5. AWOS information (system level, frequency,
phone number, etc.) concerning specific locations is
published, as the systems become operational, in the
A/FD, and where applicable, on published Instru-
ment Approach Procedures. Selected individual
systems may be incorporated into nationwide data
collection and dissemination networks in the future.
c. AWOS Broadcasts. Computer-generated
voice is used in AWOS to automate the broadcast of
the minute-by-minute weather observations. In
addition, some systems are configured to permit the
addition of an operator-generated voice message;
e.g., weather remarks following the automated
parameters. The phraseology used generally follows
that used for other weather broadcasts. Following are
explanations and examples of the exceptions.
AIM 2/14/08
7-1-25
Meteorology
1. Location and Time. The location/name and
the phrase “AUTOMATED WEATHER OBSERVA-
TION,” followed by the time are announced.
(a) If the airport's specific location is
included in the airport's name, the airport's name is
announced.
EXAMPLE“Bremerton National Airport automated weather observa-
tion, one four five six zulu;”
“Ravenswood Jackson County Airport automated weather
observation, one four five six zulu.”
(b) If the airport's specific location is not
included in the airport's name, the location is
announced followed by the airport's name.
EXAMPLE“Sault Ste. Marie, Chippewa County International Airport
automated weather observation;”
“Sandusky, Cowley Field automated weather
observation.”
(c) The word “TEST” is added following
“OBSERVATION” when the system is not in
commissioned status.
EXAMPLE“Bremerton National Airport automated weather observa-
tion test, one four five six zulu.”
(d) The phrase “TEMPORARILY INOP-
ERATIVE” is added when the system is inoperative.
EXAMPLE“Bremerton National Airport automated weather observ-
ing system temporarily inoperative.”
2. Visibility.
(a) The lowest reportable visibility value in
AWOS is “less than 1
/4.” It is announced as
“VISIBILITY LESS THAN ONE QUARTER.”
(b) A sensor for determining visibility is not
included in some AWOS. In these systems, visibility
is not announced. “VISIBILITY MISSING” is
announced only if the system is configured with a
visibility sensor and visibility information is not
available.
3. Weather. In the future, some AWOSs are to
be configured to determine the occurrence of
precipitation. However, the type and intensity may
not always be determined. In these systems, the word
“PRECIPITATION” will be announced if precipita-
tion is occurring, but the type and intensity are not
determined.
4. Ceiling and Sky Cover.
(a) Ceiling is announced as either “CEIL-
ING” or “INDEFINITE CEILING.” With the
exception of indefinite ceilings, all automated ceiling
heights are measured.
EXAMPLE“Bremerton National Airport automated weather observa-
tion, one four five six zulu. Ceiling two thousand overcast;”
“Bremerton National Airport automated weather observa-
tion, one four five six zulu. Indefinite ceiling two_hundred,
sky obscured.”
(b) The word “Clear” is not used in AWOS
due to limitations in the height ranges of the sensors.
No clouds detected is announced as “NO CLOUDS
BELOW XXX” or, in newer systems as “CLEAR
BELOW XXX” (where XXX is the range limit of the
sensor).
EXAMPLE“No clouds below one two thousand.”
“Clear below one two thousand.”
(c) A sensor for determining ceiling and sky
cover is not included in some AWOS. In these
systems, ceiling and sky cover are not announced.
“SKY CONDITION MISSING” is announced only if
the system is configured with a ceilometer and the
ceiling and sky cover information is not available.
5. Remarks. If remarks are included in the
observation, the word “REMARKS” is announced
following the altimeter setting.
(a) Automated “Remarks.”
(1) Density Altitude.
(2) Variable Visibility.
(3) Variable Wind Direction.
(b) Manual Input Remarks. Manual input
remarks are prefaced with the phrase “OBSERVER
WEATHER.” As a general rule the manual remarks
are limited to:
(1) Type and intensity of precipitation.
(2) Thunderstorms and direction; and
AIM 2/14/08
7-1-26 Meteorology
(3) Obstructions to vision when the visibili-
ty is 3 miles or less.
EXAMPLE“Remarks ... density altitude, two thousand five hundred ...
visibility variable between one and two ... wind direction
variable between two four zero and three one zero
...observed weather ... thunderstorm moderate rain
showers and fog ... thunderstorm overhead.”
(c) If an automated parameter is “missing”
and no manual input for that parameter is available,
the parameter is announced as “MISSING.” For
example, a report with the dew point “missing” and
no manual input available, would be announced as
follows:
EXAMPLE“Ceiling one thousand overcast ... visibility three ...
precipitation ... temperature three zero, dew point missing
... wind calm ... altimeter three zero zero one.”
(d) “REMARKS” are announced in the
following order of priority:
(1) Automated “REMARKS.”
Density Altitude.
Variable Visibility.
Variable Wind Direction.
(2) Manual Input “REMARKS.”
Sky Condition.
Visibility.
Weather and Obstructions to Vision.
Temperature.
Dew Point.
Wind; and
Altimeter Setting.
EXAMPLE“Remarks ... density altitude, two thousand five hundred ...
visibility variable between one and two ... wind direction
variable between two four zero and three one zero ...
observer ceiling estimated two thousand broken ...
observer temperature two, dew point minus five.”
d. Automated Surface Observing System
(ASOS)/Automated Weather Sensor System
(AWSS). The ASOS/AWSS is the primary surface
weather observing system of the U.S. (See Key to
Decode an ASOS/AWSS (METAR) Observation,
FIG 7-1-8 and FIG 7-1-9.) The program to install
and operate these systems throughout the U.S. is a
joint effort of the NWS, the FAA and the Department
of Defense. AWSS is a follow-on program that
provides identical data as ASOS. ASOS/AWSS is
designed to support aviation operations and weather
forecast activities. The ASOS/AWSS will provide
continuous minute-by-minute observations and
perform the basic observing functions necessary to
generate an aviation routine weather report (ME-
TAR) and other aviation weather information. The
information may be transmitted over a discrete VHF
radio frequency or the voice portion of a local
NAVAID. ASOS/AWSS transmissions on a discrete
VHF radio frequency are engineered to be receivable
to a maximum of 25 NM from the ASOS/AWSS site
and a maximum altitude of 10,000 feet AGL. At many
locations, ASOS/AWSS signals may be received on
the surface of the airport, but local conditions may
limit the maximum reception distance and/or altitude.
