The purpose of this study is to assess the entrainment by rain of chaff which is used to track air motions by radar. Experiments are described where 214 individual water drops with diameters of 4.9 mm and falling at 78% of terminal velocity collide with single strands of (cylindrical) chaff fibres (diameters of 25 μm and lengths of 10.7 cm), which were falling freely at the time of collision. The length of the fibres investigated is adequate to be used by 10 cm wavelength tracking radars. On the average the water drops carried the chaff over a distance of 4.5 cm from the point of original contact. The actual distance of carry depends on the initial point of contact with respect to both the drop and the fibre; it is greatest for centered collisions.
A simple model is outlined on the basis of the equal but opposite drag forces the chaff experiences within the drop and within the air. Extrapolations for the carrying distance were then made for drops of various sizes, falling at terminal velocities, and drop spectra exhibiting a given Marshall–Palmer distribution. The main conclusion is that the average increase in the downward motion of the chaff due to rain is quite small as compared with the free fallspeed of the chaff and can be neglected in practical applications to the tracking of air motions by radar.