Abstract: To determine the size of airborne water droplets having a diameter from approximately 1 to 100 .mu.m, if such water droplets move at a velocity of 50 to 150 m/s relative to a measuring probe and strike it,in the probe body, whose front face toward the flow has a vapor-deposited platinum film, approximately 0.5 .mu.m thick, whose temperature is kept constant at above 100.degree. C. by means of a bridge circuit;an analog voltage output signal of the bridge circuit is digitized and then further processed, with the aid of an evaluation computer program, among others, andwith the aid of a formula (in accordance with equation (6)) derived from an energy balance for a droplet striking a platinum film, determines the droplet diameter from analyzed identifying variables of a droplet signal and from other known parameters.