pan[1] (see fig. 2) has suggested limiting the hot belt,
which corresponds to, but is slightly greater than, the old torrid zone,
by the two mean annual isotherms of 68 deg.--a temperature which
approximately coincides with the polar limit of the trade-winds and with
the polar limit of palms. The hot belt widens somewhat over the
continents, chiefly because of the mobility of the ocean waters, whereby
there is a tendency towards an equalization of the temperature between
equator and poles in the oceans, while the stable lands acquire a
temperature suitable to their own latitude. Furthermore, the
unsymmetrical distribution of land in the low latitudes of the northern
and southern hemispheres results in an unsymmetrical position of the hot
belt with reference to the equator, the belt extending farther north
than south of the equator. The polar limits of the temperate zones are
fixed by the isotherm of 50 deg. for the warmest month. Summer heat is
more important for vegetation than winter cold, and where the warmest
month has a temperature below 50 deg., cereals and forest trees do not
grow, and man has to adjust himself to the peculiar climatic conditions
in a very special way. The two polar caps are not symmetrical as regards
the latitudes which they occupy. The presence of extended land masses in
the high northern latitudes carries the temperature of 50 deg. in the
warmest month farther poleward there than is the case in the
corresponding latitudes occupied by the oceans of the southern
hemisphere, which warm less easily and are constantly in motion. Hence
the southern cold cap, which has its equatorial limits at about lat. 50
deg. S., is of much greater extent than the northern polar cap. The
northern temperate belt, in which the great land areas lie, is much
broader than the southern, especially over the continents. These
temperature zones emphasize the natural conditions of climate more than
is the case in any subdivision by latitude circles, and they bear a
fairly close resemblance to the old zonal classification of the Greeks.

_Classification of the Zones by Wind Belts._--The heat zones however,
emphasize the temperature to the exclusion of such important elements
as wind and rainfall. So distinctive are the larger climatic features of
the great wind belts of the world, that a classification of climates
according to wind systems has been suggested.[2] As the rain-belts of
the world are closely associated with these