Abstract:
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.
