Helicopter wake effects on stratus and stratocumulus clouds are described. Holes and troughs were created in such clouds with dimensions several hundred feet across. The velocities and penetration distances of the downwash flow beneath the HH-53B helicopter employed in the experiments were investigated by the release of a “tracing agent” from the helicopter and by hovering at various altitudes above the water surface of a Florida bayou. The downwash velocities were as large as 100 ft sec−1, near the rotor, and the downwash extended some 1300 ft beneath the helicopter (under near dry adiabatic conditions).

A suggestion is made that helicopters might be employed operationally to clear radiation-type ground fog from airfield runways. The cloud clearing capabilities of the HH-53B helicopter are discussed in support of this suggestion, by reference to observational data and to the theory of Hohler.

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