A representative picture, from the aeronautical point of view, of vertical currents above mountainous country is obtained by letting these currents act on an aircraft set to fly horizontally in still air. Pressure and temperature traces recorded by such an aircraft give the effective vertical velocities and some idea of the temperature lapse rate in the undisturbed stream. Two sets of results are given for illustration. One shows low-level lee waves which were caused presumably by a temperature inversion; the other is a case of strong turbulence side by side with a smooth wave, in which downdrafts reached 1600 ft/min in an 18-knot wind.

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