The need for prudence when flying through known areas of forecasted severe weather is well recognized. Presently, ATC procedures call for controllers to assist pilots, particularly when operating on IFR flight plans, in avoiding known areas of severe weather. It is impor tant to remember, however, that there are limitations to an air traffic controller＇s capability for providing such assistance. The controller’s primary responsibility is to provide safe sepa ration between aircraft. No additional services can be given which distract or derogate from that primary responsibility. Additionally, ATC radar equipment, communications conges tion, and traffic may also limit and reduce a controller’s ability to provide any additional service. To a large degree, the assistance that might be given by air traffic control depends upon the weather information available to controllers through weather reporting channels or by pi lots actually requesting to avoid severe weather areas. Because severe weather is extremely transitory in nature, the information available to controllers may be of only limited value, if any – unless frequently updated by pilot reports or radar weather information. In-flight reports from pilots in direct communications with controllers can give vital weather information. Pilots wishing to avoid severe weather along the route should request deviations from route and/or altitude as far in advance as possible. Obtaining clearance to avoid severe weath er can often be accommodated in the en-route areas, away from terminal areas, because there is less traffic congestion. In the terminal areas, ATC coordination is complicated by complex departure routes and adjacent airports. Consequently, controllers are less likely to be able to accommodate requests for weather detours. ATC also has other problems in handling the weather. Under certain conditions, echoes received from precipitation render ATC radar unusable because of clutter. To avoid disruption in radar service, modifications have been made too many radars to reduce precipitation clutter. This feature is known as circular polarization. Terminal radar systems use this feature to reduce clutter during moderate to heavy rain or snow storms in order to keep track of traffic. This necessary process, however, means the controller does not always see the weather on the radar scope. Moreover, if ATC is using a secondary radar system, the controller’s scope displays only those signals emitted by the airborne radar beacon transmitters. With this sys tem the radar scope does not display weather echoes at all. The controllers can, however, be alerted to turn on their normal radar to observe weather, provided this will not result in weather cluttering, rendering the scope unusable for air traffic control. The point to re member is that pilots should realize that the controller’s weather information is often far from complete.3.The controllers are more likely to clear the pilots to detour severe weather en route just because （ ） .