This research describes a preliminary effort to measure the effect of the atmosphere on the refraction of microwaves. Variations in the structure of the lower atmosphere cause small changes in the index of refraction, and these small changes might alter the angle at which the wave front arrives at the receiver. To measure the minute variations in the arrival angle, a transmitter, radiating at a frequency of 9520 megacycles was placed on Mt. Wilson. The radiated energy was focused by means of a metallic lens, and the shift of the diffraction pattern of the lens as measured in its focal plane, was taken as a measure of the change in the angle of arrival. This shift was determined by detecting the position of the upper null of the diffraction pattern. There were changes in the position of this null and these changes were, in general, consistent with the meager meteorological information available. Variations of the order of one tenth of a degree were noticed. These changes are larger, in all probability, than can be accounted for by calculations based on ray theory. Reflections from nearby buildings and trees could conceivably influence the nature of the diffraction pattern, but the exact nature or their effect would be extremely difficult to calculate.