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6/17/21 

AIM 

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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 informa-
tion for weather avoidance and flight planning. 

Meteorology 

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