[102] This designation is popularly and incorrectly applied to the comparatively limited, but very violent and destructive rotary storms or whirlwinds which originate locally on the heated plains of the Middle West of the United States, and are almost always accompanied by violent electric phenomena. These should properly be called _tornadoes_. At sea such whirlwinds give rise to waterspouts, in deserts to sand storms. In the southern hemisphere, the air-movements substantially correspond to those observed in the northern, so far as not modified by mountain chains; as is especially the case in South America. INFLUENCE OF TOPOGRAPHY. _Rains to Windward of Mountain Chains._—The surface features or topography of the regions traversed by the air currents or winds may materially modify both their direction and their physical condition, especially as to moisture and temperature. Mountain chains may deflect them, or, causing the air currents to rise on their slopes, and thus to cool by expansion, the moisture these bring with them from the sea may be partially, or sometimes almost wholly, deposited in the form of rain or snow; chiefly on the windward slopes. Then, continuing across the range, the air deprived of most of its moisture cannot readily yield up more; hence the scarcity of rain—“arid climate”—under the lee of mountain chains; as in the Great Basin between the Sierra Nevada and Cascade ranges on the one hand and the Rocky Mountains on the other, and also on the Great Plains under the lee of the latter. The abundant rainfall between the Mississippi river and the Atlantic coast is due to the moist winds coming from the warm waters of the Gulf of Mexico and Caribbean sea, whose access is not interfered with by any cross-ranges of mountains. But the Great Plains lying between the Mississippi and the Rocky Mountains are not within the sweep of the Gulf winds, whose trend is SW to NE; while they are equally out of reach of moisture from the Pacific, all that having been successively deposited on the intervening mountains; hence their deficient rainfall. Northward of the temperate zone the rainfall generally decreases as we approach the arctic regions; except where the influence of warm ocean currents to windward creates comparatively local exceptions, as in the case of Norway and Alaska. [Illustration: FIG. 52.—Composite Curve showing distribution of Rainfall in Europe, Africa and America projected on 100 Meridian W. L.]