An Investigation of Clouds and Precipitation for the Texas High Plains Page: 4
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II. THE CLOUD CENSUS
The cloud census is based on ten years (1952-1961) of three-hourly
cloud data from Amarillo (AMA), Lubbock (LBB), and Midland (MAF). The
locations of these stations are shown in Figure 1. The selection of this
particular ten-year period was based on its inclusion of periods of both
above and below-normal precipitation for the area. This is illustrated in
Figure 2 which shows yearly departures from the 40-year (1926-1965) average
precipitation as a function of time. Note that the period from 1952 to 1956
was relatively dry, while precipitation during the latter five years was,
for the most part, well above normal. A brief climatic summary for each of
the stations is given in Appendix I. The results of the cloud census for
Amarillo and Lubbock are based on the entire ten-year period. Cloud t pe
observations were not made at Midland during 1952 and 1953, thus the results
for this station are based on only eight years of data (1954-1961). Cloud
amounts for Midland are based on the entire ten-year period.
A. Definition of Cloud Types
From the macrophysical point of view, a cloud is the result of
cooling moist air to a temperature below its dewpoint. In the atmosphere,
this cooling is almost always brought about by lifting of the air, which
cools during expansion as the pressure falls. A comparison of the temperature
lapse rate in cloud-forming ascent with the temperature structure of the
environment determines whether or not the ascending mass will be buoyant.
During buoyant ascent, when the surrounding temperature is lower
than the cloudy updraft, upward acceleration is enhanced, thus favoring the
formation of clouds with large vertical extent. The term "penetrative
convection" is used to convey the idea that a small volume of cloud air pene-
trates vertically through a region of relatively stable air. Clouds formed
in this manner are called cumulus clouds. In some cases only small cumulus
clouds form. As the clouds grow to higher altitudes, clear, dry air mixes
into the clouds and causes them to be chilled by evaporation of the cloud
droplets. When this occurs, the cloud stops growing and evaporates. Small
cumulus clouds normally last only about five to fifteen minutes (Mason, 1957).
On the other hand, if the atmosphere is very moist and unstable, the rising
air may be accelerated as it moves to higher altitudes. Updrafts exceeding
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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/m1/16/: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.