nd third, the influence of land or water; in the fourth, the
effect of altitude.

_Classification of the Zones by Latitude Circles._--It is customary to
classify climates roughly into certain broad belts. These are the
climatic zones. The five zones with which we are most familiar are the
so-called torrid, the two temperate, and the two frigid zones. The
torrid, or better, the tropical zone, naming it by its boundaries, is
limited on the north and south by the two tropics of Cancer and
Capricorn, the equator dividing the zone into two equal parts. The
temperate zones are limited towards the equator by the tropics, and
towards the poles by the Arctic and Antarctic circles. The two polar
zones are caps covering both polar regions, and bounded on the side
towards the equator by the Arctic and Antarctic circles.

These five zones are classified on purely astronomical grounds. They are
really zones of solar climate. The tropical zone has the least annual
variation of insolation. It has the maximum annual amount of insolation.
Its annual range of temperature is very slight. It is the summer zone.
Beyond the tropics the contrasts between the seasons rapidly become more
marked. The polar zones have the greatest variation in insolation
between summer and winter. They also have the minimum amount of
insolation for the whole year. They may well be called the winter zones,
for their summer is so short and cool that the heat is insufficient for
most forms of vegetation, especially for trees. The temperate zones are
intermediate between the tropical and the polar in the matter of annual
amount and of annual variation of insolation. Temperate conditions do
not characterize these zones as a whole. They are rather the seasonal
belts of the world.

[Illustration: From _Grundzuge der physischen Erdkunde_, by permission
of Veit & Co.
FIG. 2.--Supan's Temperature Zones.]

_Temperature Zones._--The classification of the zones on the basis of
the distribution of sunshine serves very well for purposes of simple
description, but a glance at any isothermal chart shows that the
isotherms do not coincide with the latitude lines. In fact, in the
higher latitudes, the former sometimes follow the meridians more closely
than they do the parallels of latitude. Hence it has been suggested that
the zones be limited by isotherms rather than by parallels of latitude,
and that a closer approach be thus made to the actual conditions of
climate. Su