While the automated system and the human may
differ in their methods of data collection and
interpretation, both produce an observation quite
similar in form and content. For the “objective”
elements such as pressure, ambient temperature, dew
point temperature, wind, and precipitation accumula-
tion, both the automated system and the observer use
a fixed location and time-averaging technique. The
quantitative differences between the observer and the
automated observation of these elements are
negligible. For the “subjective” elements, however,
observers use a fixed time, spatial averaging
technique to describe the visual elements (sky
condition, visibility and present weather), while the
automated systems use a fixed location, time
averaging technique. Although this is a fundamental
change, the manual and automated techniques yield
remarkably similar results within the limits of their
respective capabilities.
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7-1-27
Meteorology
1. System Description.
(a) The ASOS/AWSS at each airport location
consists of four main components:
(1) Individual weather sensors.
(2) Data collection and processing units.
(3) Peripherals and displays.
(b) The ASOS/AWSS sensors perform the
basic function of data acquisition. They continuously
sample and measure the ambient environment, derive
raw sensor data and make them available to the
collection and processing units.
2. Every ASOS/AWSS will contain the
following basic set of sensors:
(a) Cloud height indicator (one or possibly
three).
(b) Visibility sensor (one or possibly three).
(c) Precipitation identification sensor.
(d) Freezing rain sensor (at select sites).
(e) Pressure sensors (two sensors at small
airports; three sensors at large airports).
(f) Ambient temperature/Dew point tempera-
ture sensor.
(g) Anemometer (wind direction and speed
sensor).
(h) Rainfall accumulation sensor.
3. The ASOS/AWSS data outlets include:
(a) Those necessary for on-site airport users.
(b) National communications networks.
(c) Computer-generated voice (available
through FAA radio broadcast to pilots, and dial-in
telephone line).
NOTE-
Wind direction broadcast over FAA radios is in reference
to magnetic north.
4. An ASOS/AWOS/AWSS report without
human intervention will contain only that weather
data capable of being reported automatically. The
modifier for this METAR report is “AUTO.” When
an observer augments or backs-up an ASOS/AWOS/
AWSS site, the “AUTO” modifier disappears.
5. There are two types of automated stations,
AO1 for automated weather reporting stations
without a precipitation discriminator, and AO2 for
automated stations with a precipitation discriminator.
As appropriate, “AO1” and “AO2” shall appear in
remarks. (A precipitation discriminator can deter-
mine the difference between liquid and
frozen/freezing precipitation).
NOTE-
To decode an ASOS/AWSS report, refer to FIG 7-1-8 and
FIG 7-1-9.
REFERENCE-
A complete explanation of METAR terminology is located in AIM,
Paragraph 7-1-30, Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR).
AIM 2/14/08
7-1-28 Meteorology
FIG 7-1-8
Key to Decode an ASOS/AWSS (METAR) Observation (Front)
AIM 2/14/08
7-1-29
Meteorology
FIG 7-1-9
Key to Decode an ASOS/AWSS (METAR) Observation (Back)
AIM 2/14/08
7-1-30 Meteorology
e. TBL 7-1-1 contains a comparison of weather
observing programs and the elements reported.
f. Service Standards. During 1995, a govern-
ment/industry team worked to comprehensively
reassess the requirements for surface observations at
the nation's airports. That work resulted in agreement
on a set of service standards, and the FAA and NWS
ASOS sites to which the standards would apply. The
term “Service Standards” refers to the level of detail
in weather observation. The service standards consist
of four different levels of service (A, B, C, and D) as
described below. Specific observational elements
included in each service level are listed in
TBL 7-1-2.
1. Service Level D defines the minimum
acceptable level of service. It is a completely
automated service in which the ASOS observation
will constitute the entire observation, i.e., no
additional weather information is added by a human
observer. This service is referred to as a stand alone_D
site.
2. Service Level C is a service in which the
human observer, usually an air traffic controller,
augments or adds information to the automated
observation. Service Level C also includes backup of
ASOS elements in the event of an ASOS malfunction
or an unrepresentative ASOS report. In backup, the
human observer inserts the correct or missing value
for the automated ASOS elements. This service is
provided by air traffic controllers under the Limited
Aviation Weather Reporting Station (LAWRS)
process, FSS and NWS observers, and, at selected
sites, Non-Federal Observation Program observers.
Two categories of airports require detail beyond
Service Level C in order to enhance air traffic control
efficiency and increase system capacity. Services at
these airports are typically provided by contract
weather observers, NWS observers, and, at some
locations, FSS observers.
3. Service Level B is a service in which weather
observations consist of all elements provided under
Service Level C, plus augmentation of additional data
beyond the capability of the ASOS. This category of
airports includes smaller hubs or special airports in
other ways that have worse than average bad weather
operations for thunderstorms and/or freezing/frozen
precipitation, and/or that are remote airports.
4. Service Level A, the highest and most
demanding category, includes all the data reported in
Service Standard B, plus additional requirements as
specified. Service Level A covers major aviation
hubs and/or high volume traffic airports with average
or worse weather.
