A study of the fine structure of air refractivity in the lower troposphere has been continued through the spring and summer of 1962 by the observation of weak radio reflections from clear air (radar angels), with a vertically directed radar of special design operating at 6770 Mc/s. These observations have been limited to the layer of frictional influence, within 1500 meters of the surface. The interpretation suggests that the reflecting centers are broad strata whose refractivity contrasts weakly with that of their environment, whose vertical depths are no more than a few centimeters, and which are either flat over horizontal distances of at least several meters or concave downwards with radii of curvature somewhat less than their height above ground. The incidence of transitory reflections generally follows a regular distribution in the vertical, with a maximum at 300 meters; the form of this distribution is modified by the intrusion of weather fronts, thermals, and other clear-weather structures. The transitory reflecting stratum is cut off from its generating source early in its life history, to be dispersed into its environment by molecular or eddy diffusion. The total incidence of transitory angels fluctuates quasi-periodically in time, with a period of about 10 minutes; it is suggested that this periodicity is due to the influence of internal gravity waves in the atmosphere. In contrast, persistent reflections are associated with a more stable environment; the maximum incidence is at the lowest heights observed, with a gradual decrease towards higher levels. Their relationship to clear-weather structure is less certain than for the transitory reflections. The persistent reflecting stratum must be replenished continuously by the generating source during its lifetime to offset diffusion into the environment.
Regular Distribution Inhibits Generic Numerosity Processing
