An Introduction to 
The Study of Air Mass and Isentropic Analysis 
By JEROME NAMIAS 
INTRODUCTION TO THE 5TH EDITION 
HE SYSTEM of weather analysis 
developed chiefly by the Norweg- 
ian school of meteorologists and 
referred to as “Air Mass Analysis” in 
the United States, has received wide- 
spread adoption throughout the me- 
teorological services of the world. In 
addition to the group of professional 
meteorologists who employ’ these 
methods as the foundation of their 
activities in synoptic meteorology, 
there has developed a large group of 
people whose interests are so intimate- 
ly associated with meteorology that it 
is necessary for them to possess more 
than a fragmentary knowledge of the 
physical processes of the atmosphere. 
The increasing number of aviation 
enthusiasts is only one of these groups. 
Many such people, and indeed, many 
practicing professionals at present en- 
gaged in meteorology, have not had the 
time nor the opportunity to carry on 
an organized collegiate program of 
study in modern synoptic meteorology, 
and for this reason have felt the need 
for some simplified presentation of the 
fundamentals upon which the science 
rests. It has been the purpose of this 
series of articles to fulfill this gap in 
such a manner that these students may 
be able to obtain a physical picture of 
basic weather processes without first 
having to possess a mastery of ad- 
vanced physics and mathematics. Since 
weather forecasting is still quite re- 
moved from the quantitative stage, 
and since a qualitative evaluation of 
the various entering factors consti- 
tutes a large share of the forecaster’s 
technique, it is possible to develop a 
moderate degree of forecasting ability 
with an understanding of the physical 
processes as described in these articles. 
The actual technique of air mass and 
isentropic analysis can hardly be im- 
parted adequately by written material. 
It requires a more personalized guid- 
ance by an experienced analyst. 
While textbooks in modern synoptic 
meteorology have been vastly improved 
since many of these articles were first 
written, notably by the works of Byers, 
Taylor and Sutcliffe, there has con- 
tinued a demand for these articles, and 
more recently for some similar presen- ~ 
tation dealing with the new method of 
upper-air analysis along the surfaces 
of constant entropy. Professor S. Pet- 
terssen (of M.I. T.) and the McGraw- 
Hill Company have been so kind as to 
permit me to publish here, with some 
small alterations, the chapter on 
isentropic analysis originally prepared 
for his textbook on Weather Analysis 
and Forecasting (McGraw-Hill, N. Y., 
1940). 
I. CONDITIONS OF ATMOSPHERIC STABILITY: LAPSE RATES 
A. VERTICAL DISPLACEMENT OF 
A PARTICLE 
It has long been known that verti- 
cal motions in the atmosphere are of 
great significance in that practically 
all precipitation may be ascribed to 
the condensation brought about 
through expansional cooling of rising 
air. It can be shown that the amount 
of precipitation possible through the 
mixing of currents of air possessing 
different temperature and moisture 
characteristics is very small, and 
that the precipitation resulting from 
