AIM

8/15/19

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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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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.