.
(e) Obscurations are reported when the sky
is partially obscured by a ground-based phenomena
by indicating the amount of obscuration as FEW,
SCT, BKN followed by three zeros (000). In remarks,
the obscuring phenomenon precedes the amount of
obscuration and three zeros.
EXAMPLE-
BKN000 (in body) “sky partially obscured” . . . . . . . .
FU BKN000 (in remarks) “smoke obscuring five- . . .
to seven-eighths of the
sky”
(f) When sky conditions include a layer aloft,
other than clouds, such as smoke or haze the type of
phenomena, sky cover and height are shown in
remarks.
EXAMPLE-
BKN020 (in body) “ceiling two thousand . . . . . . . .
broken”
RMK FU BKN020 “broken layer of smoke . . . . . . . .
aloft, based at
two thousand”
(g) Variable ceiling. When a ceiling is
below three thousand and is variable, the remark
“CIG” will be shown followed with the lowest and
highest ceiling heights separated by a “V.”
EXAMPLE-
CIG 005V010 “ceiling variable . . . . . . . . . . . .
between five hundred and
one thousand”
(h) Second site sensor. When an automated
station uses meteorological discontinuity sensors,
remarks will be shown to identify site specific sky
conditions which differ and are lower than conditions
reported in the body.
EXAMPLE-
CIG 020 RY11 “ceiling two thousand at . . . . . . . . . . .
runway one one”
(i) Variable cloud layer. When a layer is
varying in sky cover, remarks will show the
variability range. If there is more than one cloud
layer, the variable layer will be identified by
including the layer height.
EXAMPLE-
SCT V BKN “scattered layer variable to . . . . . . . . . . . . .
broken”
BKN025 V OVC “broken layer at . . . . . . . . .
two thousand five hundred
variable to overcast”
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7-1-64 Meteorology
(j) Significant clouds. When significant
clouds are observed, they are shown in remarks,
along with the specified information as shown below:
(1) Cumulonimbus (CB), or Cumulonim-
bus Mammatus (CBMAM), distance (if known),
direction from the station, and direction of
movement, if known. If the clouds are beyond
10_miles from the airport, DSNT will indicate
distance.
EXAMPLE-
CB W MOV E “cumulonimbus west moving . . . . . . .
east”
CBMAM DSNT S “cumulonimbus mammatus . . . .
distant south”
(2) Towering Cumulus (TCU), location, (if
known), or direction from the station.
EXAMPLE-
TCU OHD “towering cumulus overhead” . . . . . . . . .
TCU W “towering cumulus west” . . . . . . . . . . . .
(3) Altocumulus Castellanus (ACC), Stra-
tocumulus Standing Lenticular (SCSL),
Altocumulus Standing Lenticular (ACSL), Cirrocu-
mulus Standing Lenticular (CCSL) or rotor clouds,
describing the clouds (if needed) and the direction
from the station.
EXAMPLE-
ACC W “altocumulus castellanus west” . . . . . . . . . . . . .
ACSL SW-S “standing lenticular . . . . . . . . .
altocumulus southwest
through south”
APRNT ROTOR CLD S “apparent rotor cloud south”
CCSL OVR MT E “standing lenticular . . . . .
cirrocumulus over the
mountains east”
10. Temperature/Dew Point. Temperature
and dew point are reported in two, two-digit groups
in degrees Celsius, separated by a solidus (“/”).
Temperatures below zero are prefixed with an “M.”
If the temperature is available but the dew point is
missing, the temperature is shown followed by a
solidus. If the temperature is missing, the group is
omitted from the report.
EXAMPLE15/08 “temperature one five, . . . . . . . . . . . . . .
dew point 8”
00/M02 “temperature zero, . . . . . . . . . . . .
dew point minus 2”
M05/ “temperature minus five, . . . . . . . . . . . . . . .
dew point missing”
11. Altimeter. Altimeter settings are reported
in a four-digit format in inches of mercury prefixed
with an “A” to denote the units of pressure.
EXAMPLE-
A2995 - “Altimeter two niner niner five”
12. Remarks. Remarks will be included in all
observations, when appropriate. The contraction
“RMK” denotes the start of the remarks section of a
METAR report.
Except for precipitation, phenomena located within
5_statute miles of the point of observation will be
reported as at the station. Phenomena between 5 and
10 statute miles will be reported in the vicinity, “VC.”
Precipitation not occurring at the point of observation
but within 10 statute miles is also reported as in the
vicinity, “VC.” Phenomena beyond 10 statute miles
will be shown as distant, “DSNT.” Distances are in
statute miles except for automated lightning remarks
which are in nautical miles. Movement of clouds or
weather will be indicated by the direction toward
which the phenomena is moving.
(a) There are two categories of remarks:
(1) Automated, manual, and plain
language.
(2) Additive and automated maintenance
data.
(b) Automated, Manual, and Plain Lan-
guage. This group of remarks may be generated
from either manual or automated weather reporting
stations and generally elaborate on parameters
reported in the body of the report. (Plain language
remarks are only provided by manual stations).
(1) Volcanic eruptions.
(2) Tornado, Funnel Cloud, Waterspout.
(3) Station Type (AO1 or AO2).
(4) PK WND.
(5) WSHFT (FROPA).
(6) TWR VIS or SFC VIS.
(7) VRB VIS.
(8) Sector VIS.
(9) VIS @ 2nd Site.
(10) (freq) LTG (type) (loc).
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Meteorology
(11) Beginning/Ending of Precipitation/
TSTMS.
(12) TSTM Location MVMT.
(13) Hailstone Size (GR).
