The trade-wind belt broadens in crossing the southern portion of the North American continent, and on the west coast and during its greatest northward migration reaches to southern California. As we have seen, the lowlands not adjacent to mountains in the Caribbean region receive little or no rain from the trade-winds, for the reason that the prevailing air-currents are moving from cooler to warmer regions, and therefore have their capacity for moisture increased as they advance more rapidly than their thirst can be satiated. The trade-winds are thus normally drying winds. The same principle holds true for continents as well as islands. The trade-winds on reaching the eastern border of the Mexican plateau are forced upward and part with much of their moisture in the form of rain and snow, and on descending to the lower lands bordering the Pacific are desiccating winds. The conditions are thus much the same as on the lowlands situated to the leeward of the mountains of the West Indies. The narrow fringe of low-lying country on the west border of the main body of Mexico, together with the peninsula of Lower California and a portion of the southern extremity of California, are arid, although in immediate proximity to the ocean. The leading characteristics of the climate of this, the arid portion of the tropical province, are its small rainfall, clear atmosphere, high mean annual temperature, moderate seasonal changes, and active evaporation. The most typical portion of the arid region bordering the Pacific from California southward lies in northwestern Mexico, where the mean annual precipitation on the western slope of the central table-land is from 15 to 20 inches, but gradually diminishes as one descends to the lower lands to the westward to 10 inches, and even to 5 inches or less. A great portion of the lowlands is practically a desert, although, as is common on the desert-like tracts of this continent, it is sparingly covered with detached clumps of cacti, yuccas, and other similar plants which live with the minimum of water. The hot, arid lands of the southwest just referred to are similar in position, in reference to the relation of land areas to the trade-wind belts, as other trade-wind deserts, as they are termed, such as the Sahara, which occur on the leeward side of continents where they are crossed by these drying winds.