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Inquiry has been received as to the possible effect on the indications by cup anemometers of wind velocities due to the bombardment of rain drops under conditions of comparatively high winds and very heavy rains. This is an interesting question which has perhaps not been very carefully investigated experimentally. Nevertheless, a seemingly conclusive answer is possible on analytical grounds. Two obvious effects require consideration. First, the cups become thoroughly wetted by the addition of a certain amount of water and therefore slightly heavier. The second effect arises from the collisions with numerous water drops striking against both the concave and convex faces of the cups. By measurement it is found that the actual weight of water on any of the cup wheels of the Weather Bureau anemometers amounts to 6.1 grams, which is 2.2 percent of the weight of the standard light-weight cups and, of course, a still smaller percentage of that of the henvier cup systems. Wind-tunnel tests with steady wind velocities show conclusively that when the length of the arms and other dinlensions are the same the run of these anemometers is wholly independent of small changes in the mere weight of the cups. In the case of gusty wyinds the lag of the anemometer in following the wind is, of course, greater the greater the weight of the cups. For this reason heavy rain tends to make the cups lag begnd the true wind velocity more than they do in the same w n d without rain, and conversely when the wind velocity is falling off rapidly the cups overrun slightly. These effects are quite inconsequential on the average run of the cups. In other words, the effect of the increased weight of the cups due to rainfall may be regarded as negligible and unimportant,. As to the effect of the bombardment of the cups by the raindrops, reasoning indicates that the convex cups which are advancing in the wind collide with the raindrops at relatively high velocities. We are thinking here of the horizontal coniponent of the motion of the raindrops with reference to the surfaces of the cups a t the instant of c.ollision. The concave cups, however, are advancing with the wind and t,he kinetic energy involved in the collision of the raindrops and these cups because of the smaller relative velocities is certainly much less than the corresponding kinetic e,nergies involved in the collisions of the raindrops with the conves cups. The ultimate result of these collisions must therefore be a tendency to retard the speed of rotation of the cups. As already stated, no wind tunnel or other measurements are availble by which the amount of this retardation can be evaluated. On the other hand, we do have carefully made measureine,nts of the effects of moderate degrees of friction in retarding the niotion of the cups as compnred to the almost coinplet,e absence of fricbion. These friction tests show that even quite appreciable amounts of friction have an inappreciable or very small effect in retarding the speed of the cups in high winds and it is certain that the eflects of the bombardment of the raindrops is quite appreciably less than the moderate amounts of friction which have been subjected to tests. The final conc,lusions of the, fore.going analysis are therefore (1) that ra.in tends to make the anemometer more sluggish by a very small a,mount in responding to fluctuating gustmy winds, although the average run of the cups in such gust,y winds is not appreciably affected and (2) that heavy rains in high winds retard the run of the cups substantially as does fiiction, but that the amount of this retardation, while not esa.ctly known, certainly is inconsequential and unimportant . We may conclude this consideration by asking how the cups would behave if esposed in perfectly still air during a heavy downpour of rain. A little analysis indicates that the nction of the rain on each one of the cups would tend more or less to turn it bac.kwa.rd. The combined force on all the cups might even be sufficient to actually turn a nearly frictionless anemometer cup wheel backward. In this way, too, falling rain tends to oppose motions caused by the wind.

WHAT IS THE EFFECT OF HEAVY RAINS WITH HIGH WINDS ON THE RUN OF CUP-WHEEL ANEMOMETERS ?