(14) Virga.
(15) VRB CIG (height).
(16) Obscuration.
(17) VRB Sky Condition.
(18) Significant Cloud Types.
(19) Ceiling Height 2nd Location.
(20) PRESFR PRESRR.
(21) Sea-Level Pressure.
(22) ACFT Mishap (not transmitted).
(23) NOSPECI.
(24) SNINCR.
(25) Other SIG Info.
(c) Additive and Automated Maintenance
Data.
(1) Hourly Precipitation.
(2) 3- and 6-Hour Precipitation Amount.
(3) 24-Hour Precipitation.
(4) Snow Depth on Ground.
(5) Water Equivalent of Snow.
(6) Cloud Type.
(7) Duration of Sunshine.
(8) Hourly Temperature/Dew Point
(Tenths).
(9) 6-Hour Maximum Temperature.
(10) 6-Hour Minimum Temperature.
(11) 24-Hour Maximum/Minimum
Temperature.
(12) Pressure Tendency.
(13) Sensor Status.
PWINO
FZRANO
TSNO
RVRNO
PNO
VISNO
Examples of METAR reports and explanation:
METAR KBNA 281250Z 33018KT 290V360
1/2SM R31/2700FT SN BLSN FG VV008 00/M03
A2991 RMK RAE42SNB42
METAR aviation routine weather . . . . . .
report
KBNA Nashville, TN . . . . . . . .
281250Z date 28th , time 1250 UTC . . . . . .
(no modifier) This is a manually generated . .
report, due to the absence of
“AUTO” and “AO1 or AO2”
in remarks
33018KT wind three three zero at one . . . . .
eight
290V360 wind variable between . . . . . .
two nine zero and three six
zero
1/2SM visibility one half . . . . . . . .
R31/2700FT Runway three one RVR two . . .
thousand seven hundred
SN moderate snow . . . . . . . . . . .
BLSN FG visibility obscured by . . . . .
blowing snow and fog
VV008 indefinite ceiling eight . . . . . . .
hundred
00/M03 temperature zero, dew point . . . . . . .
minus three
A2991 altimeter two niner niner one . . . . . . . .
RMK remarks . . . . . . . .
RAE42 rain ended at four two . . . . . . .
SNB42 snow began at four two . . . . . . .
METAR KSFO 041453Z AUTO VRB02KT 3SM
BR CLR 15/12 A3012 RMK AO2
METAR aviation routine weather . . . . . .
report
KSFO San Francisco, CA . . . . . . . .
041453Z date 4th , time 1453 UTC . . . . . .
AUTO fully automated; no human . . . . . . .
intervention
VRB02KT wind variable at two . . . .
3SM visibility three . . . . . . . . .
BR visibility obscured by mist . . . . . . . . . .
CLR no clouds below one two . . . . . . . . .
thousand
15/12 temperature one five, dew . . . . . . . . .
point one two
AIM 2/14/08
7-1-66 Meteorology
A3012 altimeter three zero one two . . . . . . . .
RMK remarks . . . . . . . .
AO2 this automated station has a . . . . . . . . .
weather discriminator (for
precipitation)
SPECI KCVG 152224Z 28024G36KT 3/4SM
+TSRA BKN008 OVC020CB 28/23 A3000 RMK
TSRAB24 TS W MOV E
SPECI (nonroutine) aviation special . . . . . . .
weather report
KCVG Cincinnati, OH . . . . . . .
152228Z date 15th , time 2228 UTC . . . . . .
(no modifier) This is a manually generated . .
report due to the absence of
“AUTO” and “AO1 or AO2”
in remarks
28024G36KT wind two eight zero at . .
two four gusts three six
3/4SM visibility three fourths . . . . . . . .
+TSRA thunderstorms, heavy rain . . . . . . .
BKN008 ceiling eight hundred broken
OVC020CB two thousand overcast . . .
cumulonimbus clouds
28/23 temperature two eight, . . . . . . . . .
dew point two three
A3000 altimeter three zero zero zero . . . . . . . .
RMK remarks . . . . . . . .
TSRAB24 thunderstorm and rain began . . . . .
at two four
TS W MOV E thunderstorm west moving
east
c. Aerodrome Forecast (TAF). A concise state-
ment of the expected meteorological conditions at an
airport during a specified period (usually 24 hours).
TAFs use the same codes as METAR weather reports.
They are scheduled four times daily for 24-hour
periods beginning at 0000Z, 0600Z, 1200Z, and
1800Z. TAFs are issued in the following format:
TYPE OF REPORT/ICAO STATION IDENTIFIER/
DATE AND TIME OF ORIGIN/VALID PERIOD
DATE AND TIME/FORECAST METEOROLOG-
ICAL CONDITIONS
NOTE-
The “/” above and in the following descriptions are for
separation purposes in this publication and do not appear
in the actual TAFs.
TAF
KOKC 051130Z 051212 14008KT 5SM BR
BKN030 TEMPO 1316 1 1
/2SM BR
_____FM1600 16010KT P6SM SKC
_____FM2300 20013G20KT 4SM SHRA OVC020
PROB40 0006 2SM TSRA OVC008CB BECMG
0608 21015KT P6SM NSW SCT040
TAF format observed in the above example:
TAF = type of report
KOKC = ICAO station identifier
051130Z = date and time of origin
051212 = valid period date and times
14008KT 5SM BR BKN030 = forecast meteorologi-
cal conditions
Explanation of TAF elements:
1. Type of Report. There are two types of TAF
issuances, a routine forecast issuance (TAF) and an
amended forecast (TAF AMD). An amended TAF is
issued when the current TAF no longer adequately
describes the on-going weather or the forecaster feels
the TAF is not representative of the current or
expected weather. Corrected (COR) or delayed
(RTD) TAFs are identified only in the communica-
tions header which precedes the actual forecasts.
