An Investigation of Clouds and Precipitation for the Texas High Plains Page: 75
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75
cirriform clouds, the data for cloud frequency are shown graphicalLy in Figures
22a, 22b, and 22c. Cirrus clouds were omitted because of the observational
difficulties which would tend to make these data non-representative.
In each case, solid lines in the figures represent cloud frequency
for the dry months and the dashed lines denote cloud frequency for the wet
months. In the case of stratus, the frequency was higher for the wet month at
both Amarillo and Lubbock. The large differences in the spring are accounted
for in large part by early morning stratus indicative of low-level moisture
during the wet months. This is true to a lesser extent of the other months
also. Fall stratus occurs also in association with cyclonic activity. It is
obvious that stratus is characteristic of the wet months and is practically
nonexistent during dry months.
In the case of stratocumulus, there is a significant difference between
wet and dry months in the spring and fall with low frequencies and smaller differ-
ences in the summer. The only case in which the dry month frequency was higher
than the wet month frequency was in August at Lubbock. Recall that this month,
August 1957, was really not characteristic of wet Augusts in the plains.
As expected, cumulonimbus development is considerably greater during
the wet months. On only one occasion, August at Amarillo, could the difference
be considered insignificant. Once again the exception occurs during August.
Only 33% of the Augusts at Amarillo had less precipitation than August 1957.
It is interesting to note in Figure 22b that cumulus tends to have a
higher summer frequency during the dry months, especially at Amarillo. Even
during the spring there is very little difference between wet and dry months.
This indicates that convective development was initiated during the dry months
but, in most instances, was not strong enough to produce cumulonimbus clouds
and subsequent precipitation. One might speculate at this point on the cloud
modification possibilities offered by this situation, where cumulus clouds are
plentiful but are not able by natural means to reach the precipitation stage.
It is true that the natural precipitation efficiency (fraction of condensate
that reaches the ground as precipitation) tends to be quite low for isolated
cumulus clouds over flat terrain (Braham, 1952). This is due in part to the
dry environment through which these clouds must penetrate. Entrainment of dry
ambient air causes evaporation of the cloud droplets and consequently halts
the growth of the cloud. It may be that these adverse environmental interactions
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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/m1/87/?q=%22~1~1%22~1: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.