TBL 7-1-1
Weather Observing Programs
Element Reported AWOS-A AWOS-1 AWOS-2 AWOS-3 ASOS Manual
Altimeter X X X X X X
Wind X X X X X
Temperature/
Dew Point
X X X X X
Density Altitude X X X X
Visibility X X X X
Clouds/Ceiling X X X
Precipitation X X
Remarks X X
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7-1-31
Meteorology
TBL 7-1-2
SERVICE LEVEL A
Service Level A consists of all the elements of
Service Levels B, C and D plus the elements
listed to the right, if observed.
10 minute longline RVR at precedented sites or
additional visibility increments of 1/8, 1/16 and 0
Sector visibility
Variable sky condition
Cloud layers above 12,000 feet and cloud types
Widespread dust, sand and other obscurations
Volcanic eruptions
SERVICE LEVEL B
Service Level B consists of all the elements of
Service Levels C and D plus the elements listed to
the right, if observed.
Longline RVR at precedented sites
_(may be instantaneous readout)
Freezing drizzle versus freezing rain
Ice pellets
Snow depth & snow increasing rapidly remarks
Thunderstorm and lightning location remarks
Observed significant weather not at the station
remarks
SERVICE LEVEL C
Service Level C consists of all the elements of Service
Level D plus augmentation and backup by a human
observer or an air traffic control specialist on location
nearby. Backup consists of inserting the correct value if
the system malfunctions or is unrepresentative.
Augmentation consists of adding the elements listed to
the right, if observed. During hours that the observing
facility is closed, the site reverts to Service Level D.
Thunderstorms
Tornadoes
Hail
Virga
Volcanic ash
Tower visibility
Operationally significant remarks as deemed
appropriate by the observer
SERVICE LEVEL D
This level of service consists of an ASOS continually
measuring the atmosphere at a point near the runway. The
ASOS senses and measures the weather parameters listed to
the right.
Wind
Visibility
Precipitation/Obstruction to vision
Cloud height
Sky cover
Temperature
Dew point
Altimeter
7-1-13. Weather Radar Services
a. The National Weather Service operates a
network of radar sites for detecting coverage,
intensity, and movement of precipitation. The
network is supplemented by FAA and DOD radar
sites in the western sections of the country. Local
warning radar sites augment the network by operating
on an as needed basis to support warning and forecast
programs.
b. Scheduled radar observations are taken hourly
and transmitted in alpha-numeric format on weather
telecommunications circuits for flight planning
purposes. Under certain conditions, special radar
reports are issued in addition to the hourly
transmittals. Data contained in the reports are also
collected by the National Center for Environmental
Prediction and used to prepare national radar
summary charts for dissemination on facsimile
circuits.
c. A clear radar display (no echoes) does not mean
that there is no significant weather within the
coverage of the radar site. Clouds and fog are not
detected by the radar. However, when echoes are
present, turbulence can be implied by the intensity of
the precipitation, and icing is implied by the presence
of the precipitation at temperatures at or below zero
degrees Celsius. Used in conjunction with other
weather products, radar provides invaluable informa-
tion for weather avoidance and flight planning.
AIM 2/14/08
7-1-32 Meteorology
FIG 7-1-10
NEXRAD Coverage
AIM 2/14/08
7-1-33
Meteorology
FIG 7-1-11
NEXRAD Coverage
AIM 2/14/08
7-1-34 Meteorology
FIG 7-1-12
NEXRAD Coverage
AIM 2/14/08
7-1-35
Meteorology
d. All En Route Flight Advisory Service facilities
and AFSSs have equipment to directly access the
radar displays from the individual weather radar sites.
Specialists at these locations are trained to interpret
the display for pilot briefing and inflight advisory
services. The Center Weather Service Units located in
ARTCCs also have access to weather radar displays
and provide support to all air traffic facilities within
their center's area.
e. Additional information on weather radar
products and services can be found in AC 00-45,
Aviation Weather Services.
REFERENCE-
Pilot/Controller Glossary Term- Precipitation Radar Weather
Descriptions.
AIM, Thunderstorms, Paragraph 7-1-28.
A/FD, Charts, NWS Upper Air Observing Stations and Weather Network
for the location of specific radar sites.
7-1-14. ATC Inflight Weather Avoidance
Assistance
a. ATC Radar Weather Display.
1. ATC radars are able to display areas of
precipitation by sending out a beam of radio energy
that is reflected back to the radar antenna when it
strikes an object or moisture which may be in the form
of rain drops, hail, or snow. The larger the object is,
or the more dense its reflective surface, the stronger
the return will be presented. Radar weather
processors indicate the intensity of reflective returns
in terms of decibels (dBZ). ATC systems cannot
detect the presence or absence of clouds. The ATC
systems can often determine the intensity of a
precipitation area, but the specific character of that
area (snow, rain, hail, VIRGA, etc.) cannot be
determined. For this reason, ATC refers to all
weather_areas displayed on ATC radar scopes as
“precipitation.”
2. All ATC facilities using radar weather
processors with the ability to determine precipitation
intensity, will describe the intensity to pilots as:
(a) “LIGHT” (< 30 dBZ)
(b) “MODERATE” (30 to 40 dBZ)
(c) “HEAVY” (> 40 to 50 dBZ)
(d) “EXTREME” (> 50 dBZ)
3. ATC facilities that, due to equipment
limitations, cannot display the intensity levels of
precipitation, will describe the location of the
precipitation area by geographic position, or position
relative to the aircraft. Since the intensity level is not
available, the controller will state “INTENSITY
UNKNOWN.”
4. ARTCC facilities normally use a Weather and
Radar Processor (WARP) to display a mosaic of data
obtained from multiple NEXRAD sites. There is a
time delay between actual conditions and those
displayed to the controller. For example, the
precipitation data on the ARTCC controller's display
could be up to 6 minutes old. When the WARP is not
available, a second system, the narrowband Air Route
Surveillance Radar (ARSR) can display two distinct
levels of precipitation intensity that will be described
to pilots as “MODERATE” (30 to 40 dBZ) and
“HEAVY TO EXTREME” ( > 40 dBZ ). The WARP
processor is only used in ARTCC facilities.
