ity in the equatorial belt of weak pressure gradients and high
temperature, and are commonly associated with the rainy season, being
most common at the beginning and end of the regular rains. In many
places, thunderstorms occur daily throughout their season, with
extraordinary regularity and great intensity.

_Cloudiness._--Taken as a whole, the tropics are not favoured with such
clear skies as is often supposed. Cloudiness varies about as does the
rainfall. The maximum is in the equatorial belt of calms and rains,
where the sky is always more or less cloudy. The minimum is in the trade
latitudes, where fair skies as a whole prevail. The equatorial cloud
belt moves north and south after the sun. Wholly clear days are very
rare in the tropics generally, especially near the equator, and during
the rainy season heavy clouds usually cover the sky. Wholly overcast,
dull days, such as are common in the winter of the temperate zone, occur
frequently only on tropical coasts in the vicinity of cold ocean
currents, as on the coast of Peru and on parts of the west coast of
Africa.

_Intensity of Sky-Light and Twilight._--The light from tropical skies by
day is trying, and the intense insolation, together with the reflection
from the ground, increases the general dazzling glare under a tropical
sun. During much of the time smoke from forest and prairie fires (in the
dry season), dust (in deserts), and water-vapour give the sky a pale
whitish appearance. In the heart of the trade-wind belts at sea the sky
is of a deeper blue. Twilight within the tropics is shorter than in
higher latitudes, but the coming on of night is less sudden than is
generally assumed.

_Climatic Subdivisions._--The rational basis for a classification of the
larger climatic provinces of the torrid zone is found in the general
wind systems, and in their control over rainfall. Following this scheme
there are: (1) the equatorial belt; (2) the trade-wind belts; (3) the
monsoon belts. In each of these subdivisions there are modifications due
to marine and continental influences. In general, both seasonal and
diurnal phenomena are more marked in continental interiors than on the
oceans, islands and windward coasts. Further, the effect of altitude is
so important that another group should be added to include (4) mountain
climates.

[Illustration: FIG. 4.--Annual march of temperature: equatorial type. A,
Africa, interior; B, Batavia; J, Jaluit, Marshall Islands.