Thermodynamics applied to air mass analysis
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Since the beginning of 1929 systematic work has been carried out at the Massachusetts Institute of Technology to develop practical methods for the identification and characterization of air masses with the aid of upper air soundings of pressure, temperature and humidity. A brief report on this work was published in October 1930. It was then shown that by plotting against each other two meteorological elements, which under certain well defined conditions are recognized as conservative, namely, specific humidity and potential temperature, curves are obtained which, in winter time, to a high extent remain unchanged and characteristic of the individual air masses. In view of this property the curves were named "invariant curves." Since the invariance is restricted to the winter season, but the curves always may be advantageously used to determine the vertical structure and life history of air masses, they shall, in the following, be referred to as "characteristic curves." The report also stated that by means of characteristic curves a new method had been created of indicating certain differences in stability between the principal American air masses. These differences may be expressed in terms of the variation with elevation of specific entropy, and it was therefore decided to continue the investigation and to include in it a study of the equivalent-potential temperature, which, in an easily comprehensible form, measures the specific entropy of moist air. The excellent results obtained by Robitzsch through the introduction of equivalent-potential temperature into practical meteorological work lent additional support to this decision.
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