5. ATC radar is not able to detect turbulence.
Generally, turbulence can be expected to occur as the
rate of rainfall or intensity of precipitation increases.
Turbulence associated with greater rates of rainfall/
precipitation will normally be more severe than any
associated with lesser rates of rainfall/precipitation.
Turbulence should be expected to occur near
convective activity, even in clear air. Thunderstorms
are a form of convective activity that imply severe or
greater turbulence. Operation within 20 miles of
thunderstorms should be approached with great
caution, as the severity of turbulence can be markedly
greater than the precipitation intensity might indicate.
b. Weather Avoidance Assistance.
1. To the extent possible, controllers will issue
pertinent information on weather or chaff areas and
assist pilots in avoiding such areas when requested.
Pilots should respond to a weather advisory by either
acknowledging the advisory or by acknowledging the
advisory and requesting an alternative course of
action as follows:
(a) Request to deviate off course by stating
the number of miles and the direction of the requested
deviation. In this case, when the requested deviation
is approved, navigation is at the pilot's prerogative,
but must maintain the altitude assigned by ATC and
to remain within the specified mileage of the original
course.
(b) Request a new route to avoid the affected
area.
(c) Request a change of altitude.
AIM 2/14/08
7-1-36 Meteorology
(d) Request radar vectors around the affected
areas.
2. For obvious reasons of safety, an IFR pilot
must not deviate from the course or altitude or flight
level without a proper ATC clearance. When weather
conditions encountered are so severe that an
immediate deviation is determined to be necessary
and time will not permit approval by ATC, the pilot's
emergency authority may be exercised.
3. When the pilot requests clearance for a route
deviation or for an ATC radar vector, the controller
must evaluate the air traffic picture in the affected
area, and coordinate with other controllers (if ATC
jurisdictional boundaries may be crossed) before
replying to the request.
4. It should be remembered that the controller's
primary function is to provide safe separation
between aircraft. Any additional service, such as
weather avoidance assistance, can only be provided
to the extent that it does not derogate the primary
function. It's also worth noting that the separation
workload is generally greater than normal when
weather disrupts the usual flow of traffic. ATC radar
limitations and frequency congestion may also be a
factor in limiting the controller's capability to
provide additional service.
5. It is very important, therefore, that the request
for deviation or radar vector be forwarded to ATC as
far in advance as possible. Delay in submitting it may
delay or even preclude ATC approval or require that
additional restrictions be placed on the clearance.
Insofar as possible the following information should
be furnished to ATC when requesting clearance to
detour around weather activity:
(a) Proposed point where detour will
commence.
(b) Proposed route and extent of detour
(direction and distance).
(c) Point where original route will be
resumed.
(d) Flight conditions (IFR or VFR).
(e) Any further deviation that may become
necessary as the flight progresses.
(f) Advise if the aircraft is equipped with
functioning airborne radar.
6. To a large degree, the assistance that might be
rendered by ATC will depend upon the weather
information available to controllers. Due to the
extremely transitory nature of severe weather
situations, the controller's weather information may
be of only limited value if based on weather observed
on radar only. Frequent updates by pilots giving
specific information as to the area affected, altitudes,
intensity and nature of the severe weather can be of
considerable value. Such reports are relayed by radio
or phone to other pilots and controllers and also
receive widespread teletypewriter dissemination.
7. Obtaining IFR clearance or an ATC radar
vector to circumnavigate severe weather can often be
accommodated more readily in the en route areas
away from terminals because there is usually less
congestion and, therefore, offer greater freedom of
action. In terminal areas, the problem is more acute
because of traffic density, ATC coordination
requirements, complex departure and arrival routes,
adjacent airports, etc. As a consequence, controllers
are less likely to be able to accommodate all requests
for weather detours in a terminal area or be in a
position to volunteer such routing to the pilot.
Nevertheless, pilots should not hesitate to advise
controllers of any observed severe weather and
should specifically advise controllers if they desire
circumnavigation of observed weather.
c. Procedures for Weather Deviations and
Other Contingencies in Oceanic Controlled
Airspace.
1. When the pilot initiates communications with
ATC, rapid response may be obtained by stating
“WEATHER DEVIATION REQUIRED” to indicate
priority is desired on the frequency and for ATC
response.
2. The pilot still retains the option of initiating
the communications using the urgency call “PAN-
PAN” 3 times to alert all listening parties of a special
handling condition which will receive ATC priority
for issuance of a clearance or assistance.
3. ATC will:
(a) Approve the deviation.
(b) Provide vertical separation and then
approve the deviation; or
(c) If ATC is unable to establish vertical
separation, ATC shall advise the pilot that standard
separation cannot be applied; provide essential traffic
AIM 2/14/08
7-1-37
Meteorology
information for all affected aircraft, to the extent
practicable; and if possible, suggest a course of
action. ATC may suggest that the pilot climb or
descend to a contingency altitude (1,000 feet above or
below that assigned if operating above FL 290;
500_feet above or below that assigned if operating at
or below FL 290).
PHRASEOLOGY-
STANDARD SEPARATION NOT AVAILABLE, DEVIATE
AT PILOT'S DISCRETION; SUGGEST CLIMB (or
descent) TO (appropriate altitude); TRAFFIC (position
and altitude); REPORT DEVIATION COMPLETE.
4. The pilot will follow the ATC advisory
altitude when approximately 10 NM from track as
well as execute the procedures detailed in para-
graph_7-1-14c5.
5. If contact cannot be established or revised
ATC clearance or advisory is not available and
deviation from track is required, the pilot shall take
the following actions:
(a) If possible, deviate away from an
organized track or route system.
