ASOS SERVICE LEVELS
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 following weather parameters: Wind, Visibility, Precipitation/Obstructions to Vision, Cloud Height, Sky Cover, Temperature, Dew Point and Altimeter. A site ranked as service level D has a stand-alone ASOS.
Service Level C.
This level consists of all the elements of Service Level D, plus augmentation and backup by a human observer on location nearby. Backup consists of inserting the correct value if the system malfunctions or is unrepresentative. Augmentation consists of adding the following elements if they are observed: Thunderstorms, Tornadoes, Hail, Virga, Volcanic Ash, Tower Visibility and any operationally significant remarks as deemed appropriate by the observer. During the hours that the observing facility is closed, the site reverts to Service Level D, stand-alone status. Generally speaking, air traffic control tower specialists provide Service Level C.
Service Level B.
This level consists of all of the elements of service levels C and D plus the augmentation of the following elements: Longline RVR at precedented sites (may be an instantaneous readout), Freezing Drizzle versus Freezing Rain, Ice Pellets, Snow Depth and Snow Increasing Rapidly remarks, Thunderstorm and Lightning Location remarks, Observed Significant Weather Not at the Station remarks. Ruing the hours that the air traffic control tower is closed, the site may revert to Service Level D. Generally speaking, contract weather observers provide Service Level B.
Service Level A.
This level consists of all of the elements of service levels B, C and D plus the augmentation of the following elements: 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 and Volcanic Eruptions.