AIM 

4/20/23 

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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. 

Meteorology

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