The phenomenon of ultrasonic intensity fluctuations has been studied experimentally outdoors under various kinds of weather conditions. There seem to be three types of fluctuations. Their causes are suggested by correlations of ultrasonic data with simultaneous micrometeorological observations. Two types of fluctuations are of the mean intensity. Those of the first are usually fairly slow with a period of several minutes and with a range of intensity variation of 10 to 30 db. Apparently they are observed only near boundaries of “shadow zones” and are caused by the motions of the “shadows” as the vertical gradients of the velocity of sound change. The second type is ordinarily faster, with a frequency of 2 or 3 per minute, and an intensity range of only 2 to 5 db. They are observed at a distance from shadow boundaries where the fluctuations of gradients can produce only changes in the convergence and divergence of rays with consequent variations in intensity. Third type fluctuations are usually extremely rapid, as frequent as several times per second, with an amplitude of variation of several db, and there is evidence that they are caused by the random motion, between source and receiver, of inhomogeneities or discrete bodies of air in the atmosphere. Typical diurnal cycles of ultrasonic fluctuations and of related weather conditions, as well as of micrometeorological phenomena, have been observed.