Chopping a Hole in Fog

In desperate and imaginative efforts to clear fog from airports, highways and other critical areas, meteorologists have used giant fans, rotating racks strung with nylon strands and chemicals dropped from planes or spewed up ward from strange machines on the ground. Now the U.S. Air Force thinks that it has found a practical new weap on in the continuing fight against fog: the helicopter.

During research into the meteorology of fogs, scientists at the Air Force Cam bridge Research Laboratories in Bed ford, Mass., concluded that there might be a simple way to disperse mists that develop when moist air near the ground is cooled at night. The researchers reasoned that if the warm, drier air above could somehow be driven down into the moist blanket of fog, it would cause suspended water droplets to evaporate, thus clearing the air.

For recent tests of their theory, the Air Force scientists chose Smith Moun tain Airport, near Roanoke, Va., which is often socked in by moist, low-lying mist. The Air Force made available twin-turbine CH-3E helicopters for use as air-mixing devices.

In a routine developed during three foggy weeks of tests, a helicopter was flown to an altitude of 500 ft. above the mist, where it hovered until the turbulence of its downdraft traced a cir cular outline about 5,000 ft. in diameter on the upper layer of fog. The chopper then descended to 100 ft. above the fog and, at a speed of 30 m.p.h., began to fly in a gradually enlarging spiral pattern until it reached the edge of the circular outline. Within a minute, the fog began to fade away at the center of the circle. Ten minutes later, a clearing nearly a mile in diameter had been opened above the airport.

Air Force scientists are conducting further experiments in California to refine their fog-dispersing system, but they say that it has already proved practical under combat conditions in Southeast Asia. Twice, after Air Force planes were forced down and obscured by low-lying cloud banks in enemy-infested territory, rescue helicopters spiraled overhead until they had cleared holes in the clouds. They then lowered lines and rescued the downed pilots, who thus became the first beneficiaries of a novel procedure that Air Force scientists hope will soon become routine.

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