background image

6/17/21 

AIM 

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, Para 7

1

26, Thunderstorms. 

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

7

1

12.  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. 

Meteorology 

7

1

35