eir equatorward margins, by the migrating belt of
equatorial rains and by monsoons; near lands they are often interfered
with by land and sea breezes; in certain regions they are invaded by
violent cyclonic storms. The trades, except where they blow on to
windward coasts or over mountains, are drying winds. They cause the
deserts of northern Africa and of the adjacent portions of Asia; of
Australia, South Africa and southern South America. The monsoons on the
southern and eastern coasts of Asia are the best known winds of their
class. In the northern summer the south-west monsoon, warm and sultry,
blows over the latitudes from about 10 deg. N. to and beyond the
northern tropic, between Africa and the Philippines, giving rains over
India, the East Indian archipelago and the eastern coasts of China. In
winter, the north-east monsoon, the normal cold-season outflow from Asia
combined with the north-east trade, and generally cool and dry, covers
the same district, extending as far north as latitude 30 deg. Crossing
the equator, these winds reach northern Australia and the western
islands of the South Pacific as a north-west rainy monsoon, while this
region in the opposite season has the normal south-east trade. Other
monsoons are found in the Gulf of Guinea and in equatorial Africa.
Wherever they occur, they control the seasonal changes.

Tropical rains are in the main summer rains, coming when the normal
trade gives way to the equatorial belt of rains, or when the summer
monsoon sets in. There are, however, many cases of a rainy season when
the sun is low, expecially on windward coasts in the trades. Tropical
rains come usually in the form of heavy downpours and with a well-marked
diurnal period, the maximum varying with the locality between noon and
midnight. Local influences are, however, very important, and in many
places night rainfall maxima are found.

_Land and Sea Breezes._--The sea breeze is an important climatic feature
on many tropical coasts. With its regular occurrence, and its cool,
clean air, it serves to make many districts habitable for white
settlers, and has deservedly won the name of "the doctor." On not a few
coasts, the sea breeze is a true prevailing wind. The location of
dwellings is often determined by the exposure of a site to the sea
breeze.

_Thunderstorms._--Local thunderstorms are frequent in the humid portions
of the tropics. They have a marked diurnal periodicity, find their best
opportun