(b) Broadcast aircraft position and intentions
on the frequency in use, as well as on frequency
121.5_MHz at suitable intervals stating: flight
identification (operator call sign), flight level, track
code or ATS route designator, and extent of deviation
expected.
(c) Watch for conflicting traffic both visually
and by reference to TCAS (if equipped).
(d) Turn on aircraft exterior lights.
(e) Deviations of less than 10 NM or
operations within COMPOSITE (NOPAC and
CEPAC) Airspace, should REMAIN at ASSIGNED
altitude. Otherwise, when the aircraft is approximate-
ly 10 NM from track, initiate an altitude change based
on the following criteria:
TBL 7-1-3
Route
Centerline/Track
Deviations
>10 NM
Altitude Change
East
000 - 179_M
Left
Right
Descend 300 Feet
Climb 300 Feet
West
180-359_M
Left
Right
Climb 300 Feet
Descend 300 Feet
Pilot Memory Slogan: “East right up,
West right down.”
(f) When returning to track, be at assigned
flight level when the aircraft is within approximately
10 NM of centerline.
(g) If contact was not established prior to
deviating, continue to attempt to contact ATC to
obtain a clearance. If contact was established,
continue to keep ATC advised of intentions and
obtain essential traffic information.
7-1-15. Runway Visual Range (RVR)
There are currently two configurations of RVR in the
NAS commonly identified as Taskers and New
Generation RVR. The Taskers are the existing
configuration which uses transmissometer technolo-
gy. The New Generation RVRs were deployed in
November 1994 and use forward scatter technology.
The New Generation RVRs are currently being
deployed in the NAS to replace the existing Taskers.
a. RVR values are measured by transmissometers
mounted on 14-foot towers along the runway. A full
RVR system consists of:
1. Transmissometer projector and related items.
2. Transmissometer receiver (detector) and
related items.
3. Analogue recorder.
4. Signal data converter and related items.
5. Remote digital or remote display program-
mer.
b. The transmissometer projector and receiver are
mounted on towers 250 feet apart. A known intensity
of light is emitted from the projector and is measured
by the receiver. Any obscuring matter such as rain,
snow, dust, fog, haze or smoke reduces the light
intensity arriving at the receiver. The resultant
intensity measurement is then converted to an RVR
value by the signal data converter. These values are
displayed by readout equipment in the associated air
traffic facility and updated approximately once every
minute for controller issuance to pilots.
c. The signal data converter receives information
on the high intensity runway edge light setting in use
(step_3, 4, or 5); transmission values from the
transmissometer and the sensing of day or night
conditions. From the three data sources, the system
will compute appropriate RVR values.
d. An RVR transmissometer established on a
250_foot baseline provides digital readouts to a
AIM 2/14/08
7-1-38 Meteorology
minimum of 600 feet, which are displayed in 200 foot
increments to 3,000 feet and in 500 foot increments
from 3,000 feet to a maximum value of 6,000 feet.
e. RVR values for Category IIIa operations extend
down to 700 feet RVR; however, only 600 and
800_feet are reportable RVR increments. The
800_RVR reportable value covers a range of 701 feet
to 900 feet and is therefore a valid minimum
indication of Category IIIa operations.
f. Approach categories with the corresponding
minimum RVR values. (See TBL 7-1-4.)
TBL 7-1-4
Approach Category/Minimum RVR Table
Category Visibility (RVR)
Nonprecision 2,400 feet
Category I 1,800 feet
Category II 1,200 feet
Category IIIa 700 feet
Category IIIb 150 feet
Category IIIc 0 feet
g. Ten minute maximum and minimum RVR
values for the designated RVR runway are reported in
the body of the aviation weather report when the
prevailing visibility is less than one mile and/or the
RVR is 6,000 feet or less. ATCTs report RVR when
the prevailing visibility is 1 mile or less and/or the
RVR is 6,000 feet or less.
h. Details on the requirements for the operational
use of RVR are contained in FAA AC 97-1, “Runway
Visual Range (RVR).” Pilots are responsible for
compliance with minimums prescribed for their class
of operations in the appropriate CFRs and/or
operations specifications.
i. RVR values are also measured by forward
scatter meters mounted on 14-foot frangible
fiberglass poles. A full RVR system consists of:
1. Forward scatter meter with a transmitter,
receiver and associated items.
2. A runway light intensity monitor (RLIM).
3. An ambient light sensor (ALS).
4. A data processor unit (DPU).
5. Controller display (CD).
j. The forward scatter meter is mounted on a
14-foot frangible pole. Infrared light is emitted from
the transmitter and received by the receiver. Any
obscuring matter such as rain, snow, dust, fog, haze
or smoke increases the amount of scattered light
reaching the receiver. The resulting measurement
along with inputs from the runway light intensity
monitor and the ambient light sensor are forwarded to
the DPU which calculates the proper RVR value. The
RVR values are displayed locally and remotely on
controller displays.
k. The runway light intensity monitors both the
runway edge and centerline light step settings (steps_1
through 5). Centerline light step settings are used for
CAT IIIb operations. Edge Light step settings are
used for CAT I, II, and IIIa operations.
l. New Generation RVRs can measure and display
RVR values down to the lowest limits of
Category_IIIb operations (150 feet RVR). RVR
values are displayed in 100 feet increments and are
reported as follows:
1. 100-feet increments for products below
800_feet.
2. 200-feet increments for products between
800 feet and 3,000 feet.
3. 500-feet increments for products between
3,000 feet and 6,500 feet.
4. 25-meter increments for products below
150_meters.
5. 50-meter increments for products between
150 meters and 800 meters.
6. 100-meter increments for products between
800 meters and 1,200 meters.
7. 200-meter increments for products between
1,200 meters and 2,000 meters.
