HYDROMETEOROLOGY IN THE UNITED STATES 
cally tenable. This concept requires considerable theo- 
retical investigation. 
Adjustments for Other Factors. In order to reach a 
project drainage area where rain is to fall, air often 
must first be lifted over a rise in the ground surface. As 
in the case of the single-barrier, orographic-rain prob- 
lem, moisture is removed from the air durig its passage 
over the higher ground. Thus, in treating situations 
where either a mountain barrier or a long, gentle rise 
in ground is in the way of air flow toward a basin, it is 
assumed that a depth of precipitable water contained in 
a saturated, pseudoadiabatic column of height equal to 
that of the barrier is removed from the total quantity 
of moisture initially available for precipitation. The 
further assumption is made that the barrier—although 
it may be effective in producing more frequent storms— 
does not act to alter the wind structure of a transposed 
storm. It is evident that use cannot be made of the 
barrier adjustment when a project basin is so close to 
the downwind side of a barrier that spillover is effective 
in increasing the precipitation within the basin. The 
approximate reduction effect of a barrier is one per cent 
for each 100 ft of barrier height. Statistical tests of the 
propriety of the barrier adjustment indicate that the 
adjustment is of the right order of magnitude. 
Wind adjustments have been attempted, but the 
results lack the consistency which marks the moisture 
and barrier adjustments. Two principal difficulties arise 
in this connection. First, because minor topographic 
effects have such a profound influence on the surface 
wind, it has not been possible to determine reliable 
wind indices for either the strengths of the currents in 
storms or the maximum strengths which can be ex- 
pected in a given locality. Second, it is likely that 
changes in index-station wind would produce funda- 
mental changes in the nature of a storm by causing 
important alterations in the space and time distributions 
of winds. 
McCormick [13] has recently shown that there is a 
significant downward trend of peak rainfall depths as 
latitude increases. The effect increases as the area over 
which the depths are averaged increases; for point- 
rainfall it is so small as not to be statistically significant, 
but for depths over an area of 2000 square miles the re- 
duction is in the neighborhood of 2 per cent per degree of 
latitude between latitudes 30°N and 60°N. Variation 
with rainfall duration was Jess significant, although 
there was a tendency for the importance to increase 
with duration. It is possible that there is a simple 
physical explanation for the latitude effect. For instance, 
if the Coriolis acceleration is dominant in the balance 
of forces in maximum storms, then, since wind appears 
to the first, power in the equation of continuity, rainfall 
depths should vary inversely with the sine of the 
latitude. 
McCormick also found the quite unexpected, yet 
statistically significant, fact that the maximum ob- 
served depths of rainfall in the United States decrease 
sharply south of latitude 30°. It is possible that existence 
of the northern Gulf of Mexico coast—also at latitude 
30°—is responsible for this behavior. On the other hand, 
1041 
it is possible that the 30°-peak of rainfall is associated 
with extreme southward displacements of the meander- 
ing jet stream of middle latitudes. Rainfall variations 
with latitude appear to be important. Thus, with north- 
ward or southward transpositions of storms, a latitude 
adjustment may be in order. The proper nature of the 
adjustment, however, must await further physical and 
statistical research. 
When allowance is made for latitude and atmospheric 
moisture content of the major storms of record in the 
United States, a map of the peak values discloses several 
meridional maxima, the chief of which extends north- 
south approximately along the 98° meridian. It may be 
more than coincidence that the western extremity of the 
Gulf of Mexico is at about the same meridian. There 
also may be a certain critical wave length—eastward 
from the Rockies—of the zonal west winds which, if 
attained, can be associated with storms which produce 
greater rainfalls than do storms associated with other 
wave lengths. The problem of longitudinal variation of 
maximum rainfall deserves critical attention in the 
light of its possible use in connection with storm trans- 
position. 
Statistical treatments of many other variables should 
be carried through to determine the existence and 
probable nature of consistent variations. Hand in hand 
with the statistics, however, theoretical research should 
attempt to place the variations on sound physical 
bases and to determine the degree to which one type of 
adjustment is dependent upon another. The ultimate 
objective of the adjustment theory is the expression of 
rainfall as the product of a number of independent 
factors, each a function of a particular (measurable) 
variable, such that all storms can be reduced to a 
common denominator. 
Isobar-Isotherm Relationships. For storms in which 
the rain-producing air enters at one side of the rainfall 
area and leaves at the other, it follows that the rainfall 
depths will be greater the more the inflow winds are 
confined to low levels and the outflow winds to high 
levels. Under the assumption that the geostrophic wind- 
shear equation holds approximately, the wind-stratifi- 
cation system of a heavy rainstorm is such that air 
entering the system does so with higher temperatures 
to the left of the current axis and lower temperatures 
to the right and leaves with the reverse distribution of 
temperature. A different sort of rainfall-favoring pat- 
tern, in which mass convergence takes place at low 
levels and mass divergence aloft, is characterized, on the 
inflow side, by anticyclonically curved isobars at low 
levels and cyclonically curved isobars aloft and, on the 
outflow side, by anticyclonically curved isobars aloft 
and cyclonically curved isobars below. 
The patterns described above may be combined to 
form an isobar-isotherm configuration which is theo- 
retically an important rain producer and which has been 
observed on synoptic maps in varying degrees. For 
example, combination of a warm, A-shaped trough in 
Texas and a cold cyclone in more northern states meets 
most of the requirements. With such a configuration, 
the heaviest rain should fall to the east of the line 
