belt the cloudiness is high throughout the year,
averaging .7 to .8, with a relatively small annual period. The curve
following, E (fig. 6), is fairly typical, but the annual period varies
greatly under local controls.

[Illustration: FIG. 6.--Annual march of cloudiness in the tropics. E,
Equatorial type; M, Monsoon type.]

At greater distances from the equator than about 10 deg. or 12 deg. the
sun is still vertical twice a year within the tropics, but the interval
between these two dates is so short that the two rainy seasons merge
into one, in summer, and there is also but one dry season, in winter.
This is the so-called _tropical type_ of rainfall, and is found where
the trade belts are encroached upon by the equatorial rains during the
migration of these rains into each hemisphere. It is illustrated in the
curves for Sao Paulo, Brazil, and for the city of Mexico (fig. 5). The
mean annual rainfall at Sao Paulo is 54.13 in. and at Mexico 22.99 in.
The districts of tropical rains of this type lie along the equatorial
margins of the torrid zone, outside of the latitudes of the _equatorial_
type of rainfall. The rainy season becomes shorter with increasing
distance from the equator. The weather of the opposite seasons is
strongly contrasted. The single dry season lasts longer than either dry
season in the equatorial belt, reaching eight months in typical cases,
with the wet season lasting four months. The lowlands often become dry
and parched during the long dry trade-wind season (winter) and
vegetation withers away, while grass and flowers grow in great abundance
and all life takes on new activity during the time when the equatorial
rainy belt with its calms, variable winds and heavy rains is over them
(summer). The Sudan lies between the Sahara and the equatorial forests
of Africa. It receives rains, and its vegetation grows actively, when
the doldrum belt is north of the equator (May-August). But when the
trades blow (December-March) the ground is parched and dusty. The
Venezuelan _llanos_ have a dry season in the northern winter, when the
trade blows. The rains come in May-October. The _campos_ of Brazil,
south of the equator, have their rains in October-April, and are dry the
remainder of the year. The Nile overflow results from the rainfall on
the mountains of Abyssinia during the northward migration of the belt of
equatorial rains.

[Illustration: FIG. 7.--Annual march of temperature: tropical type. W,
Wa