Abstract
Photometric measurements of the light scattered from the twilight sky, in the direction of the sun's vertical at angle an of 70 deg. from the zenith, were made during IGY-IGC period 1957-59, at Mt. Abu, India, with a photometer, the telescope of which covered a circular field of sky of about 1 deg. in the diameter. Observations have revealed that the discontinuities in the slopes of intensity curves are observed when the centre of the sun was between 5 and 6 deg. below the observer's horizon. These discontinuities are clearly displayed as maxima on the curves of logarithmic gradients of intensity represented by d In I/dh ( h is the effective height of the earth's shadow with proper allowance for atmospheric refraction and screening by the atmosphere near the earth) against ?, the solar depression below the observer's horizon; and these are attributed to changes in the structure of the atmosphere due to changes in the dust content of the atmosphere. At Abu, the average height of maximum dust accumulation is about 20 km with the lowest of 14 km being found in June and highest of 23 km in Nov.-Dec. DOI: 10.1111/j.2153-3490.1970.tb01939.x
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