METEOROLOGICAL ANALYSIS IN MIDDLE LATITUDES 
in more complicated synoptic situations and at high 
tropospheric levels is not so easy. What Riehl and La- 
Seur did, essentially, was to determine an appropriate 
lapse rate between 700 and 300 mb for each typical 
synoptie flow pattern. Then using the 700-mb chart as 
a guide and the proper, calculated, lapse rates, Riehl and 
LaSeur were enabled to obtain reasonable estimations 
of upper-atmospheric pressures up to 300 mb. One 
decided advantage of their technique is that the lapse 
rates, being statistically determined, are obtained inde- 
pendently of the mean temperatures in the layer from 
700 to 300 mb. Since, however, these two quantities are 
so closely related, mean temperatures may be used as 
a check on the accuracy of the extrapolated hypsometri- 
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tion, the direction in which that pressure system is 
moving. Notwithstanding the very definite limits of the 
spatial range throughout which the deductions drawn 
by this method can be validly applied, an analysis, 
which for practical purposes is unique, can be obtained 
for a considerable region about each point where the 
extrapolation with respect to time is carried out. This 
area for which the extrapolation is valid varies with the 
synoptic situation. Details of the interpretations of the 
changes of various data with time are found in the 
studies of single-station analysis made at the Uni- 
versity of Chicago in the early 1940’s [14]. More recent 
applications of this sort of extrapolation for daily anal- 
ysis have been made in such diverse regions as the 
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Fie. 1.—Example of observed thickness patterns in the vicinity of several different frontal systems. Red lines are 1000-700-mb 
thickness; black lines, sea-level isobars. 
cal values of the 300-mb surface, contributing in no 
small way to the value of the subsequent analysis. 
There is still another approach to the problem of the 
analysis of regions of exiguous data, based not on 
horizontal nor vertical extrapolation, but rather on 
extrapolation with respect to time. When the surface 
data are particularly scant, vertical extrapolation is 
possible only at widely separated points and is therefore 
incapable of providing sufficient data to insure a unique 
analysis. Likewise, under these particular circumstances 
the technique of extrapolation based on the horizontal 
distortion of thickness lines is of but meager produc- 
tivity. When these are the circumstances, reliance upon 
extrapolation at a point with respect to time produces 
the optimum analytical results. 
The quintessence of the information derived from 
the technique of time extrapolation is the structure of 
the pressure system passing a single point and, in addi- 
Aleutians, where time cross sections were used to aid in 
analyzing the vast unpeopled expanses of the Pacific 
Ocean, and in the tropics. Riehl [18] recently published 
a paper elucidating the use of extrapolation with respect 
to time in the solution of problems of tropical analysis. 
In the field of analytical research, time cross sections 
are tools which have shed light on countless perplexing 
questions. In research, one particular class of time cross 
section has enjoyed widespread and manifold uses [10]. 
This consists of a graphical representation of some 
element (¢.g., pressure) plotted on a diagram in which 
time is used as an ordinate and a spatial unit (e.g., longi- 
tude) is the abscissa. In this hypothetical case, lines 
could be drawn on the graph portraying the position of 
troughs and ridges with respect to time. The divergent 
uses to which this sort of analysis has been put attest 
to its value in clarifying significant meteorological re- 
lationships. 
