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reassess the requirements for surface observations at
the nation’s airports. That work resulted in agreement
on a set of service standards, and the FAA and NWS
ASOS sites to which the standards would apply. The
term “Service Standards” refers to the level of detail
in weather observation. The service standards consist
of four different levels of service (A, B, C, and D) as
described below. Specific observational elements
included in each service level are listed in
TBL 7−1−2.
1. Service Level D defines the minimum
acceptable level of service. It is a completely
automated service in which the ASOS observation
will constitute the entire observation, i.e., no
additional weather information is added by a human
observer. This service is referred to as a stand alone D
site.
2. Service Level C is a service in which the
human observer, usually an air traffic controller,
augments or adds information to the automated
observation. Service Level C also includes backup of
ASOS elements in the event of an ASOS malfunction
or an unrepresentative ASOS report. In backup, the
human observer inserts the correct or missing value
for the automated ASOS elements. This service is
provided by air traffic controllers under the Limited
Aviation Weather Reporting Station (LAWRS)
process, FSS and NWS observers, and, at selected
sites, Non−Federal Observation Program observers.
Two categories of airports require detail beyond
Service Level C in order to enhance air traffic control
efficiency and increase system capacity. Services at
these airports are typically provided by contract
weather observers, NWS observers, and, at some
locations, FSS observers.
3. Service Level B is a service in which weather
observations consist of all elements provided under
Service Level C, plus augmentation of additional data
beyond the capability of the ASOS. This category of
airports includes smaller hubs or special airports in
other ways that have worse than average bad weather
operations for thunderstorms and/or freezing/frozen
precipitation, and/or that are remote airports.
4. Service Level A, the highest and most
demanding category, includes all the data reported in
Service Standard B, plus additional requirements as
specified. Service Level A covers major aviation
hubs and/or high volume traffic airports with average
or worse weather.
TBL 7−1−1
Weather Observing Programs
Element Reported AWOS−A AWOS−1 AWOS−2 AWOS−3 ASOS Manual
Altimeter X X X X X X
Wind X X X X X
Temperature/
Dew Point
X X X X X
Density Altitude X X X X
Visibility X X X X
Clouds/Ceiling X X X
Precipitation X X
Remarks X X
AIM 8/5/04
7−1−28 Meteorology
TBL 7−1−2
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 continually
measuring the atmosphere at a point near the runway. The
ASOS senses and measures the weather parameters listed to
the right.
Wind
Visibility
Precipitation/Obstruction to vision
　
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