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G. J. Stensland and R. G. Semonin

Concern about the apparent increase in the acidity of rainfall from the 1950s to the 1970s prompted reexamination of data from the intermittent, short-term sampling networks that are the basis of the trend estimates. A reassessment of precipitation chemistry data for the mid-1950s reveals excessively high values of calcium and magnesium in comparison with current measurements. The most likely explanation is the severe drought and duststorms that much of the United States experienced in the 1950s. When these excess soil loadings are adjusted within reason to nondrought conditions, newly calculated pH values for this period are not much different from those in recent years. These results suggest that the downward pH trend due to the increase in acid-forming emissions since the mid-1950s is much smaller than previously estimated.

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R. G. Semonin and R. E. McCrady

Abstract

A semiautomatic aerosol sampler, designed to use Millipore filter material, is described. The operating mechanism is an adaptation of an automatic slide-changer assembly. By enclosing the sampler in an aerodynamic housing, the device is suitable for light aircrat sampling operations. Minor modifications of the instrument will permit ground sampling for air pollution research.

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F. A. Huff and R. G. Semonin

Abstract

Assuming that successful precipitation modification could be achieved under atmospheric conditions favorable for the development of natural precipitation, a three-phase study was made to acquire information on the potential for alleviation of moderate to severe droughts in Illinois. The first phase involved time-space analyses of monthly precipitation characteristics in previous major droughts of 12- to 24-month duration. In the second phase, detailed analyses were made of various storm properties during the 1953–54 drought, one of the worst on record in Illinois. The third phase involved use of a one-dimensional cloud model to investigate cloud seeding potential in the 1953–54 drought. Overall, results indicated that atmospheric conditions are favorable for natural precipitation development frequently enough in most droughts that successful cloud seeding operations could contribute occasionally to temporary alleviation of water shortages over portions of an extensive drought region, particularly with respect to agricultural needs.

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Robert Cataneo, John R. Adam, and Richard G. Semonin

Abstract

In the development of raindrops from cloud droplets in warm rain, the collision-coalescence process is considered to be the main growth mechanism for droplets of unequal size greater than 20 μm in diameter. However, due to the wake effect, the possibility of equal-sized droplets colliding does exist for some maximum vertical separation of the droplets. An empirical study has been performed which led to the determination of the maximum vertical separation required, as a function of droplet size, for equal-sized droplets to be influenced by the wake effect.

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S. A. Changnon Jr., R. G. Semonin, and F. A. Huff

Abstract

METROMEX was the first major field program aimed at studying the reality and causes of urban rainfall anomalies suggested in several climatological studies. The results from the 1971–74 METROMEX data portray statistically significant increases in summer rainfall, heavy (>2.5 cm) rainstorms, thunderstorms and hail in and just east (downstorm) of St. Louis. Examination of the rainfall yield of individual showers (cells), the spatial distribution of echo (rain) developments, and areal distribution of afternoon rain clearly point to the urban-industrial complex as the site for the favored initiation of the rain process under certain conditions. The greater frequency of rain initiations over the urban and industrial areas appear to be tied to three urban-related factors including thermodynamic effects leading to more clouds and greater incloud instability, mechanical and thermodynamic effects that produce confluence zones where clouds initiate, and enhancement of the coalescence process due to giant nuclei. Case studies reveal that once additional cells are produced, nature, coupled with the increased likelihood for merger with more storms per unit area, takes over and produces heavier rainfalls. Hence, the city is a focal point for both rain initiation and rain enhancement under conditions when rain is likely.

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