s not the equator, but latitude 10 deg. N. The highest
mean annual temperatures, shown by the isotherm of 85 deg., are in Central
Africa, in India, the north of Australia and Central America, but, with
the exception of the first, these areas are small. The temperatures
average highest where there is little rain. In June, July and August
there are large districts in the south of Asia and north of Africa with
temperatures over 90 deg.

Over nearly all of the zone the mean annual range of temperature is less
than 10 deg., and over much of it, especially on the oceans, it is less
than 5 deg. Even near the margins of the zone the ranges are less than
25 deg., as at Calcutta, Hong-Kong, Rio de Janeiro and Khartum. The mean
daily range is usually larger than the mean annual. It has been well
said that "night is the winter of the tropics." Over an area covering
parts of the Pacific and Indian Oceans from Arabia to the Caroline
Islands and from Zanzibar to New Guinea, as well as on the Guiana coast,
the minimum temperatures do not normally fall below 68 deg. Towards the
margins of the zone, however, the minima on the continents fall to or
even below 32 deg. Maxima of 115 deg. and even over 120 deg. occur over
the deserts of northern Africa. A district where the mean maxima exceed
113 deg. extends from the western Sahara to north-western India, and
over Central Australia. Near the equator the maxima are therefore not as
high as those in many so-called "temperate" climates. The tropical
oceans show remarkably small variations in temperature. The "Challenger"
results on the equator showed a daily range of hardly 0.7 deg. in the
surface water temperature, and P. G. Schott determined the annual range
as 4.1 deg. on the equator, 4.3 deg. at latitude 10 deg., and 6.5 deg.
at latitude 20 deg.

_The Seasons._--In a true tropical climate the seasons are not
classified according to temperature, but depend on rainfall and the
prevailing winds. The life of animals and plants in the tropics, and of
man himself, is regulated very largely, in some cases almost wholly, by
rainfall. Although the tropical rainy season is characteristically
associated with a vertical sun, that season is not necessarily the
hottest time of the year. It often goes by the name of winter for this
reason. Towards the margins of the zone, with increasing annual ranges
of temperature, seasons in the extra-tropical sense gradually appear.

_Physiological Effects of Heat