34 JEROME NAMIAS 
AVERAGE U.S. PRECIPITATION 
3.5) 
3.0) 
2.5) 
2.04 
INCHES 
0.5) 
=! 
JAN FEB MAR APR MAY JUNE = JULY 
AUG SEPT. OCT.T INOV. DEC JAN FEB 
Fra. 2—Annual course of average monthly precipitation (period 1895-1952) 
received over the United States and that received in October 1952, the 
driest month of record 
almost complete absence of cyclones over the 
country; the few which did form or enter were 
very weak and soon died (ends of arrows). 
Another of the interesting characteristics of the 
October 1952 regime was the small variability of 
five-day mean patterns during the month. In 
other words, here was a pattern not only recur- 
rent but also persistent. Yet there was consider- 
able day-to-day activity in the form of rapidly 
moving fronts and anticyclones into and through 
the United States. 
Even though it was well predicted, it is not 
possible in this short report to describe how this 
peculiar pattern became established, for this was 
contingent upon a long sequence of events inter- 
woven with the fabric of climatological (seasonal) 
response. However, it is quite clear that during 
this particular month anomalous factors of many 
kinds must have been operating in concert with 
normal climatological factors. Thus we are deal- 
ing with a type of resonance phenomenon. Such 
cases obviously deserve much greater study from 
those who aspire to modify weather on a large 
scale. 
A sharp change from a dry to a wet regime— 
Occasionally extended dry periods terminate 
abruptly and a regime appreciably wetter than 
normal ensues. The layman used to look upon 
these events as nature’s way of providing com- 
pensation; but in recent years such events are 
as likely to be attributed to man’s interference 
with the atmosphere, perhaps by atomic bombs 
or through the machinations of rainmakers. It is 
therefore important that the meteorologist, at 
least, be aware of the fact that nature frequently 
provides mechanisms for establishing mm sequence 
two quite contrasting weather regimes. 
In the following case an abnormally dry situa- 
tion over the Ohio Valley in December 1958 
(Fig. 5) was replaced by a flood-producing re- 
gime in January. Actually most of the rainfall 
occurred in the latter part of January. We shall 
first describe, in terms of 15-day means, the de- 
velopment of the mid-tropospheric general cir- 
culation which led to both regimes, next show 
how a general instability in pattern was created, 
and finally indicate how the pattern was trans- 
formed into a fairly stable but radically different 
one. 
The sequence of 15-day mean 700-mb maps 
from the last half of November 1958 to the last 
half of January 1959 is shown in Figure 6. Wind 
speed profiles corresponding to these charts are 
shown in Figure 7. Note that the wave length 
between United States and east Pacific troughs 
gradually increased from longitude 73° (at 
40°N) to 99° as the peak strength of the west- 
erlies increased up through the last half of De- 
cember. Following the sharp decline in the west- 
erlies during the first half of January, a radically 
different wave pattern emerged over the western 
hemisphere during the last half of January, when 
a new trough formed in the area extending from 
the Great Lakes southwestward to Texas. Ap- 
parently, in accord with Rossby’s [1939] expres- 
sion of stationary wave lengths related to zonal 
