Synoptic and Planetary Scale Phenomena Leading to the 
Formation and Recurrence of Precipitation 
JEROME NAMIAS 
U. S. Weather Bureau, Washington, D. C. 
Abstract—This paper describes certain macroscale features (in both space and time) 
that play a dominant role in setting the stage for vertical motions, without which it is 
impossible to consider precipitation problems. First, an empirical relationship of broad- 
scale total precipitation fields to the geometry of time-averaged flow patterns in mid- 
troposphere is discussed and to some extent interpreted in terms of the interaction of 
synoptic and planetary scale systems. Contrasting ‘regimes’ of the order of a month, in 
which weather processes persistently recur, are illustrated. The amazingly stable regime 
of October 1952, the driest month in the meteorological history of the United States, is 
one of these. Another is the period December 1958 through January 1959, when a pre- 
vailingly dry regime changed abruptly into a wet one. The large-scale physical and 
synoptic characteristics of these phenomena are discussed in the light of the fine balance 
of the planetary circulation and against the background of seasonal change. Finally, the 
various precipitation forms of one of the major storms associated with the January 
rain-producing pattern which led to flood rains over the Ohio Valley are interpreted 
with the help of electronically computed charts of vertical motion. 
INTRODUCTION 
Of all the problems of prediction with which 
the synoptic meteorologist or forecaster has to 
deal, those associated with the formation of pre- 
cipitation are probably the most formidable and 
frustrating. For many years, precipitation was 
(and frequently still is) predicted mainly by re- 
lating it to the geometry of isobaric patterns at 
one or more levels as well as to the anticipated 
evolution of these patterns. Alternatively, polar 
front and air mass concepts provided a more 
explicitly physical basis for rainfall prognosis. 
These concepts forced the forecaster to think in 
terms of vertical motion, the main causative fac- 
tor, although this thinking was still indirect and 
qualitative. During the past decade, as a result 
of a great world-wide scientific endeavor to ob- 
jectivize weather prediction, vertical motion has 
begun to receive attention as a direct forecasting 
parameter. The development of high-speed com- 
puting methods is greatly expediting progress in 
this effort. 
At the conclusion of this report some mention 
will be made of the current use of computed ver- 
tical motions in explaining areas of precipitation ; 
a detailed account of this basic problem will be 
found in the following paper by Smagorinsky. 
The present paper will be concerned mainly with 
the large-scale or macro-scale features of the 
general circulation, the centers of action, which 
are perhaps best brought into focus by averaging 
over periods of time long compared to the life of 
an individual cyclone. When such a class of slow- 
evolving atmospheric motions are examined 
against the background of climatology they throw 
a great deal of light upon fundamental problems 
underlying the recurrence of dry or wet regimes 
and the often sharp breaks that occur as one re- 
gime changes to another of entirely different 
character. Furthermore, these large-scale time- 
averaged states appear to set the stage for the 
birth and growth of cyclones in certain areas, but 
not in others. For complete under§tanding of the 
physies of precipitation, certainly of its duration 
and its longer period characteristics, it is difficult 
to see how the large-scale background and its his- 
torical development can be ignored. 
PRECIPITATION Fretps RELATED TO Mip- 
TrRoposPHERIC TIME-AVERAGED 
Firow ParrerNs 
A general model—When time averages of mid- 
tropospheric contour charts for periods ranging 
from a few days to a month or a season are pre- 
pared and related to abnormalities of precipita- 
tion, it soon is apparent that over most areas in 
mid-latitudes heavy amounts are usually found 
on the forward side of troughs and lght on the 
back side. A schematic model resulting from sta- 
tistical studies by Klein [1948] employing five- 