7-1-16. Reporting of Cloud Heights
a. Ceiling, by definition in the CFRs and as used
in aviation weather reports and forecasts, is the height
above ground (or water) level of the lowest layer of
clouds or obscuring phenomenon that is reported as
“broken,” “overcast,” or “obscuration,” e.g., an
aerodrome forecast (TAF) which reads “BKN030”
refers to height above ground level. An area forecast
which reads “BKN030” indicates that the height is
above mean sea level.
AIM 2/14/08
7-1-39
Meteorology
REFERENCE-
AIM, Key to Aerodrome Forecast (TAF) and Aviation Routine Weather
Report (METAR), Paragraph 7-1-30, defines “broken,” “overcast,” and
“obscuration.”
b. Pilots usually report height values above MSL,
since they determine heights by the altimeter. This is
taken in account when disseminating and otherwise
applying information received from pilots. (“Ceil-
ing” heights are always above ground level.) In
reports disseminated as PIREPs, height references
are given the same as received from pilots, that is,
above MSL.
c. In area forecasts or inflight advisories, ceilings
are denoted by the contraction “CIG” when used with
sky cover symbols as in “LWRG TO CIG OVC005,”
or the contraction “AGL” after, the forecast cloud
height value. When the cloud base is given in height
above MSL, it is so indicated by the contraction
“MSL” or “ASL” following the height value. The
heights of clouds tops, freezing level, icing, and
turbulence are always given in heights above ASL or
MSL.
7-1-17. Reporting Prevailing Visibility
a. Surface (horizontal) visibility is reported in
METAR reports in terms of statute miles and
increments thereof; e.g., 1
/16,
1
/8,
3/
16,
1
/4,
5/
16,
3/
8,
1/
2,
5/
8,
3
/4,
7
/8, 1, 1 1
/8, etc. (Visibility reported by an
unaugmented automated site is reported differently
than in a manual report, i.e., ASOS: 0, 1
/16,
1/
8,
1
/4,
1/
2,
3
/4, 1, 1 1
/4, 1 1/
2, 1 3/
4, 2, 2 1/
2, 3, 4, 5, etc., AWOS: M1 /4,
1
/4,
1/
2,
3
/4, 1, 1 1
/4, 1 1/
2, 1 3/
4, 2, 2 1/
2, 3, 4, 5, etc.)
Visibility is determined through the ability to see and
identify preselected and prominent objects at a
known distance from the usual point of observation.
Visibilities which are determined to be less than
7_miles, identify the obscuring atmospheric condi-
tion; e.g., fog, haze, smoke, etc., or combinations
thereof.
b. Prevailing visibility is the greatest visibility
equalled or exceeded throughout at least one half of
the horizon circle, not necessarily contiguous.
Segments of the horizon circle which may have a
significantly different visibility may be reported in
the remarks section of the weather report; i.e., the
southeastern quadrant of the horizon circle may be
determined to be 2 miles in mist while the remaining
quadrants are determined to be 3 miles in mist.
c. When the prevailing visibility at the usual point
of observation, or at the tower level, is less than
4_miles, certificated tower personnel will take
visibility observations in addition to those taken at the
usual point of observation. The lower of these two
values will be used as the prevailing visibility for
aircraft operations.
7-1-18. Estimating Intensity of Rain and
Ice Pellets
a. Rain
1. Light. From scattered drops that, regardless
of duration, do not completely wet an exposed surface
up to a condition where individual drops are easily
seen.
2. Moderate. Individual drops are not clearly
identifiable; spray is observable just above pave-
ments and other hard surfaces.
3. Heavy. Rain seemingly falls in sheets;
individual drops are not identifiable; heavy spray to
height of several inches is observed over hard
surfaces.
b. Ice Pellets
1. Light. Scattered pellets that do not com-
pletely cover an exposed surface regardless of
duration. Visibility is not affected.
2. Moderate. Slow accumulation on ground.
Visibility reduced by ice pellets to less than 7 statute
miles.
3. Heavy. Rapid accumulation on ground.
Visibility reduced by ice pellets to less than 3 statute
miles.
7-1-19. Estimating Intensity of Snow or
Drizzle (Based on Visibility)
a. Light. Visibility more than 1
/2 statute mile.
b. Moderate. Visibility from more than 1
/4_stat-
ute mile to 1
/2 statute mile.
c. Heavy. Visibility 1
/4 statute mile or less.
7-1-20. Pilot Weather Reports (PIREPs)
a. FAA air traffic facilities are required to solicit
PIREPs when the following conditions are reported
or forecast: ceilings at or below 5,000 feet; visibility
at or below 5 miles (surface or aloft); thunderstorms
AIM 2/14/08
7-1-40 Meteorology
and related phenomena; icing of light degree or
greater; turbulence of moderate degree or greater;
wind shear and reported or forecast volcanic ash
clouds.
b. Pilots are urged to cooperate and promptly
volunteer reports of these conditions and other
atmospheric data such as: cloud bases, tops and
layers; flight visibility; precipitation; visibility
restrictions such as haze, smoke and dust; wind at
altitude; and temperature aloft.
c. PIREPs should be given to the ground facility
with which communications are established;
i.e.,_EFAS, AFSS/FSS, ARTCC, or terminal ATC.
One of the primary duties of EFAS facilities, radio
call “FLIGHT WATCH,” is to serve as a collection
point for the exchange of PIREPs with en route
aircraft.
d. If pilots are not able to make PIREPs by radio,
reporting upon landing of the inflight conditions
encountered to the nearest AFSS/FSS or Weather
Forecast Office will be helpful. Some of the uses
made of the reports are:
1. The ATCT uses the reports to expedite the
flow of air traffic in the vicinity of the field and for
hazardous weather avoidance procedures.
2. The AFSS/FSS uses the reports to brief other
pilots, to provide inflight advisories, and weather
avoidance information to en route aircraft.
3. The ARTCC uses the reports to expedite the
flow of en route traffic, to determine most favorable
altitudes, and to issue hazardous weather information
within the center's area.
