and Humidity._--Tropical heat is
associated with high relative humidity except over deserts and in dry
seasons. The air is therefore muggy and oppressive. The high
temperatures are disagreeable and hard to bear. The "hot-house air" has
an enervating effect. Energetic physical and mental action are often
difficult or even impossible. The tonic effect of a cold winter is
lacking. The most humid districts in the tropics are the least desirable
for persons from higher latitudes; the driest are the healthiest. The
most energetic natives are the desert-dwellers. The monotonously
enervating heat of the humid tropics makes man sensitive to slight
temperature changes. The intensity of direct insolation, as well as of
radiation from the earth's surface, may produce heat prostration and
sunstroke. "Beware of the sun" is a good rule in the tropics.

_Pressure._--The uniform temperature distribution in the tropics
involves uniform pressure distribution. Pressure gradients are weak. The
annual fluctuations are slight, even on the continents. The diurnal
variation of the barometer is so regular and so marked that, as von
Humboldt said, the time of day can be told within about twenty minutes
if the reading of the barometer be known.

_Winds and Rainfall._--Along the barometric equator, where the pressure
gradients are weakest, is the equatorial belt of calms, variable winds
and rains--the doldrums. This belt offers exceptionally favourable
conditions for abundant rainfall, and is one of the rainiest regions of
the world, averaging probably about 100 in. Here the sky is prevailingly
cloudy; the air is hot and oppressive; heavy showers and thunderstorms
are frequent, chiefly in the afternoon and evening. Here are the dense
tropical forests of the Amazon and of equatorial Africa. This belt of
calms and rains shifts north and south of the equator after the sun. In
striking contrast are the easterly trade winds, blowing between the
tropical high pressure belts and the equatorial belt of low pressure. Of
great regularity, and contributing largely to the uniformity of tropical
climates, the trades have long been favourite sailing routes because of
the steadiness of the wind, the infrequency of storms, the brightness of
the skies and the freshness of the air. The trades are subject to many
variations. Their northern and southern margins shift north and south
after the sun; at certain seasons they are interrupted, often over wide
areas near th