Science for beginners . d less India hurricanes occasionally visit the Atlantic coastof the United States. Sometimes they enter the Gulf ofMexico and pass northward over the Mississippi or OhioValleys. Such storms damaged Galveston, Texas, in Sep-tember, 1900, and August, 1915, and the city of New Orleansin September, 1915. (3) The westerly air currents of the temperate zones arefull of broad waves or surges of unequal pressure. Thesewaves are the highs and lows of the weather map, which 206 THE WEATHER control the weather of the temperate zones in the ways we havestudied. Fig. 16
Science for beginners . d less India hurricanes occasionally visit the Atlantic coastof the United States. Sometimes they enter the Gulf ofMexico and pass northward over the Mississippi or OhioValleys. Such storms damaged Galveston, Texas, in Sep-tember, 1900, and August, 1915, and the city of New Orleansin September, 1915. (3) The westerly air currents of the temperate zones arefull of broad waves or surges of unequal pressure. Thesewaves are the highs and lows of the weather map, which 206 THE WEATHER control the weather of the temperate zones in the ways we havestudied. Fig. 162 shows the usual paths of the lows (storms)in various portions of the earth. 218. Effects of the Suns Annual Migration.—The yearlyshift, or migration, of the sun north and south of the equator,from summer to winter, and back, affects the weather of afew portions of the earth in an interesting way. In summerthe land warms more than water. That warms and expandsthe lower air most over the land. This expansion lifts the. Fig. 164.—A summer month. Low pressure in interior. Storm tracksfar to north. Prevailing southerly winds. Rain distributed far inland. upper air over the land higher than that over the in turn causes some of the upper air to run off the landout over the ocean. This leaves less air, and therefore lesspressure, over the land, and adds more air, and therefore morepressure, over the oceans. This greater pressure over theoceans (Fig. 163) crowds the bottom air toward the land,so the prevailing winds at the ground blow inland in sum-mer. The most notable winds of this sort are the monsoonsof India, that carry heavy summer rains to the northern in-terior of that country. In the United States, the prevailing GENERAL CIRCULATION OF THE ATMOSPHERE 207 south winds of summer over the Mississippi Valley are causedin the same way. They are of great benefit, as they carrymoisture from the Gulf and Atlantic far northward to supply
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