ATC Inflight Weather Avoidance  Assistance ATC Radar Weather Displays ATC radar systems 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, or the denser its reflective surface, the stronger  the return. Radar weather processors indicate the intensity  of reflective returns in terms of decibels with respect to the  radar reflectively factor (dBZ).  ATC systems cannot detect the presence or absence of  clouds. ATC radar 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.” All ATC facilities using radar weather processors with the  ability to determine precipitation intensity describes the  intensity to pilots as: ARTCC controllers do not use the term “LIGHT” because  their systems do not display “LIGHT” precipitation  intensities. ATC facilities that, due to equipment limitations,  cannot display the intensity levels of precipitation, 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 states, “INTENSITY UNKNOWN.” ARTCC facilities normally use a Weather and Radar  Processor (WARP) to display a mosaic of data obtained from  multiple NEXRAD sites. The WARP processor is only used  in ARTCC facilities.  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 secondary  system, the narrowband ARSR is utilized. The ARSR system  can display two distinct levels of precipitation intensity that  is described to pilots as “MODERATE” (30 to 40 dBZ) and  “HEAVY to EXTREME” (>40 dBZ).  IFR pilots shall not deviate from their assigned course or  altitude without an ATC clearance. Plan ahead for possible  course deviations because hazardous convective conditions  can develop quite rapidly. This is important to consider  because the precipitation data displayed on ARTCC radar  scopes can be up to 6 minutes old, and thunderstorms can  develop at rates exceeding 6,000 feet per minute (fpm). When  encountering weather conditions that threaten the safety of  the aircraft, the pilot may exercise emergency authority as  ATC radar systems cannot 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 is normally 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 implies 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. ATC’s first duty priority is to separate aircraft and issue  safety alerts. ATC provides additional services to the extent  possible, contingent upon higher priority duties and other  factors including limitations of radar, volume of traffic,  frequency congestion, and workload. Subject to the above  factors/limitations, controllers issue pertinent information  on weather or chaff areas; and if requested, assist pilots, to  the extent possible, in avoiding areas of precipitation. Pilots  should respond to a weather advisory by acknowledging the  advisory and, if desired, requesting an alternate course of  action, such as: 1.  Request to deviate off course by stating the direction  and number of degrees or miles needed to deviate from  the original course; 2.  Request a change of altitude; or  3.  Request routing assistance to avoid the affected  area. Because ATC radar systems cannot detect the  presence or absence of clouds and turbulence, such  assistance conveys no guarantee that the pilot will not  encounter hazards associated with convective activity.  Pilots wishing to circumnavigate precipitation areas  by a specific distance should make their desires  clearly known to ATC at the time of the request for  services. Pilots must advise ATC when they can  resume normal navigation. Weather Avoidance Assistance 2-11 1. “LIGHT”  (< 30 dBZ) 2. “MODERATE”  (30 to 40 dBZ) 3. “HEAVY”  (>40 to 50 dBZ) 4. “EXTREME”  (>50 dBZ)