An Investigation of Clouds and Precipitation for the Texas High Plains Page: 62
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water with the expected values derived from Baker's data for some of the major
precipitation periods. The results of this study are shown in Tables 3, 4, and
5 for spring, fall, and summer, respectively. Reitan (1957) has shown that the
summer rainfall in Arizona and the southwest is primarily determined by the
moisture content of the air over the state. An inspection of the tables
indicates that above-normal precipitable water is also characteristic of signi-
ficant rainfall in the Texas plains. Column 8 in the tables gives the percent-
age of time that precipitable water would be expected to be greater than the
observed value based on the distribution derived by Baker. On only 10% of the
days was the value of precipitable water less than the median of the distribution.
On the average, the probability of a day with greater precipitable water was 230
for spring and summer and 15% for the fall.
E. Atmospheric Stability
In dealing with atmospheric motions responsible for the production
of clouds and precipitation, it is important to realize the role played by
stability. Showalter (1953) has derived an extremely simple, thermodynamically
sound and easily understood tool for making a very rapid check on convective
instability.
The Showalter stability index is computed as follows: The 850 mb
parcel is lifted dry adiabatically to saturation and then pseudo-adiabatically
to 500 mb. The lifted 500 mb temperature is then subtracted algebraically
from the observed 500 mb temperature. A negative number indicates instability
(rising air warmer than its surroundings) and a positive number indicates sta-
bility in the atmospheric layer from 850 mb to 500 mb. Experience has indicated
that any positive index of 30 or less is very likely to be associated with
showers and quite likely to produce thunderstorms. Thunderstorms have increasing
probability as the index falls from 10 to -20. Negative values of -30 or greater
may be indicative of severe thunderstorms.
Stability indices were computed for the selected rain periods discussed
earlier and are given in column 9 of Tables 3, 4, and 5. Note that the values
are lowest during summer and highest in the fall. Admittedly, the determination
of the Showalter index in the colder months of the year is somewhat dubious,
but it does serve to illustrate a comparison between wet and dry periods. Sta-
bility during dry periods will be discussed in Sections IV and V. Based on the
criterion that any positive index of 30 or less is very likely to be associated
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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/m1/74/: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.