An Investigation of Clouds and Precipitation for the Texas High Plains Page: 42
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III. CL OUL AND PRECIPITATION
The search for an adequate physical explanation of the formation of
precipitation elements has been an area of intense investigation for many
years. Bergeron (1935) concludes that of several competing mechanisms, none
is adequate to account for the release of precipitation. He postulated that
the only factor that could explain the growth of precipitation-size drops
was the appearance of a few ice crystals among a large population of super-
cooled droplets in those parts of the cloud where the temperature was below
-100C. In this region, there would exist a supersaturation with respect to
ice so that the ice crystals would tend to grow rapidly. Condensation would
take place continually on the ice, while at the same time liquid water would
evaporate from the droplets. This process would continue until the liquid
phase was entirely consumed. Thus, Bergeron postulated that every raindrop
(d>0.5 mm) originates as an ice particle which grows in this way and therefore
all rain clouds must extend well above the level of the 00C isotherm.
The Bergeron theory, with support from Findeisen, came to be generally
accepted during the following decade. However, it was obvious to most meteor-
ologists that, in tropical regions, showers often fell from clouds whose tops
did not reach the freezing level. Suggestions that rain could be produced
by wholly liquid clouds began to become more prominent. In recent years,
careful visual and radar observations have accumulated which show beyond all
doubt that heavy rain may fall from clouds which are entirely beneath the level
of the 00C isotherm.
Thus it appears that droplet growth to precipitation size can be due
to either or both of two mechanisms
1. the Bergeron process for clouds extending well above the
freezing level, and,
2. pure coalescence for clouds not extending above the freezing
level.
In most instances it appears that a combination of the above mechanisms is
operative. It may, therefore, be a difficult task to decide which of the two
possible mechanisms is responsible for the initiation of the precipitation
process. During the rainy seasons in the Texas plains, it is probable that
both producing processes are operative.42
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Haragan, Donald R. An Investigation of Clouds and Precipitation for the Texas High Plains, report, March 1970; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth839452/m1/54/?rotate=270: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.