Juxy 19, 1918] 
tains, where it is not needed for agriculture, 
it is deficient on the lowlands, where man has 
to irrigate. However, the aridity of parts of 
the West has some compensation in the ex- 
tensive forests of tremendous trees on the 
soaking slopes of the Pacific. 
SEASONAL DISTRIBUTION OF RAINFALL 
In some respects, the distribution of rain- 
fall throughout the year is more important 
than the amount. On this depends the rain- 
fall usable for agriculture, and likewise the 
effects of rainfall on soil. Thus the 25 inches 
of rainfall in Nebraska are as useful for crops 
as 40 inches in Virginia. In fact, the extra 
15 inches-in Virginia may do more harm than 
good, on poorly kept farms at least, by wash- 
ing and leaching the soil. 
Rainfall comes (1) in general cyclonic 
rains, (2) in local convectional (thunder) 
showers, and (3) in topographically produced 
falls. The cyclonic rains are greatest with 
frequent strong cyclones in regions where 
there is abundant moisture. The thunder- 
showers are most numerous in mid-summer* 
unless at this time the supply of moisture is 
not abundant. The topographically produced 
rains are heaviest when there is the greatest 
cooling of the moist winds. In the United 
States, general cyclonic rains on the Pacific 
coast and in the eastern third of the country 
are heaviest in the colder months. Thunder- 
storms are common in summer in the wetter 
parts of the country west of the Sierra- 
Neyada-Cascades. Topographically produced 
rains are important on the Pacific, Gulf and 
Atlantic coasts, and on the windward sides 
of mountains; they are essentially early winter 
rains. 
Professor Ward has picked out 14 well- 
recognizable rainfall types in the United 
States; and he has made composite curves and 
discussed each.5 The rains east of the Rockies 
tend to be heaviest in summer, and those west, 
in winter. The type covering the greatest 
region is the continental “ Missouri” type. It 
4See Mo. Weather Rev., Vol. 43, 1915, pp. 322- 
340, 13 charts; and pp. 619-620. 
5 Geogr. Review, Vol. 4, 1917, pp. 131-144. 
SCIENCE 71 
has a summer rainy season with a maximum 
of over 4 inches in June and a minimum of 
1 inch in January. This shades off into many 
types on all sides. The Ohio type may be 
considered as the Missouri type with 1 to 2. 
inches of cyclonic rain added through the 
cold half of the year. The New England type 
has still more of the cyclonic winter rain- 
fall, with 8 to 4 inches of rain every month. 
Farther south the Atlantic has an intensifica- 
tion of the July and August rainfall with the 
very favorable moisture conditions for thun- 
derstorms and with the occasional heavy rain 
of tropical cyclones. The Tennessee type in- 
cludes so much rainfall from the strong ey- 
clonic action in February and March that the 
principal maximum, 43 inches comes at this 
time; and there still is the summer maximum. 
The Gulf coast is always moist. There are 
three types of rainfall—different combinations 
of thunderstorm and cyclonic rains—all with 
maximum intensity in the warmer half of the 
year. 
In the East Rocky Mountain Foothills type, 
the rainfall in spring starts off like the Mis- 
souri type, but the winter snows are insuffi- 
cient to supply moisture for increasing thun- 
derstorm rains beyond May. The winters are 
dry in spite of numerous cyclones, because the 
air can contain so little moisture at the low 
temperatures. West of the crest of the Rock- 
ies, the moisture from the Pacific is precipi- 
tated topographically most in winter. In the 
plateau region, summer convection, especially 
before the ground is thoroughly dried, brings 
another maximum early in summer. In the 
south, however, the winter precipitation is so 
light and so soon evaporated that the summer 
showers do not occur till July when moisture 
arrives in sufficient quantity from the Gulf of 
California and the Pacific. On the north 
Pacifie coast where there is much cyclonic ac- 
tivity throughout the winter the maximum 
comes in December (over 7 inches) when the 
topographic rainfall tends to be heaviest. In 
the south, cyclonic activity is more important 
than the cooling of on-shore winds in produc- 
ing rainfall, so the heaviest rains in the 
“ Southern Pacific ” type occur from January 