4. The NWS uses the reports to verify or amend
conditions contained in aviation forecast and
advisories. In some cases, pilot reports of hazardous
conditions are the triggering mechanism for the
issuance of advisories. They also use the reports for
pilot weather briefings.
5. The NWS, other government organizations,
the military, and private industry groups use PIREPs
for research activities in the study of meteorological
phenomena.
6. All air traffic facilities and the NWS forward
the reports received from pilots into the weather
distribution system to assure the information is made
available to all pilots and other interested parties.
e. The FAA, NWS, and other organizations that
enter PIREPs into the weather reporting system use
the format listed in TBL 7-1-5. Items 1 through 6 are
included in all transmitted PIREPs along with one or
more of items 7 through 13. Although the PIREP
should be as complete and concise as possible, pilots
should not be overly concerned with strict format or
phraseology. The important thing is that the
information is relayed so other pilots may benefit
from your observation. If a portion of the report needs
clarification, the ground station will request the
information. Completed PIREPs will be transmitted
to weather circuits as in the following examples:
AIM 2/14/08
7-1-41
Meteorology
TBL 7-1-5
PIREP Element Code Chart
PIREP ELEMENT PIREP CODE CONTENTS
1. 3-letter station identifier XXX Nearest weather reporting location to the reported phenomenon
2. Report type UA or UUA Routine or Urgent PIREP
3. Location /OV In relation to a VOR
4. Time /TM Coordinated Universal Time
5. Altitude /FL Essential for turbulence and icing reports
6. Type Aircraft /TP Essential for turbulence and icing reports
7. Sky cover /SK Cloud height and coverage (sky clear, few, scattered, broken, or
overcast)
8. Weather /WX Flight visibility, precipitation, restrictions to visibility, etc.
9. Temperature /TA Degrees Celsius
10. Wind /WV Direction in degrees magnetic north and speed in knots
11. Turbulence /TB See AIM paragraph 7-1-23
12. Icing /IC See AIM paragraph 7-1-21
13. Remarks /RM For reporting elements not included or to clarify previously
reported items
EXAMPLE1. KCMH UA /OV APE 230010/TM 1516/FL085/TP
BE20/SK BKN065/WX FV03SM HZ FU/TA 20/TB LGT
NOTE1. One zero miles southwest of Appleton VOR; time
1516_UTC; altitude eight thousand five hundred; aircraft
type BE200; bases of the broken cloud layer is six thousand
five hundred; flight visibility 3 miles with haze and smoke;
air temperature 20 degrees Celsius; light turbulence.
EXAMPLE2. KCRW UV /OV KBKW 360015-KCRW/TM
1815/FL120//TP BE99/SK IMC/WX RA/TA M08 /WV
290030/TB LGT-MDT/IC LGT RIME/RM MDT MXD
ICG DURC KROA NWBND FL080-100 1750Z
NOTE2. From 15 miles north of Beckley VOR to Charles-
ton_VOR; time 1815 UTC; altitude 12,000 feet; type
aircraft, BE-99; in clouds; rain; temperature minus
8_Celsius; wind 290 degrees magnetic at 30 knots; light to
moderate turbulence; light rime icing during climb
northwestbound from Roanoke, VA, between 8,000 and
10,000 feet at 1750_UTC.
7-1-21. PIREPs Relating to Airframe Icing
a. The effects of ice on aircraft are cumulative-
thrust is reduced, drag increases, lift lessens, and
weight increases. The results are an increase in stall
speed and a deterioration of aircraft performance. In
extreme cases, 2 to 3 inches of ice can form on the
leading edge of the airfoil in less than 5 minutes. It
takes but 1
/2 inch of ice to reduce the lifting power of
some aircraft by 50 percent and increases the
frictional drag by an equal percentage.
b. A pilot can expect icing when flying in visible
precipitation, such as rain or cloud droplets, and the
temperature is between +02 and -10 degrees Celsius.
When icing is detected, a pilot should do one of two
things, particularly if the aircraft is not equipped with
deicing equipment; get out of the area of
precipitation; or go to an altitude where the
temperature is above freezing. This “warmer”
altitude may not always be a lower altitude. Proper
preflight action includes obtaining information on the
freezing level and the above freezing levels in
precipitation areas. Report icing to ATC, and if
operating IFR, request new routing or altitude if icing
will be a hazard. Be sure to give the type of aircraft to
ATC when reporting icing. The following describes
how to report icing conditions.
1. Trace. Ice becomes perceptible. Rate of
accumulation slightly greater than sublimation.
Deicing/anti-icing equipment is not utilized unless
encountered for an extended period of time (over
1_hour).
2. Light. The rate of accumulation may create
a problem if flight is prolonged in this environment
AIM 2/14/08
7-1-42 Meteorology
(over 1 hour). Occasional use of deicing/anti-icing
equipment removes/prevents accumulation. It does
not present a problem if the deicing/anti-icing
equipment is used.
3. Moderate. The rate of accumulation is such
that even short encounters become potentially
hazardous and use of deicing/anti-icing equipment or
flight diversion is necessary.
4. Severe. The rate of accumulation is such that
deicing/anti-icing equipment fails to reduce or
control the hazard. Immediate flight diversion is
necessary.
EXAMPLE-
Pilot report: give aircraft identification, location,
time_(UTC), intensity of type, altitude/FL, aircraft
type,_indicated air speed (IAS), and outside air
temperature_(OAT).
NOTE1. Rime ice. Rough, milky, opaque ice formed by the
instantaneous freezing of small supercooled water
droplets.
2. Clear ice. A glossy, clear, or translucent ice formed by
the relatively slow freezing of large supercooled water
droplets.
3. The OAT should be requested by the AFSS/FSS or ATC
if not included in the PIREP.
7-1-22. Definitions of Inflight Icing Terms
See TBL 7-1-6, Icing Types, and TBL 7-1-7, Icing
Conditions.
