AIM 

4/20/23

 

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Meteorology

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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 or AWOS
continually measuring the atmosphere at a point near the
runway. The ASOS or AWOS senses and measures the
weather parameters listed to the right.

Wind
Visibility
Precipitation/Obstruction to vision
Cloud height
Sky cover
Temperature
Dew point
Altimeter

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11. 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 information for weather avoidance and flight planning.