wind systems, a
classification of the zones by winds also emphasizes the conditions of
rainfall. In such a scheme the tropical zone is bounded on the north and
south by the margins of the trade-wind belts, and is therefore larger
than the classic torrid zone. This trade-wind zone is somewhat wider on
the eastern side of the oceans, and properly includes within its limits
the equable marine climates of the eastern margins of the ocean basins,
even as far north as latitude 30 deg. or 35 deg. Most of the eastern
coasts of China and of the United States are thus left in the more
rigorous and more variable conditions of the north temperate zone.
Through the middle of the trade-wind zone extends the sub-equatorial
belt, with its migrating calms, rains and monsoons. On the polar margins
of the trade-wind zone lie the sub-tropical belts, of alternating trades
and westerlies. The temperate zones embrace the latitudes of the stormy
westerly winds, having on their equator-ward margins the sub-tropical
belts, and being somewhat narrower than the classic temperate zones.
Towards the poles there is no obvious limit to the temperate zones, for
the prevailing westerlies extend beyond the polar circles. These circles
may, however, serve fairly well as boundaries, because of their
importance from the point of view of insolation. The polar zones in the
wind classification, therefore, remain just as in the older scheme.

_Need of a Classification of Climates._--A broad division of the earth's
surface into zones is necessary as a first step in any systematic study
of climate, but it is not satisfactory when a more detailed discussion
is undertaken. The reaction of the physical features of the earth's
surface upon the atmosphere complicates the climatic conditions found in
each of the zones, and makes further subdivision desirable. The usual
method is to separate the _continental_ (near sea-level) and the
_marine_. An extreme variety of the continental is the _desert_; a
modified form, the _littoral_; while altitude is so important a control
that _mountain_ and _plateau_ climates are always grouped by themselves.

_Marine or Oceanic Climate._--Land and water differ greatly in their
behaviour regarding absorption and radiation. The former warms and cools
readily, and to a considerable degree; the latter, slowly and but
little. The slow changes in temperature of the ocean waters involve a
retardation in the times of occurrence of the maxima a