2. ICAO Station Identifier. The TAF code
uses ICAO 4-letter location identifiers as described
in the METAR section.
3. Date and Time of Origin. This element is
the date and time the forecast is actually prepared.
The format is a two-digit date and four-digit time
followed, without a space, by the letter “Z.”
4. Valid Period Date and Time. The UTC
valid period of the forecast is a two-digit date
followed by the two-digit beginning hour and
two-digit ending hour. In the case of an amended
forecast, or a forecast which is corrected or delayed,
the valid period may be for less than 24 hours. Where
an airport or terminal operates on a part-time basis
(less than 24 hours/day), the TAFs issued for those
locations will have the abbreviated statement “NIL
AMD SKED AFT (closing time) Z” added to the end
of the forecasts. For the TAFs issued while these
locations are closed, the word “NIL” will appear in
place of the forecast text. A delayed (RTD) forecast
will then be issued for these locations after two
complete observations are received.
AIM 2/14/08
7-1-67
Meteorology
5. Forecast Meteorological Conditions. This
is the body of the TAF. The basic format is:
W I N D / V I S I B I L I T Y / W E AT H E R / S K Y
CONDITION/OPTIONAL DATA (WIND SHEAR)
The wind, visibility, and sky condition elements are
always included in the initial time group of the
forecast. Weather is included only if significant to
aviation. If a significant, lasting change in any of the
elements is expected during the valid period, a new
time period with the changes is included. It should be
noted that with the exception of a “FM” group the
new time period will include only those elements
which are expected to change, i.e., if a lowering of the
visibility is expected but the wind is expected to
remain the same, the new time period reflecting the
lower visibility would not include a forecast wind.
The forecast wind would remain the same as in the
previous time period.
Any temporary conditions expected during a specific
time period are included with that time period. The
following describes the elements in the above format.
(a) Wind. This five (or six) digit group
includes the expected wind direction (first 3 digits)
and speed (last 2 digits or 3 digits if 100 knots or
greater). The contraction “KT” follows to denote the
units of wind speed. Wind gusts are noted by the letter
“G” appended to the wind speed followed by the
highest expected gust.
A variable wind direction is noted by “VRB” where
the three digit direction usually appears. A calm wind
(3 knots or less) is forecast as “00000KT.”
EXAMPLE18010KT wind one eight zero at one zero (wind is . . . . .
blowing from 180).
35012G20KT wind three five zero at one two gust two . .
zero.
(b) Visibility. The expected prevailing visi-
bility up to and including 6 miles is forecast in statute
miles, including fractions of miles, followed by “SM”
to note the units of measure. Expected visibilities
greater than 6 miles are forecast as P6SM (plus
six_statute miles).
EXAMPLE1 /2SM - visibility one-half
4SM - visibility four
P6SM - visibility more than six
(c) Weather Phenomena. The expected
weather phenomena is coded in TAF reports using the
same format, qualifiers, and phenomena contractions
as METAR reports (except UP).
Obscurations to vision will be forecast whenever the
prevailing visibility is forecast to be 6 statute miles or
less.
If no significant weather is expected to occur during
a specific time period in the forecast, the weather
phenomena group is omitted for that time period. If,
after a time period in which significant weather
phenomena has been forecast, a change to a forecast
of no significant weather phenomena occurs, the
contraction NSW (No Significant Weather) will
appear as the weather group in the new time period.
(NSW is included only in BECMG or TEMPO
groups).
NOTE-
It is very important that pilots understand that NSW only
refers to weather phenomena, i.e., rain, snow, drizzle, etc.
Omitted conditions, such as sky conditions, visibility,
winds, etc., are carried over from the previous time group.
(d) Sky Condition. TAF sky condition
forecasts use the METAR format described in the
METAR section. Cumulonimbus clouds (CB) are the
only cloud type forecast in TAFs.
When clear skies are forecast, the contraction “SKC”
will always be used. The contraction “CLR” is never
used in the TAF.
When the sky is obscured due to a surface-based
phenomenon, vertical visibility (VV) into the
obscuration is forecast. The format for vertical
visibility is “VV” followed by a three-digit height in
hundreds of feet.
NOTE-
As in METAR, ceiling layers are not designated in the TAF
code. For aviation purposes, the ceiling is the lowest
broken or overcast layer or vertical visibility into a
complete obscuration.
SKC “sky clear” . . . . . . . . . . . . . .
SCT005 BKN025CB “five hundred scattered,
ceiling two thousand
five hundred broken
cumulonimbus clouds”
VV008 “indefinite ceiling . . . . . . . . . . . .
eight hundred”
AIM 2/14/08
7-1-68 Meteorology
(e) Optional Data (Wind Shear). Wind
shear is the forecast of nonconvective low level winds
(up to 2,000 feet). The forecast includes the letters
“WS” followed by the height of the wind shear, the
wind direction and wind speed at the indicated height
and the ending letters “KT” (knots). Height is given
in hundreds of feet (AGL) up to and including
2,000_feet. Wind shear is encoded with the
contraction “WS,” followed by a three-digit height,
slant character “/,” and winds at the height indicated
in the same format as surface winds. The wind shear
element is omitted if not expected to occur.
