136 
AIR MASS ANALYSIS 
xX. ISENTROPIC ANALYSIS * 
JEROME NAMIAS 
INTRODUCTION 
N AIR MASS and frontal analysis 
] use is made of “indirect aerology”’ 
—a technique of deducing chiefly 
from observations of clouds and hy- 
drometeors the nature and structure 
of the atmosphere above the ground. 
This method supplies a good deal of 
information essential to weather anal- 
ysis and forecasting, and is especially 
valuable where direct observations of 
the upper air are lacking. Where a 
fairly dense network of upper-air 
soundings is available, however, in- 
direct aerology naturally must give 
way to the consideration of observed 
conditions. In the United States we 
are fortunate in being able to make 
use each day of some thirty radio- 
sonde observations, and it appears 
that this number will increase from 
time to time. Indirect aerology is 
thus being forced more and more into 
the background; but there arises the 
problem of how to use effectively 
these upper-air data in the daily rou- 
tine of analysis and forecasting. The 
current practice is all too often to 
carry out an analysis of surface 
weather maps, afterwards using the 
upper-air data merely as a check. 
This form of treatment, it hardly need 
be stated, rarely leads to the develop- 
ment of new and directly usable ideas 
for interpreting the soundings. 
Appreciable progress in the use of 
upper-air data has recently been made 
by Rossby and his colleagues [3]* at 
the Massachusetts Institute of Tech- 
nology; their technique, called isen- 
tropic analysis, goes further than 
merely supplementing the surface 
analysis of air masses and fronts; it 
brings to light much entirely new 
knowledge of the physical processes 
at work in the atmosphere. Isentropic 
analysis has already become an in- 
tegral part of the modern forecaster’s 
technique, and is now widely used in 
the United States. 
$1. BASIS FOR THE ANALYSIS 
In Articles II to IV we discussed the 
need in synoptic meteorology for con- 
servative elements by means of which 
parcels of air may be identified from 
day to day. It was pointed out that 
two of the most conservative of these 
elements are potential temperature 
and mixing ratio or specific humidity, 
both of which do not change during 
adiabatic processes as long as the air 
remains unsaturated, and as long as 
turbulent redistribution of heat and 
moisture may be neglected. These 
two quantities are used as coordinates 
of the Rossby-diagram [1]*, and if the 
points of an aerological sounding are 
plotted on such a diagram we obtain 
1See references at end of this chapter. 
the “characteristic” curve. This curve 
is unaltered during any adiabatic 
process involving the same particles, 
for in this case the individual points 
of the curve stay fixed. Thus the 
characteristic curve may be used to 
identify vertical air columns as they 
move over the surface of the earth. 
Under the assumption made above, 
each potential temperature in a series 
of characteristic curves obtained at 
different stages in the history of a 
moving air column will then be char- 
acterized by a practically unchang- 
*Reproduced, with changes, from the MS 
of the chapter under the same title which ap- 
pears in the book ‘‘Weather Analysis and 
Forecasting,’’ by kind permission of Prof. 
Sverre Peterssen and the McGraw-Hill Book 
Company. 
