f summarizing the interaction of the climatic elements under
the controls of land, water and altitude. But in any detailed study some
scheme of classification is needed in which similar climates in
different parts of the world are grouped together, and in which their
geographic distribution receives particular consideration. An almost
infinite number of classifications might be proposed; or we may take as
the basis of subdivision either the special conditions of one climatic
element, or similar conditions of a combination of two or more elements.
Or we may take a botanical or a zoological basis. Of the various
classifications which have been suggested, that of Supan gives a very
rational, simple and satisfactory scheme of grouping. In this scheme
there are thirty-five so-called climatic provinces.[3] It emphasizes the
essentials of each climate, and serves to impress these essentials upon
the mind by means of a compact, well-considered verbal summary in the
case of each province described. Obviously, no classification of
climates which is at all complete can approach the simplicity of the
ordinary classification of the zones.

_The Characteristics of the Torrid Zone._

_General: Climate and Weather._--The dominant characteristic of the
torrid zone is the simplicity and uniformity of its climatic features.
The tropics lack the proverbial uncertainty and changeableness of the
weather of higher latitudes. Weather and climate are essentially
synonymous terms. Periodic phenomena, depending upon the daily and
annual march of the sun, are dominant. Non-periodic weather changes are
wholly subordinate. In special regions only, and at special seasons, is
the regular sequence of weather temporarily interrupted by an occasional
tropical cyclone. These cyclones, although comparatively infrequent, are
notable features of the climate of the areas in which they occur,
generally bringing very heavy rains. The devastation produced by one of
these storms often affects the economic condition of the people in the
district of its occurrence for many years.

_Temperature._--The mean temperature is high, and very uniform over the
whole zone. There is little variation during the year. The mean annual
isotherm of 68 deg. is a rational limit at the polar margins of the zone,
and the mean annual isotherm of 80 deg. encloses the greater portion of the
land areas, as well as much of the tropical oceans. The warmest latitude
circle for the year i