WS010/18040KT - “LOW LEVEL WIND SHEAR
AT ONE THOUSAND, WIND ONE EIGHT ZERO
AT FOUR ZERO”
d. Probability Forecast. The probability or
chance of thunderstorms or other precipitation events
occurring, along with associated weather conditions
(wind, visibility, and sky conditions).
The PROB30 group is used when the occurrence of
thunderstorms or precipitation is 30-39% and the
PROB40 group is used when the occurrence of
thunderstorms or precipitation is 40-49%. This is
followed by a four-digit group giving the beginning
hour and ending hour of the time period during which
the thunderstorms or precipitation are expected.
NOTE-
Neither PROB30 nor PROB40 will be shown during the
first six hours of a forecast.
EXAMPLE-
PROB40 2102 1
/2SM +TSRA “chance between . . . .
2100Z and 0200Z of
visibility one-half
statute mile in
thunderstorms and
heavy rain.”
PROB30 1014 1SM RASN “chance between . . . . . .
1000Z and 1400Z of
visibility one statute
mile in mixed rain
and snow.”
e. Forecast Change Indicators. The following
change indicators are used when either a rapid,
gradual, or temporary change is expected in some or
all of the forecast meteorological conditions. Each
change indicator marks a time group within the TAF
report.
1. From (FM) group. The FM group is used
when a rapid change, usually occurring in less than
one hour, in prevailing conditions is expected.
Typically, a rapid change of prevailing conditions to
more or less a completely new set of prevailing
conditions is associated with a synoptic feature
passing through the terminal area (cold or warm
frontal passage). Appended to the “FM” indicator is
the four-digit hour and minute the change is expected
to begin and continues until the next change group or
until the end of the current forecast.
A “FM” group will mark the beginning of a new line
in a TAF report (indented 5 spaces). Each “FM” group
contains all the required elements-wind, visibility,
weather, and sky condition. Weather will be omitted
in “FM” groups when it is not significant to aviation.
FM groups will not include the contraction NSW.
EXAMPLE-
FM0100 14010KT P6SM SKC - “after 0100Z, wind
one_four zero at one zero, visibility more than six, sky
clear.”
2. Becoming (BECMG) group. The BECMG
group is used when a gradual change in conditions is
expected over a longer time period, usually two
hours. The time period when the change is expected
is a four-digit group with the beginning hour and
ending hour of the change period which follows the
BECMG indicator. The gradual change will occur at
an unspecified time within this time period. Only the
changing forecast meteorological conditions are
included in BECMG groups. The omitted conditions
are carried over from the previous time group.
EXAMPLE-
OVC012 BECMG 1416 BKN020 - “ceiling one thousand
two hundred overcast. Then a gradual change to ceiling
two thousand broken between 1400Z and 1600Z.”
3. Temporary (TEMPO) group. The TEMPO
group is used for any conditions in wind, visibility,
weather, or sky condition which are expected to last
for generally less than an hour at a time (occasional),
and are expected to occur during less than half the
time period. The TEMPO indicator is followed by a
four-digit group giving the beginning hour and
ending hour of the time period during which the
temporary conditions are expected. Only the
changing forecast meteorological conditions are
included in TEMPO groups. The omitted conditions
are carried over from the previous time group.
AIM 2/14/08
7-1-69
Meteorology
EXAMPLE1. SCT030 TEMPO 1923 BKN030 - “three thousand
scattered with occasional ceilings three thousand broken
between 1900Z and 2300Z.”
2. 4SM HZ TEMPO 0006 2SM BR HZ - “visibility four in
haze with occasional visibility two in mist and haze
between 0000Z and 0600Z.”
AIM 2/14/08
7-2-1
Altimeter Setting Procedures
Section 2. Altimeter Setting Procedures
7-2-1. General
a. The accuracy of aircraft altimeters is subject to
the following factors:
1. Nonstandard temperatures of the atmosphere.
2. Nonstandard atmospheric pressure.
3. Aircraft static pressure systems (position
error); and
4. Instrument error.
b. EXTREME CAUTION SHOULD BE EXER-
CISED WHEN FLYING IN PROXIMITY TO
OBSTRUCTIONS OR TERRAIN IN LOW TEM-
PERATURES AND PRESSURES. This is especially
true in extremely cold temperatures that cause a large
differential between the Standard Day temperature
and actual temperature. This circumstance can cause
serious errors that result in the aircraft being
significantly lower than the indicated altitude.
NOTE-
Standard temperature at sea level is 15 degrees Celsius
(59_degrees Fahrenheit). The temperature gradient from
sea level is minus 2 degrees Celsius (3.6 degrees
Fahrenheit) per 1,000 feet. Pilots should apply corrections
for static pressure systems and/or instruments, if
appreciable errors exist.
c. The adoption of a standard altimeter setting at
the higher altitudes eliminates station barometer
errors, some altimeter instrument errors, and errors
caused by altimeter settings derived from different
geographical sources.
7-2-2. Procedures
The cruising altitude or flight level of aircraft shall be
maintained by reference to an altimeter which shall
be set, when operating:
a. Below 18,000 feet MSL.
1. When the barometric pressure is
31.00_inches Hg. or less. To the current reported
altimeter setting of a station along the route and
within 100 NM of the aircraft, or if there is no station
within this area, the current reported altimeter setting
of an appropriate available station. When an aircraft
is en route on an instrument flight plan, air traffic
controllers will furnish this information to the pilot at
least once while the aircraft is in the controllers area
of jurisdiction. In the case of an aircraft not equipped
with a radio, set to the elevation of the departure
airport or use an appropriate altimeter setting
available prior to departure.
