Page:Popular Science Monthly Volume 76.djvu/252

248 In a true tropical climate seasons, in our sense, do not exist. The variations in temperature throughout the year are so slight that the seasons are not classified according to temperature, but depend on rainfall and the prevailing winds. The life of animals and of plants in the tropics, and of man himself, is regulated very largely, in some cases almost entirely, by rainfall. Although the tropical rainy season is characteristically associated with a vertical sun (i. e., "summer"), that season is not necessarily the hottest time in the year. In fact, the temperature is usually somewhat lower under the clouds, and hence the rainy season often goes by the name of "winter."

Within the tropics the equatorial belt of calms and variable winds—the "doldrums" of sailors—offers exceptionally favorable conditions for abundant rainfall. The rainfall is so heavy that the surface waters of the ocean are actually fresher than in the latitudes to the north and south. The sky is prevailingly cloudy; the air, hot and oppressive; heavy showers and thunderstorms are frequent. In the latitudes of this belt are the dense tropical forests of the Amazon and of equatorial Africa. Here drought and frost need not be feared. The belt of calms and rains shifts north and south of the equator after the sun. It is dreaded by seamen because sailing vessels are apt to be delayed in crossing it, but delays of great length are infrequent nowadays, since a careful study of the prevailing winds over the ocean areas has shown navigators at what points the belt of calms is narrowest, and where the crossing may be the most successfully made.

In striking contrast are the easterly trade winds, which blow toward the equator from about latitude 30° north and south. Of great regularity, embracing about one half of the earth's surface, the trades have long been favorite sailing routes because of the steadiness of their winds, the infrequency of their storms, the brightness of their skies and the freshness of the air, all of which are in pleasing contrast with the muggy, oppressive calms of the doldrums. All sailing routes which pass through the trade wind belts in any ocean are controlled by these winds. Steady winds like the trades certainly tempted the early navigators to put to sea. The famous voyage of Columbus was facilitated, if not made possible, by the northeast trade. The easy outward voyages of the early Spanish adventurers and colonists took them naturally to that portion of the Americas where they found even-tempered climates in which they and their descendants could live comfortably. The monsoons of India have from the earliest days of trade with the east been important agents in aiding commerce.

The most desirable house sites in the tropics are very commonly on the top of some elevation, exposed to the trades. The sea breeze, also, is an important climatic feature on many tropical coasts. "With its regular occurrence, and its cool, clean air, it serves to make many districts habitable for white settlers, and has deservedly won the name of