TBL 7-1-6
Icing Types
Clear Ice See Glaze Ice.
Glaze Ice Ice, sometimes clear and smooth, but usually containing some air pockets, which results in a
lumpy translucent appearance. Glaze ice results from supercooled drops/droplets striking a
surface but not freezing rapidly on contact. Glaze ice is denser, harder, and sometimes more
transparent than rime ice. Factors, which favor glaze formation, are those that favor slow
dissipation of the heat of fusion (i.e., slight supercooling and rapid accretion). With larger
accretions, the ice shape typically includes “horns” protruding from unprotected leading edge
surfaces. It is the ice shape, rather than the clarity or color of the ice, which is most likely to
be accurately assessed from the cockpit. The terms “clear” and “glaze” have been used for
essentially the same type of ice accretion, although some reserve “clear” for thinner accretions
which lack horns and conform to the airfoil.
Intercycle Ice Ice which accumulates on a protected surface between actuation cycles of a deicing system.
Known or Observed or
Detected Ice Accretion
Actual ice observed visually to be on the aircraft by the flight crew or identified by on-board
sensors.
Mixed Ice Simultaneous appearance or a combination of rime and glaze ice characteristics. Since the
clarity, color, and shape of the ice will be a mixture of rime and glaze characteristics, accurate
identification of mixed ice from the cockpit may be difficult.
Residual Ice Ice which remains on a protected surface immediately after the actuation of a deicing system.
Rime Ice A rough, milky, opaque ice formed by the rapid freezing of supercooled drops/droplets after
they strike the aircraft. The rapid freezing results in air being trapped, giving the ice its opaque
appearance and making it porous and brittle. Rime ice typically accretes along the stagnation
line of an airfoil and is more regular in shape and conformal to the airfoil than glaze ice. It is
the ice shape, rather than the clarity or color of the ice, which is most likely to be accurately
assessed from the cockpit.
Runback Ice Ice which forms from the freezing or refreezing of water leaving protected surfaces and
running back to unprotected surfaces.
Note-
Ice types are difficult for the pilot to discern and have uncertain effects on an airplane in flight. Ice type definitions will
be included in the AIM for use in the “Remarks” section of the PIREP and for use in forecasting.
AIM 2/14/08
7-1-43
Meteorology
TBL 7-1-7
Icing Conditions
Appendix C Icing Conditions Appendix C (14 CFR, Part 25 and 29) is the certification icing condition standard
for approving ice protection provisions on aircraft. The conditions are specified in
terms of altitude, temperature, liquid water content (LWC), representative droplet
size (mean effective drop diameter ), and cloud horizontal extent.
Forecast Icing Conditions Environmental conditions expected by a National Weather Service or an
FAA-approved weather provider to be conducive to the formation of inflight icing
on aircraft.
Freezing Drizzle (FZDZ) Drizzle is precipitation at ground level or aloft in the form of liquid water drops
which have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle
is drizzle that exists at air temperatures less than 0_C (supercooled), remains in
liquid form, and freezes upon contact with objects on the surface or airborne.
Freezing Precipitation Freezing precipitation is freezing rain or freezing drizzle falling through or outside
of visible cloud.
Freezing Rain (FZRA) Rain is precipitation at ground level or aloft in the form of liquid water drops which
have diameters greater than 0.5 mm. Freezing rain is rain that exists at air
temperatures less than 0_C (supercooled), remains in liquid form, and freezes upon
contact with objects on the ground or in the air.
Icing in Cloud Icing occurring within visible cloud. Cloud droplets (diameter < 0.05 mm) will be
present; freezing drizzle and/or freezing rain may or may not be present.
Icing in Precipitation Icing occurring from an encounter with freezing precipitation, that is, supercooled
drops with diameters exceeding 0.05 mm, within or outside of visible cloud.
Known Icing Conditions Atmospheric conditions in which the formation of ice is observed or detected in
flight.
Note-
Because of the variability in space and time of atmospheric conditions, the existence
of a report of observed icing does not assure the presence or intensity of icing
conditions at a later time, nor can a report of no icing assure the absence of icing
conditions at a later time.
Potential Icing Conditions Atmospheric icing conditions that are typically defined by airframe manufacturers
relative to temperature and visible moisture that may result in aircraft ice accretion
on the ground or in flight. The potential icing conditions are typically defined in the
Airplane Flight Manual or in the Airplane Operation Manual.
Supercooled Drizzle Drops
(SCDD)
Synonymous with freezing drizzle aloft.
Supercooled Drops or /Droplets Water drops/droplets which remain unfrozen at temperatures below 0 _C.
Supercooled drops are found in clouds, freezing drizzle, and freezing rain in the
atmosphere. These drops may impinge and freeze after contact on aircraft surfaces.
Supercooled Large Drops (SLD) Liquid droplets with diameters greater than 0.05 mm at temperatures less than
0_C, i.e., freezing rain or freezing drizzle.
AIM 2/14/08
7-1-44 Meteorology
7-1-23. PIREPs Relating to Turbulence
a. When encountering turbulence, pilots are
urgently requested to report such conditions to ATC
as soon as practicable. PIREPs relating to turbulence
should state:
1. Aircraft location.
2. Time of occurrence in UTC.
3. Turbulence intensity.
4. Whether the turbulence occurred in or
near clouds.
5. Aircraft altitude or flight level.
6. Type of aircraft.
7. Duration of turbulence.
EXAMPLE1. Over Omaha, 1232Z, moderate turbulence in clouds at
Flight Level three one zero, Boeing 707.
2. From five zero miles south of Albuquerque to three zero
miles north of Phoenix, 1250Z, occasional moderate chop
at Flight Level three three zero, DC8.
b. Duration and classification of intensity should
be made using TBL 7-1-8.
TBL 7-1-8
