An Investigation of Clouds and Precipitation for the Texas High Plains Page: 43
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43
Once an ice crystal growing in a dense supercooled cloud has become
appreciably larger than a cloud droplet, its growth will be greatly accelerated
by accretion of these droplets which will freeze on impact to form ice pellets
of irregular shape (Mason, 1957). It appears that most rain of extra-tropical
origin which falls from thick layered clouds is initiated almost entirely by
the growth of ice crystals, which however, must aggregate to form snow flakes
if they are to produce raindrops of the observed size.
There are, in general, three types of precipitation, all of which
are characteristic to some extent of rainfall in the Texas High Plains. The
first of these is the intermittent or continuous precipitation from a contin-
uous cloud cover of the altostratus or nimbostratus type. This kind of precip-
itation is caused by the slow upglide of a large mass of air, due to convergence
in the horizontal wind field. The second type is the showers or squalls of
short duration that begin and end suddenly. This kind of precipitation, which
originates from cumulonimbus clouds, is indicative of instability and is caused
by the rapid rising of small bodies of air through the atmosphere (thermals).
The third type is drizzle which falls from low stratus cloud Layers. This
kind of precipitation is indicative of stable air masses and _s not connected
with any appreciable vertical velocity. In fact, the small drops are able
to fall out of the cloud because of the absence of an appreciable upward motion.
A. Texas Precipitation Seasons
In the study of possible methods of modifying Texas precipitation,
it is important to recognize that the sequence of meteorological events leading
to precipitation in one season of the year may not be the same as that producing
precipitation in another season of the same year (Staley, 1959). Over most of
Texas the heaviest rainfall occurs in the spring and fall as shown by the bar
graphs in Figure 15. The graphs at Amarillo, Lubbock, and Midland are charac-
teristic of the plains area. Note that a dominant portion of the annual
precipitation falls in the seven month period from April thru October. Since
this is the critical period as regards water supply, it will receive the major
emphasis in this research.
The heavy rainfall in April and May is usually the result of convective
activity set off by squall lines moving out ahead of frontal disturbances. It
is doubtful that large-scale attempts at increasing this precipitation would be
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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/m1/55/: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.