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A. S. Dennis

Abstract

The distribution in time of individual lightning flashes recorded visually during 20 storms in New Mexico and on 23-cm radar screens during several storms in the south-central United States has been analyzed. The logarithms of the intervals between flashes within a given storm are normally distributed with a standard deviation σ of approximately one natural-log unit. The available data do not reject the hypothesis that there are no statistically significant variations in σ among storms. The autocorrelation of the intervals between flashes in one storm is very small. The implications of these findings are briefly explored.

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A. S. Dennis

Abstract

No abstract available.

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A. S. Dennis

Abstract

Examination of radar records shows that snow trails frequently occur around the tops of showers. Evidence has been sought that showers are occasionally initiated by the seeding of supercooled cumulus clouds by snow. Certain observed radar patterns suggest strongly that showers were produced in this manner. As a check, plan diagrams with height contours have been synthesized and used to organize vertical sections obtained on different bearings. The constructed plan diagrams show showers occurring in regions of snow trails more frequently than elsewhere.

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A. Koscielski and A. S. Dennis

Abstract

First radar echoes on a randomized cloud seeding project in North Dakota appeared closer to cloud base and at higher temperatures on seed days than on no-seed days. The average first echo temperature was near −11°C on no-seed days and −7°C on seed days.

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Dennis J. Musil and A. S. Dennis

Abstract

No abstract available.

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A. S. Dennis and Alexander Koscielski

Abstract

The height and temperature of first radar echoes observed on 49 test cases in a randomized cloud seeding project have been related to seeding treatment and to five cloud parameters, namely, cloud base height, cloud base temperature, maximum radar echo height, and the height and speed of maximum updraft as computed with a cloud model.

First echo height above cloud base in unseeded clouds averaged 11,000 ft and was strongly correlated with maximum updraft speed. First echo height in clouds seeded with silver iodide averaged 6900 ft. It was most closely correlated with cloud base temperature, suggesting that first echoes tend to appear at some fixed temperature. First echoes in salt seeded clouds appeared on an average 5300 ft above cloud base and showed no significant correlation with any of the cloud parameters mentioned.

It is concluded that both salt and silver iodide seeding can cause first echoes to appear closer to cloud base in unseeded clouds, with salt probably having a more pronounced effect in summertime convective clouds.

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A. S. Dennis and Alexander Koscielski

Abstract

A 3-year randomized crossover seeding experiment has been conducted in South Dakota to test effects of artificial nucleation upon supercooled convective clouds of spring and early summer. The associated rainfall observations have been analyzed by several statistical techniques. The principal conclusions are: 1) on days with isolated showers, rainfall has been heavier in the seeded target area than in the unseeded target area; 2) on days with widespread convective activity and southwesterly winds aloft, rainfall has been lighter in the seeded target area than in the unseeded target area, except for the regions 10–20 miles east of the Black Hills; and 3) on days with widespread convective activity and northwesterly winds aloft, rainfall has been lighter in the seeded than in the unseeded target area.

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A. S. Dennis and H. D. Orville

Abstract

No abstract available.

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K. R. Biswas and A. S. Dennis

Abstract

Salt seeding took place below one end of a line of stratocumulus clouds with 350 lb of NaCl released. Cloud base was 9000 ft and cloud tops were at 15 to 18,000 ft above sea level. Cloud top temperature was near −2C and updraft speeds below the hue were near 3 m sec−1. The resulting shower was monitored by radar with the total rainfall being estimated at 280 acre feet. No rain fell from the unseeded portion of the cloud line or from any other clouds within 50 mi.

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Martin R. Schock and A. S. Dennis

Abstract

Conditions within 16 cumulus congestus clouds were measured on aircraft penetrations near the −5C level and precipitation developments during the subsequent 45 min were noted. Significant correlations exist between the in-cloud conditions and the time of onset and intensity of precipitation. The degree of spatial correlation among the fields of temperature, vertical motion and water content within a given cloud proved more important to subsequent developments than did the mean values of these variables.

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