In Central America the influence of mountains on the climate is much the same as in the West India islands; in fact, the narrow rugged belt of land connecting the two Americas may, from our present point of view, be considered as a great island situated mainly within the trade-wind belt, and similar to Jamaica, for example, in its influences on the winds blowing across it. The eastern slopes of the Central American mountains, together with the adjacent lowlands, with the exception, principally, of Yucatan, are well watered and clothed with dense vegetation, while on the western slopes, and especially over the narrow fringe of lowland adjacent to the Pacific, the rainfall is less than on the Caribbean coast, and the forests are open with many grass-covered areas which are favourable for agriculture. In the mountainous portions of the West Indies and of Central America, on account of the more healthy conditions on the leeward or drier sides, as compared with the windward or humid slopes of the mountains, the towns and the principal portion of the white inhabitants are located on their western borders. Owing to the great humidity and the long-continued high temperature during the hotter portion of the year throughout the tropical province, much of the lowland to the eastward of the high mountains is swampy and unhealthy. This low region in Mexico and Central America is known as the _tierra caliente_, or hot country; on the mountains and plateaus, or in general where the elevation is between 5,000 and 7,000 feet, is the cooler and remarkably salubrious _tierra templada_; and at still higher elevations occur the _tierra frie_. Owing to the decrease in temperature with elevation, and the fact that the moist warm air is forced to rise, and in consequence expands on passing over the highland, the rainfall probably increases with elevation through the three zones just referred to. While the tropical province is characterized by the uniformity with which its atmospheric changes occur, it is nevertheless in part subject to occasional and exceedingly violent cyclonic storms termed hurricanes, which begin in the torrid zone, travel northward (Fig. 26, page 210), and make their influence felt in more than one of the climatic provinces into which North America is here divided. Thunder-storms, frequently of great violence, also occur, especially in the Central American region in summer, when the doldrums migrate northward.