A study of radar angels at 6800 Mc/s has been carried out at London, Canada, over a period of 1 year. Examination of the clear-air reflections from heights between 300 and 1500 meters above the vertically directed radar has shown that persistent angels occur for surface air temperatures between 30° and 50° F, and that transitory angels are present for surface temperatures above 20° F that are outside of this range. The maximum incidence of angels increases with type of air mass in the order cA, mA, mP, mT, although the most common duration is approximately the same in these air masses. If the angel is due to reflection at a horizontal flat stratum, the power reflection coefficient of the stratum lies between 10−16 and 10−14 for transitory angels and is as large as 10−11 for persistent angels. It is shown that transitory angels may arise through eddy mixing in the lower troposphere when refractivity contrasts of about one part per million occur, but that the reflecting stratum must be no more than a few centimeters in depth. It appears that persistent angels are associated with a high concentration of moisture at the ground, but their physical origin is not clear.
This paper describes a seven-pound device that indicates changes in the refractive index of a gas. Because it is capable of detecting refractivity fluctuations that are as small as one part per million, it is useful in studying the irregularities in the refractive index of the troposphere that are important in short radio-wave transmission. The refractometer is self-contained, so that it may be used in controlled or in free ascent through the atmosphere. The device is essentially a high frequency oscillator whose frequency is governed by the capacitance of an air sensing condenser. Special provision is made for reliable operation over the wide range of temperatures normally found in the troposphere.
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.
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