2. When the barometric pressure exceeds
31.00 inches Hg. The following procedures will be
placed in effect by NOTAM defining the geographic
area affected:
(a) For all aircraft. Set 31.00 inches for en
route operations below 18,000 feet MSL. Maintain
this setting until beyond the affected area or until
reaching final approach segment. At the beginning of
the final approach segment, the current altimeter
setting will be set, if possible. If not possible,
31.00_inches will remain set throughout the ap-
proach. Aircraft on departure or missed approach will
set 31.00 inches prior to reaching any mandatory/
crossing altitude or 1,500 feet AGL, whichever is
lower. (Air traffic control will issue actual altimeter
settings and advise pilots to set 31.00 inches in their
altimeters for en route operations below 18,000 feet
MSL in affected areas.)
(b) During preflight, barometric altimeters
shall be checked for normal operation to the extent
possible.
(c) For aircraft with the capability of setting
the current altimeter setting and operating into
airports with the capability of measuring the current
altimeter setting, no additional restrictions apply.
(d) For aircraft operating VFR, there are no
additional restrictions, however, extra diligence in
flight planning and in operating in these conditions is
essential.
(e) Airports unable to accurately measure
barometric pressures above 31.00 inches of Hg. will
report the barometric pressure as “missing” or “in
excess of 31.00 inches of Hg.” Flight operations to
and from those airports are restricted to VFR weather
conditions.
AIM 2/14/08
7-2-2 Altimeter Setting Procedures
(f) For aircraft operating IFR and unable to set
the current altimeter setting, the following restric-
tions apply:
(1) To determine the suitability of depar-
ture alternate airports, destination airports, and
destination alternate airports, increase ceiling
requirements by 100 feet and visibility requirements
by 1
/4 statute mile for each 1
/10 of an inch of Hg., or
any portion thereof, over 31.00 inches. These
adjusted values are then applied in accordance with
the requirements of the applicable operating
regulations and operations specifications.
EXAMPLE-
Destination altimeter is 31.28 inches, ILS DH 250 feet
(200-1
/2). When flight planning, add 300-3
/4 to the
weather requirements which would become 500-11 /4.
(2) On approach, 31.00 inches will remain
set. Decision height (DH) or minimum descent
altitude shall be deemed to have been reached when
the published altitude is displayed on the altimeter.
NOTE-
Although visibility is normally the limiting factor on an
approach, pilots should be aware that when reaching DH
the aircraft will be higher than indicated. Using the
example above the aircraft would be approximately
300_feet higher.
(3) These restrictions do not apply to
authorized Category II and III ILS operations nor do
they apply to certificate holders using approved QFE
altimetry systems.
(g) The FAA Regional Flight Standards
Division Manager of the affected area is authorized to
approve temporary waivers to permit emergency
resupply or emergency medical service operation.
b. At or above 18,000 feet MSL. To 29.92_inch-
es of mercury (standard setting). The lowest usable
flight level is determined by the atmospheric pressure
in the area of operation as shown in TBL 7-2-1.
TBL 7-2-1
Lowest Usable Flight Level
Altimeter Setting
(Current Reported)
Lowest Usable
Flight Level
29.92 or higher 180
29.91 to 29.42 185
29.41 to 28.92 190
28.91 to 28.42 195
28.41 to 27.92 200
c. Where the minimum altitude, as prescribed in
14_CFR Section 91.159 and 14_CFR Section 91.177,
is above 18,000 feet MSL, the lowest usable flight
level shall be the flight level equivalent of the
minimum altitude plus the number of feet specified in
TBL 7-2-2.
TBL 7-2-2
Lowest Flight Level Correction Factor
Altimeter Setting Correction Factor
29.92 or higher none
29.91 to 29.42 500 feet
29.41 to 28.92 1000 feet
28.91 to 28.42 1500 feet
28.41 to 27.92 2000 feet
27.91 to 27.42 2500 feet
EXAMPLE-
The minimum safe altitude of a route is 19,000 feet MSL
and the altimeter setting is reported between 29.92 and
29.42 inches of mercury, the lowest usable flight level will
be 195, which is the flight level equivalent of 19,500 feet
MSL (minimum altitude plus 500 feet).
AIM 2/14/08
7-2-3
Altimeter Setting Procedures
7-2-3. Altimeter Errors
a. Most pressure altimeters are subject to
mechanical, elastic, temperature, and installation
errors. (Detailed information regarding the use of
pressure altimeters is found in the Instrument Flying
Handbook, Chapter IV.) Although manufacturing
and installation specifications, as well as the periodic
test and inspections required by regulations (14 CFR
Part 43, Appendix E), act to reduce these errors, any
scale error may be observed in the following manner:
1. Set the current reported altimeter setting on
the altimeter setting scale.
2. Altimeter should now read field elevation if
you are located on the same reference level used to
establish the altimeter setting.
