A weather phenomenon called a microburst is potentially lethal when airplanes encounter it at low altitude and occurs more frequently than generally recognized, two scientists told Congress yesterday.

Further, they said that microbursts can happen anywhere and that they think one occurred at Andrews Air Force Base Aug. 1 moments after Air Force One landed with President Reagan on board. On that day, a vicious front moved through Washington, blowing down trees and bringing heavy rains. The media plane trailing Air Force One from Atlanta was diverted to Washington National Airport because of the weather.

John McCarthy, a scientist at the National Center for Atmospheric Research in Boulder, Colo., testified that records from the Andrews tower show winds briefly exceeding 112 mph at 2:12 p.m., then registering more normal levels. Air Force One landed at 2:05 p.m., according to the base's press office.

"That suggests a microburst-type event," McCarthy said.

A microburst is a downdraft that may be no more than two miles wide. As winds approach the ground, they spread out in all directions. An aircraft encountering a microburst at low altitude flies first into a headwind, then almost immediately into a tailwind possibly strong enough to push a jetliner to the ground.

Microbursts are blamed for two major airline accidents, including the crash immediately after takeoff of a Pan American flight in New Orleans on July 9, 1982, that killed 153 people.

McCarthy and fellow scientist Theodore Fujita have researched microbursts extensively. They recommended strongly to the House Public Works investigations subcommittee that microburst warning systems be improved and expanded at the nation's major airports and that pilots and controllers be alerted to dangers and warning signs of microbursts.