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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
weatherareas 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” (< 30 dBZ)
(b) “MODERATE” (30 to 40 dBZ)
(c) “HEAVY” (> 40 to 50 dBZ)
(d) “EXTREME” (> 50 dBZ)
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
the number of miles and the direction of the requested
deviation. In this case, when the requested deviation
is approved, navigation is at the pilot's prerogative,
but must maintain the altitude assigned by ATC and
to remain within the specified mileage of the original
course.
(b) Request a new route to avoid the affected
area.
(c) Request a change of altitude.
AIM 2/14/08
7-1-36 Meteorology
(d) Request radar vectors around the affected
areas.
2. For obvious reasons of safety, an IFR pilot
must not deviate from the course or altitude or flight
level without a proper ATC clearance. When weather
conditions encountered are so severe that an
immediate deviation is determined to be necessary
and time will not permit approval by ATC, the pilot's
emergency authority may be exercised.
3. When the pilot requests clearance for a route
deviation or for an ATC radar vector, the controller
must evaluate the air traffic picture in the affected
area, and coordinate with other controllers (if ATC
jurisdictional boundaries may be crossed) before
replying to the request.
4. It should be remembered that the controller's
primary function is to provide safe separation
between aircraft. Any additional service, such as
weather avoidance assistance, can only be provided
to the extent that it does not derogate the primary
function. It's also worth noting that the separation
workload is generally greater than normal when
weather disrupts the usual flow of traffic. ATC radar
　
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