3. Note the variation between the known field
elevation and the altimeter indication. If this variation
is in the order of plus or minus 75 feet, the accuracy
of the altimeter is questionable and the problem
should be referred to an appropriately rated repair
station for evaluation and possible correction.
b. Once in flight, it is very important to obtain
frequently current altimeter settings en route. If you
do not reset your altimeter when flying from an area
of high pressure into an area of low pressure, your
aircraft will be closer to the surface than your
altimeter indicates. An inch error in the altimeter
setting equals 1,000 feet of altitude. To quote an old
saying: “GOING FROM A HIGH TO A LOW,
LOOK OUT BELOW.”
c. Temperature also has an effect on the accuracy
of altimeters and your altitude. The crucial values to
consider are standard temperature versus the ambient
(at altitude) temperature. It is this “difference” that
causes the error in indicated altitude. When the air is
warmer than standard, you are higher than your
altimeter indicates. Subsequently, when the air is
colder than standard you are lower than indicated. It
is the magnitude of this “difference” that determines
the magnitude of the error. When flying into a cooler
air mass while maintaining a constant indicated
altitude, you are losing true altitude. However, flying
into a cooler air mass does not necessarily mean you
will be lower than indicated if the difference is still on
the plus side. For example, while flying at 10,000 feet
(where STANDARD temperature is -5 degrees
Celsius (C)), the outside air temperature cools from
+5 degrees C to 0 degrees C, the temperature error
will nevertheless cause the aircraft to be HIGHER
than indicated. It is the extreme “cold” difference that
normally would be of concern to the pilot. Also, when
flying in cold conditions over mountainous country,
the pilot should exercise caution in flight planning
both in regard to route and altitude to ensure adequate
en route and terminal area terrain clearance.
d. TBL 7-2-3, derived from ICAO formulas,
indicates how much error can exist when the
temperature is extremely cold. To use the table, find
the reported temperature in the left column, then read
across the top row to locate the height above the
airport/reporting station (i.e., subtract the airport/
reporting elevation from the intended flight altitude).
The intersection of the column and row is how much
lower the aircraft may actually be as a result of the
possible cold temperature induced error.
e. The possible result of the above example should
be obvious, particularly if operating at the minimum
altitude or when conducting an instrument approach.
When operating in extreme cold temperatures, pilots
may wish to compensate for the reduction in terrain
clearance by adding a cold temperature correction.
AIM 2/14/08
7-2-4 Altimeter Setting Procedures
TBL 7-2-3
ICAO Cold Temperature Error Table
Reported Temp _C
Height Above Airport in Feet
200 300 400 500 600 700 800 900 1000 1500 2000 3000 4000 5000
+10 10 10 10 10 20 20 20 20 20 30 40 60 80 90
0 20 20 30 30 40 40 50 50 60 90 120 170 230 280
-10 20 30 40 50 60 70 80 90 100 150 200 290 390 490
-20 30 50 60 70 90 100 120 130 140 210 280 420 570 710
-30 40 60 80 100 120 140 150 170 190 280 380 570 760 950
-40 50 80 100 120 150 170 190 220 240 360 480 720 970 1210
-50 60 90 120 150 180 210 240 270 300 450 590 890 1190 1500
EXAMPLE-
Temperature-10 degrees Celsius, and the aircraft altitude is 1,000 feet above the airport elevation. The chart shows that
the reported current altimeter setting may place the aircraft as much as 100 feet below the altitude indicated by the altimeter.
7-2-4. High Barometric Pressure
a. Cold, dry air masses may produce barometric
pressures in excess of 31.00 inches of Mercury, and
many altimeters do not have an accurate means of
being adjusted for settings of these levels. When the
altimeter cannot be set to the higher pressure setting,
the aircraft actual altitude will be higher than the
altimeter indicates.
REFERENCE-
AIM, Paragraph 7-2-3, Altimeter Errors.
b. When the barometric pressure exceeds
31.00_inches, air traffic controllers will issue the
actual altimeter setting, and:
1. En Route/Arrivals. Advise pilots to remain
set on 31.00 inches until reaching the final approach
segment.
2. Departures. Advise pilots to set 31.00_inch-
es prior to reaching any mandatory/crossing altitude
or 1,500 feet, whichever is lower.
c. The altimeter error caused by the high pressure
will be in the opposite direction to the error caused by
the cold temperature.
7-2-5. Low Barometric Pressure
When abnormally low barometric pressure condi-
tions occur (below 28.00), flight operations by
aircraft unable to set the actual altimeter setting are
not recommended.
NOTE-
The true altitude of the aircraft is lower than the indicated
altitude if the pilot is unable to set the actual altimeter
setting.
AIM 2/14/08
7-3-1
Wake Turbulence
Section 3. Wake Turbulence
7-3-1. General
a. Every aircraft generates a wake while in flight.
Initially, when pilots encountered this wake in flight,
the disturbance was attributed to “prop wash.” It is
known, however, that this disturbance is caused by a
pair of counter-rotating vortices trailing from the
wing tips. The vortices from larger aircraft pose
problems to encountering aircraft. For instance, the
wake of these aircraft can impose rolling moments
exceeding the roll-control authority of the encounter-
ing aircraft. Further, turbulence generated within the
vortices can damage aircraft components and
equipment if encountered at close range. The pilot
must learn to envision the location of the vortex wake
generated by larger (transport category) aircraft and
adjust the flight path accordingly.
b. During ground operations and during takeoff,
jet engine blast (thrust stream turbulence) can cause
damage and upsets if encountered at close range.
Exhaust velocity versus distance studies at various
thrust levels have shown a need for light aircraft to
maintain an adequate separation behind large turbojet
aircraft. Pilots of larger aircraft should be particularly
careful to consider the effects of their “jet blast” on
other aircraft, vehicles, and maintenance equipment
during ground operations.
7-3-2. Vortex Generation
Lift is generated by the creation of a pressure
differential over the wing surface. The lowest
pressure occurs over the upper wing surface and the
highest pressure under the wing. This pressure
differential triggers the roll up of the airflow aft of the
wing resulting in swirling air masses trailing
downstream of the wing tips. After the roll up is
completed, the wake consists of two counter-rotating
cylindrical vortices. (See FIG 7-3-1.) Most of the
energy is within a few feet of the center of each
vortex, but pilots should avoid a region within about
100 feet of the vortex core.
