LARGE-SCALE SYNOPTIC PROCESSES UNDERLYING PRECIPITATION 
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Schematic model for winter precipitation associated with 700-mb 
mean patterns (after Alezn [1948]) 
day mean maps is reproduced in Figure 1. There 
is some carry-over of heavy precipitation into 
the confluent or frontogenetic zone associated 
with the out-of-phase wave trains. A thorough 
explanation of the features of this model is diffi- 
cult because it is empirically derived and is the 
net result of many factors and scales of motion. 
For example, the band of heavy precipitation 
east of the trough is associated with: (1) the 
gradual upslide motions found to the east of pro- 
gressive planetary trough systems, (2) the more 
concentrated regions of upward motion associ- 
ated with the cyclones which in turn are generally 
related to, and interactive with, the planetary 
wave systems, and (3) the very concentrated ver- 
tical ascent associated with local convective cells 
within the unstable air masses of the cyclones. 
The magnitudes of the vertical motions produced 
by these three differing scale phenomena are re- 
spectively of the order of <1 em/see, 2-20 cm/ 
sec, and 5 to 50 m/sec. 
We shall now discuss special cases illustrating 
the role of mean planetary circulations in creating 
abnormal precipitation regimes. 
The driest month in the meteorological history 
of the United States—In October 1952 the total 
precipitation which fell over the entire United 
States was the lowest of some 60 years of record 
[Winston, 1952]. This fact was determined by 
examining a routinely computed statistic ob- 
tained by combining state-averaged rainfall into 
a country-wide average, using appropriate 
weights for areas. The relation of October 1952 
to the annual course of monthly normals (aver- 
ages for 60 years) is graphically represented in 
Figure 2. Thus the rainfall of October 1952 was 
only about one-fourth the amount normally re- 
ceived over the United States in October, the 
deficit amounting to about 3.3 x 10" tons of 
water lost to the nation. It is not surprising to see 
that this record dryness occurred at the time of 
year when the normal minimum is reached and 
when the westerlies and cyclone belts are usually 
far north and vertical stability is at a maximum. 
The drought regime of this month is nicely re- 
lated to the macro-scale average monthly features 
shown in Figure 3 by the mean 700-mb contour 
pattern. This wave pattern has the same phase 
as the normal October pattern but its amplitude 
is much greater. The net effect of this oc: 
tion is to: (1) deploy Canadian Polar air masses 
rapidly southeastward into eastern United States, 
resulting in anomalous cold air in the east with 
the polar front frequently found off the eastern 
seaboard; (2) effectively shut off most of the na- 
tion (except Florida) from access to moist air 
from the Gulf of Mexico; and (3) shunt cyclones 
away from borders of the United States. 
The last effect is strikingly illustrated by the 
tracks of cyclone centers (Fig. 4). There was an 
