AIM

8/15/19

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36

Meteorology

d.

All En Route Flight Advisory Service facilities

and FSSs 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
ARTCCs also have access to weather radar displays
and provide support to all air traffic facilities within
their center’s area.

e.

Additional information on weather radar

products and services can be found in AC 00

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45,

Aviation Weather Services.

REFERENCE

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Pilot/Controller Glossary Term

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 Precipitation  Radar  Weather

Descriptions.
AIM, Paragraph 7

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28 , Thunderstorms

Chart Supplement U.S., Charts, NWS Upper Air Observing Stations and
Weather Network for the location of specific radar sites.

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14. ATC Inflight Weather Avoidance

Assistance

a. ATC Radar Weather Display.

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.”

2.

All ATC facilities using radar weather

processors with the ability to determine precipitation
intensity, will describe the intensity to pilots as:

(a)

“LIGHT” (< 26 dBZ)

(b)

“MODERATE” (26 to 40 dBZ)

(c)

“HEAVY” (> 40 to 50 dBZ)

(d)

“EXTREME” (> 50 dBZ)

NOTE

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En route ATC radar’s Weather and Radar Processor
(WARP) does not display light precipitation intensity.

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
UNKNOWN.”

4.

ARTCC facilities normally use a Weather and

Radar Processor (WARP) to display a mosaic of data
obtained from multiple NEXRAD sites. There is a
time delay between actual conditions and those
displayed to the controller. For example, the
precipitation data on the ARTCC controller’s display
could be up to 6 minutes old. When the WARP is not
available, a second system, the narrowband Air Route
Surveillance Radar (ARSR) can display two distinct
levels of precipitation intensity that will be described
to pilots as “MODERATE” (30 to 40 dBZ) and
“HEAVY TO EXTREME” ( > 40 dBZ ). The WARP
processor is only used in ARTCC facilities.

5.

ATC radar is not able to detect turbulence.

Generally, turbulence can be expected to occur as the
rate of rainfall or intensity of precipitation increases.
Turbulence associated with greater rates of rainfall/
precipitation will normally be more severe than any
associated with lesser rates of rainfall/precipitation.
Turbulence should be expected to occur near
convective activity, even in clear air. Thunderstorms
are a form of convective activity that imply severe or
greater turbulence. Operation within 20 miles of
thunderstorms should be approached with great
caution, as the severity of turbulence can be markedly
greater than the precipitation intensity might indicate.

b. Weather Avoidance Assistance.

1.

To the extent possible, controllers will issue

pertinent information on weather or chaff areas and
assist pilots in avoiding such areas when requested.
Pilots should respond to a weather advisory by either
acknowledging the advisory or by acknowledging the
advisory and requesting an alternative course of
action as follows:

(a)

Request to deviate off course by stating

 

a

heading or degrees, direction of deviation, and
approximate number of miles. In this case, when the
requested deviation is approved, navigation is at the
pilot’s prerogative, but must maintain the altitude
assigned, and remain within the lateral restrictions
issued by ATC.