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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
Cloud height
Sky cover
Temperature
Dew point
Altimeter
30 AUG 07
AIP
United States of America
GEN 3.5-29
15 MAR 07
Federal Aviation Administration Nineteenth Edition
9. Weather Radar Services
9.1 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 radars augment the network by operating on
an as needed basis to support warning and forecast
programs.
9.2 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 is also
collected by the National Meteorological Center and
used to prepare hourly national radar summary charts
for dissemination on facsimile circuits.
9.3 All En route Flight Advisory Service facilities
and many Automated Flight Service Stations have
equipment to directly access the radar displays from
the individual weather radar sites. Specialists at these
locations are trained to interpret the display for pilot
briefing and inflight advisory services. The Center
Weather Service Units located in the ARTCCs also
have access to weather radar displays and provide
support to all air traffic facilities within their center's
area.
9.4 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.
9.5 Additional information on weather radar products
and services can be found in FAA Advisory
Circular 00-45, “Aviation Weather Services.”
REFERENCEPilot/
Controller Glossary Term- Precipitation Radar Weather
Descriptions.
AIP, Thunderstorms, GEN 3.5, Paragraph 27.
Airport/Facility Directory, Charts, NWS Upper Air Observing Stations
and Weather Network for the location of specific radar sites.
10. ATC Inflight Weather Avoidance
Assistance
10.1 ATC Radar Weather Display
10.1.1 ATC radars are able to display areas of
precipitation by sending out a beam of radio energy
that is reflected back to the radar antenna when it
strikes an object or moisture which may be in the form
of rain drops, hail, or snow. The larger the object is,
or the more dense its reflective surface, the stronger
the return will be presented. Radar weather
processors indicate the intensity of reflective returns
in terms of decibels (dBZ). ATC systems cannot
detect the presence or absence of clouds. The ATC
systems can often determine the intensity of a
precipitation area, but the specific character of that
area (snow, rain, hail, VIRGA, etc.) cannot be
determined. For this reason, ATC refers to all weather
areas displayed on ATC radar scopes as “precipitation.”
10.1.2 All ATC facilities using radar weather
processors with the ability to determine precipitation
intensity, will describe the intensity to pilots as:
10.1.2.1 “LIGHT” (< 30 dBZ)
10.1.2.2 “MODERATE” (30 to 40 dBZ)
10.1.2.3 “HEAVY” (> 40 to 50 dBZ)
10.1.2.4 “EXTREME” (> 50 dBZ)
10.1.3 ATC facilities that, due to equipment
limitations, cannot display the intensity levels of
precipitation, will describe the location of the
precipitation area by geographic position, or position
relative to the aircraft. Since the intensity level is not
available, the controller will state “INTENSITY
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