FIG 7-3-1
Wake Vortex Generation
7-3-3. Vortex Strength
a. The strength of the vortex is governed by the
weight, speed, and shape of the wing of the generating
aircraft. The vortex characteristics of any given
aircraft can also be changed by extension of flaps or
other wing configuring devices as well as by change
in speed. However, as the basic factor is weight, the
vortex strength increases proportionately. Peak
vortex tangential speeds exceeding 300 feet per
second have been recorded. The greatest vortex
strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW.
b. Induced Roll
1. In rare instances a wake encounter could
cause inflight structural damage of catastrophic
proportions. However, the usual hazard is associated
with induced rolling moments which can exceed the
roll-control authority of the encountering aircraft. In
flight experiments, aircraft have been intentionally
flown directly up trailing vortex cores of larger
aircraft. It was shown that the capability of an aircraft
to counteract the roll imposed by the wake vortex
primarily depends on the wingspan and counter-
control responsiveness of the encountering aircraft.
AIM 2/14/08
7-3-2 Wake Turbulence
2. Counter control is usually effective and
induced roll minimal in cases where the wingspan
and ailerons of the encountering aircraft extend
beyond the rotational flow field of the vortex. It is
more difficult for aircraft with short wingspan
(relative to the generating aircraft) to counter the
imposed roll induced by vortex flow. Pilots of short
span aircraft, even of the high performance type, must
be especially alert to vortex encounters.
(See FIG 7-3-2.)
FIG 7-3-2
Wake Encounter Counter Control
COUNTER
CONTROL
3. The wake of larger aircraft requires the
respect of all pilots.
7-3-4. Vortex Behavior
a. Trailing vortices have certain behavioral
characteristics which can help a pilot visualize the
wake location and thereby take avoidance precau-
tions.
1. Vortices are generated from the moment
aircraft leave the ground, since trailing vortices are a
by-product of wing lift. Prior to takeoff or touchdown
pilots should note the rotation or touchdown point of
the preceding aircraft. (See FIG 7-3-4.)
2. The vortex circulation is outward, upward
and around the wing tips when viewed from either
ahead or behind the aircraft. Tests with large aircraft
have shown that the vortices remain spaced a bit less
than a wingspan apart, drifting with the wind, at
altitudes greater than a wingspan from the ground. In
view of this, if persistent vortex turbulence is
encountered, a slight change of altitude and lateral
position (preferably upwind) will provide a flight
path clear of the turbulence.
3. Flight tests have shown that the vortices from
larger (transport category) aircraft sink at a rate of
several hundred feet per minute, slowing their
descent and diminishing in strength with time and
distance behind the generating aircraft. Atmospheric
turbulence hastens breakup. Pilots should fly at or
above the preceding aircraft's flight path, altering
course as necessary to avoid the area behind and
below the generating aircraft. (See FIG 7-3-3.)
However, vertical separation of 1,000 feet may be
considered safe.
4. When the vortices of larger aircraft sink close
to the ground (within 100 to 200 feet), they tend to
move laterally over the ground at a speed of 2 or
3_knots. (See FIG 7-3-5.)
FIG 7-3-3
Wake Ends/Wake Begins
Touchdown Rotation
Wake Ends Wake Begins
AIM 2/14/08
7-3-3
Wake Turbulence
FIG 7-3-4
Vortex Flow Field
AVOID
Nominally 500-1000 Ft.
Sink Rate
Min.
Several Hundred Ft.,/FIG 7-3-5
Vortex Movement Near Ground - No Wind
No Wind
3K 3K
FIG 7-3-6
Vortex Movement Near Ground - with Cross Winds
6K
(3K + 3K)
3K Wind
0 (3K - 3K)
AIM 2/14/08
7-3-4 Wake Turbulence
5. There is a small segment of the aviation
community that have become convinced that wake
vortices may “bounce” up to twice their nominal
steady state height. With a 200-foot span aircraft, the
“bounce” height could reach approximately 200 feet
AGL. This conviction is based on a single
unsubstantiated report of an apparent coherent
vortical flow that was seen in the volume scan of a
research sensor. No one can say what conditions
cause vortex bouncing, how high they bounce, at
what angle they bounce, or how many times a vortex
may bounce. On the other hand, no one can say for
certain that vortices never “bounce.” Test data have
shown that vortices can rise with the air mass in which
they are embedded. Wind shear, particularly, can
cause vortex flow field “tilting.” Also, ambient
thermal lifting and orographic effects (rising terrain
or tree lines) can cause a vortex flow field to rise.
Notwithstanding the foregoing, pilots are reminded
that they should be alert at all times for possible wake
vortex encounters when conducting approach and
landing operations. The pilot has the ultimate
responsibility for ensuring appropriate separations
and positioning of the aircraft in the terminal area to
avoid the wake turbulence created by a preceding
aircraft.
b. A crosswind will decrease the lateral movement
of the upwind vortex and increase the movement of
the downwind vortex. Thus a light wind with a cross
runway component of 1 to 5 knots could result in the
upwind vortex remaining in the touchdown zone for
a period of time and hasten the drift of the downwind
vortex toward another runway. (See FIG 7-3-6.)
Similarly, a tailwind condition can move the vortices
of the preceding aircraft forward into the touchdown
zone. THE LIGHT QUARTERING TAILWIND
REQUIRES MAXIMUM CAUTION. Pilots should
be alert to large aircraft upwind from their approach
and takeoff flight paths. (See FIG 7-3-7.)
