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AIM

10/12/17

7−1−36

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 00−45,

Aviation Weather Services.

REFERENCE−

Pilot/Controller Glossary Term− Precipitation  Radar  Weather

Descriptions.

AIM, Paragraph 7−1−28 , Thunderstorms

Chart Supplement U.S., Charts, NWS Upper Air Observing Stations and

Weather Network for the location of specific radar sites.

7−1−14. 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−

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.

3/15/07

7110.65R CHG 2

AIM

3/29/18