An Investigation of Clouds and Precipitation for the Texas High Plains Page: 15
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15
This reflects the winter maximum and. fall minimum in sky cover shown in
Figure 3.
Spring is a transition season which includes clouds due to late
winter frontal activity in addition to clouds due to convective activity which
is being inititated during this season. The secondary maximum in May, shown
in Figure 3, is mainly a result of violent convective activity in association
with squall lines characteristic of this time of year (Staley, 1959). Summer
sky cover is almost entirely due to convective activity. It is the season
with the minimum number of both clear and overcast skies; the frequency of
cumulus and cumulonimbus clouds is quite high during the summer, but rarely
do they cover the entire sky.
The highest frequency of clear skies occurs at each of the stations
during the fall. This is a season of transition between the summer convective
activity and the frontal disturbances which dominate cloud production during
the winter. Clouds which do occur are generally stratocumulus formed by the
merging of slightly developed cumulus clouds, sometimes in association with
weak frontal passages of stratus or altostratus clouds resulting from warm
moist maritime tropical air from the Gulf of Mexico overrunning cooler conti-
nental air at the ground. In the late fall, the cold air often invades the
plains region so repeatedly that there is little opportunity for return flow
of Gulf moisture; these fr ontal systems are usually called "dry northers".
Even under these conditions, however, clouds may form by upslope motion in
the cold air.
Average diurnal variations in cloud amount for the ten-year period
are shown for each month of the year at each station in Figures 7a and 7b.
Note that January, February, March, and April are all characterized by broad
daytime maxima and nighttime minima in cloud cover. The amplitude of the carve
is probably magnified somewhat by the difficulty in observing high clouds
during darkness. During February, March and April, the cloud cover at Amarillo
is greater than at Lubbock or Midland. This is accounted for in large part by
a higher frequency of occurrence for early morning stratus resulting from
nighttime cooling of moist air. The higher moisture content at Amarillo is
also responsible for the higher incidence of afternoon convective activity
which is reflected in the curves. This peak is noted to a lesser extent at
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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/m1/27/: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.