HYDROMETEOROLOGY IN THE UNITED STATES 
adiabatic atmosphere, represented by carefully selected 
surface index stations, probably does not cause im- 
portant errors, investigations into the actual structures 
in major storms may result in significant refinements 
in the rainfall equations. 
The assumption probably open to the most serious 
question concerns the space-time distributions of wind 
in the major storms of record and in the maximum 
possible storm. Observations of upper winds made 
during periods of both fair and rainy weather are re- 
quired from stations located near the tops of mountain 
ranges. Theoretical studies of the structure of air 
flowing over mountain barriers should continue, special 
effort being directed toward the theoretical determina- 
tion of wind structures over barriers during conditions 
prevalent in major storms. The effects of variations in 
the geometry of a basin on the wind flow also deserve 
much study. In other words, the validity of the common 
assumption of two-dimensional flow should be deter- 
mined. 
The effects of a mountain barrier upon the wind 
extend not only vertically but also horizontally away 
from the barrier. In the case of very simple obstacles to 
wind flow, aerodynamic theory can accurately predict 
distortions of straight flow. In meteorology, however, 
the obstacles are irregular mountain peaks, the un- 
disturbed flow pattern is in itself complex, and the fluid 
is characterized by stratifications of density and hu- 
midity. Present meteorological theory is not adequate 
for the determination of distortions in the wind pattern 
both upwind and downwind from a barrier except in the 
qualitative sense that the air begins to rise, on the 
average, before the barrier is reached, and descends for 
some time after it has been passed. It is evident that 
some of the orographic rain must be produced, and must 
fall, at some distance upwind from the barrier. If a con- 
siderable body of rainfall data is available, the “upwind 
effect” can be treated empirically. Otherwise, assump- 
tions must be made as to the structure of the wind 
system, whereupon theoretical computations can be 
made. The upwind-effect problem is only part of the 
general problem pertaining to flow of air over barriers 
upon which both statistical and theoretical investiga- 
tions should be continued. With appropriate modifi- 
cations of the Reynolds number, laboratory studies 
with models patterned after typical orographic regions 
could also lead to valuable results. 
Spillover. Since vertical motions which produce rain 
are inevitably associated with horizontal motions, it is 
evident that rain formed at a point in the atmosphere 
will, in general, reach the ground at a spot not vertically 
below the point of formation. In the case of orographic 
rainfall, where the wind systems which carry the rain- 
drops along are usually fixed with respect to topographic 
features, the effect is called spillover. The importance of 
spillover relative to other types of orographically pro- 
duced rainfall is illustrated in Fig. 38, which contains 
profiles of rainfall depths in three major storms across 
the San Joaquin Valley [29]. Theoretical computations 
of spillover are based upon the following principal 
assumptions. For each basin two critical raindrop paths 
1039 
can be defined, one at the inflow and the other at the 
outflow end of the basin, such that all rain formed 
between them will fall within the basin. Rain formed 
outside of the space between the two paths will fall out- 
side of the basin. The paths can be based on the as- 
sumption of an average raindrop terminal velocity 
corresponding to that observed in heavy rains, and the 
horizontal speed of each drop can be assumed to be 
that of its environment. As with the total orographi- 
cally produced rainfall, the reliability of the spillover 
ec) Lh hit 
UPWIND 
ia 
OROGRAP 
Pere 
INFLOW | __ 
SPILLOVER 
20 
JAN, 19-24, 1943 7 
t SS 
ATS 
A 
1 
DEC. 9-12, 1937 fk 
PRECIPITATION (IN.) 
/ 
“Ls 
(e} 25 50 75 
DISTANCE ALONG Y (MI.) 
Fig. 3.—Storm profiles, San Joaquin River Basin, California 
125 
100 
computations depends upon the accuracy with which 
the upper-air flow patterns can be reproduced. The 
importance of spillover increases as the drainage area 
decreases; for the average small basin of about 100 
square miles, spillover may produce a component as 
high as about 10 per cent of the total rain caused by 
orography. It is probable, therefore, that errors in the 
assumptions produce computational inaccuracies of only 
a few per cent. Spillover can become highly significant 
for very small basins, however, hence research on this 
problem is desirable. Refinements in the theory can un- 
doubtedly be attained through application of physical 
and statistical-mechanical approaches and by collection 
of data relative to raindrop-size spectra and fall 
velocities in rainstorms of large magnitudes. 
NONOROGRAPHIC RAINFALL 
Storm Transposition. In regions of rugged terrain, 
computations of rainfall are facilitated by an accurate 
knowledge of the dimensions of the mountain barriers 
which play such a dominant role in air flow of the lower 
atmosphere. On the other hand, tests of rainfall formulas 
for a particular basin may nearly always be carried out 
through use of precipitation data collected within that 
