Radar photographs of thunderstorms and the Florida hurricane of September 1945 were shown to illustrate structure of those storms (see Fig. 1). The relation between radar echoes and the reflecting material in the storms was discussed. Previous research has shown the intensity of the return radiation, among other things, to be proportional to the number and sixth power of the diameter of the intercepted water droplets. For radar frequencies now in usage, this means that only water drops large enough to fall out as precipitation reflect a detectable echo. The possibility of determining distribution and intensity of precipitation during a given storm by radar was discussed, but it was concluded that this cannot be done on an absolute basis at present, but that the relative intensities can be determined by suitably modified radar. However, if observed rainfall intensities are available at a small number of stations in the area swept by the radar, then it is possible to “calibrate” the radar intensities in terms of rainfall intensit