FIG 7-3-7
Vortex Movement in Ground Effect - Tailwind
Light Quartering
Tailwind
x
Tail Wind
Touchdown Point
AIM 2/14/08
7-3-5
Wake Turbulence
7-3-5. Operations Problem Areas
a. A wake encounter can be catastrophic. In 1972
at Fort Worth a DC-9 got too close to a DC-10
(two_miles back), rolled, caught a wingtip, and
cartwheeled coming to rest in an inverted position on
the runway. All aboard were killed. Serious and even
fatal GA accidents induced by wake vortices are not
uncommon. However, a wake encounter is not
necessarily hazardous. It can be one or more jolts with
varying severity depending upon the direction of the
encounter, weight of the generating aircraft, size of
the encountering aircraft, distance from the generat-
ing aircraft, and point of vortex encounter. The
probability of induced roll increases when the
encountering aircraft's heading is generally aligned
with the flight path of the generating aircraft.
b. AVOID THE AREA BELOW AND BEHIND
THE GENERATING AIRCRAFT, ESPECIALLY
AT LOW ALTITUDE WHERE EVEN A
MOMENTARY WAKE ENCOUNTER COULD BE
HAZARDOUS. This is not easy to do. Some
accidents have occurred even though the pilot of the
trailing aircraft had carefully noted that the aircraft in
front was at a considerably lower altitude. Unfortu-
nately, this does not ensure that the flight path of the
lead aircraft will be below that of the trailing aircraft.
c. Pilots should be particularly alert in calm wind
conditions and situations where the vortices could:
1. Remain in the touchdown area.
2. Drift from aircraft operating on a nearby
runway.
3. Sink into the takeoff or landing path from a
crossing runway.
4. Sink into the traffic pattern from other airport
operations.
5. Sink into the flight path of VFR aircraft
operating on the hemispheric altitude 500 feet below.
d. Pilots of all aircraft should visualize the
location of the vortex trail behind larger aircraft and
use proper vortex avoidance procedures to achieve
safe operation. It is equally important that pilots of
larger aircraft plan or adjust their flight paths to
minimize vortex exposure to other aircraft.
7-3-6. Vortex Avoidance Procedures
a. Under certain conditions, airport traffic control-
lers apply procedures for separating IFR aircraft. If a
pilot accepts a clearance to visually follow a
preceding aircraft, the pilot accepts responsibility for
separation and wake turbulence avoidance. The
controllers will also provide to VFR aircraft, with
whom they are in communication and which in the
tower's opinion may be adversely affected by wake
turbulence from a larger aircraft, the position, altitude
and direction of flight of larger aircraft followed by
the phrase “CAUTION - WAKE TURBULENCE.”
After issuing the caution for wake turbulence, the
airport traffic controllers generally do not provide
additional information to the following aircraft
unless the airport traffic controllers know the
following aircraft is overtaking the preceding
aircraft. WHETHER OR NOT A WARNING OR
INFORMATION HAS BEEN GIVEN, HOWEVER,
THE PILOT IS EXPECTED TO ADJUST AIR-
CRAFT OPERATIONS AND FLIGHT PATH AS
NECESSARY TO PRECLUDE SERIOUS WAKE
ENCOUNTERS. When any doubt exists about
maintaining safe separation distances between
aircraft during approaches, pilots should ask the
control tower for updates on separation distance and
aircraft groundspeed.
b. The following vortex avoidance procedures are
recommended for the various situations:
1. Landing behind a larger aircraft- same
runway. Stay at or above the larger aircraft's final
approach flight path-note its touchdown point-land
beyond it.
2. Landing behind a larger aircraft- when
parallel runway is closer than 2,500 feet. Consider
possible drift to your runway. Stay at or above the
larger aircraft's final approach flight path- note its
touchdown point.
3. Landing behind a larger aircraft- crossing
runway. Cross above the larger aircraft's flight path.
4. Landing behind a departing larger air-
craft- same runway. Note the larger aircraft's
rotation point- land well prior to rotation point.
5. Landing behind a departing larger air-
craft- crossing runway. Note the larger aircraft's
rotation point- if past the intersection- continue the
approach- land prior to the intersection. If larger
aircraft rotates prior to the intersection, avoid flight
AIM 2/14/08
7-3-6 Wake Turbulence
below the larger aircraft's flight path. Abandon the
approach unless a landing is ensured well before
reaching the intersection.
6. Departing behind a larger aircraft. Note
the larger aircraft's rotation point and rotate prior to
the larger aircraft's rotation point. Continue climbing
above the larger aircraft's climb path until turning
clear of the larger aircraft's wake. Avoid subsequent
headings which will cross below and behind a larger
aircraft. Be alert for any critical takeoff situation
which could lead to a vortex encounter.
7. Intersection takeoffs- same runway. Be
alert to adjacent larger aircraft operations, particular-
ly upwind of your runway. If intersection takeoff
clearance is received, avoid subsequent heading
which will cross below a larger aircraft's path.
8. Departing or landing after a larger
aircraft executing a low approach, missed
approach, or touch-and-go landing. Because
vortices settle and move laterally near the ground, the
vortex hazard may exist along the runway and in your
flight path after a larger aircraft has executed a low
approach, missed approach, or a touch-and-go
landing, particular in light quartering wind condi-
tions. You should ensure that an interval of at least
2_minutes has elapsed before your takeoff or landing.
9. En route VFR (thousand-foot altitude plus
500 feet). Avoid flight below and behind a large
aircraft's path. If a larger aircraft is observed above on
the same track (meeting or overtaking) adjust your
position laterally, preferably upwind.
