UNIT IV SECTION 7 CLIMATE AND WEATHER ARE PRODUCED BY PHYSICAL CHANGES Introduction. In general man seems to thrive best in climates in which extremes of heat and cold, dampness and dryness are avoided. A climate can be too equable; a climate of unvarying temperature, whether it be hot, cold, or ideal, is not only monotonous but depressing and enervating. Man seems to do his best work in climates where the temperature changes decidedly from season to season and from day to night. It is the object of this Section to show that climate and weather are a matter of physical cause and effect. This study of climate and the weather is generally referred to as the science of meteorology. Climate Is Controlled in Part by the Distribution of Land and Water. Continental climates show greater temperature ranges, less frequent rainfall, and more sunshine than marine climates. Desert climates represent the extreme of continental climatic conditions, with their hot days, relatively cool nights, and low average rainfall. Large bodies of water experience relatively small or slow temperature variations because of the high specific heat of water, as has previously been explained. In the daytime a large portion of the sunlight is lost by reflection from the surfaces of bodies of water, while much of that sunlight which is absorbed is used up in the process of evaporation. The amount of sunlight that is absorbed as heat is insufficient to raise the surface temperature of the ocean even a degree in a day because of the high specific heat of water and the cooling effect of evaporation. Land areas, on the other hand, reflecting less heat than water areas and being composed of materials of relatively low specific heat, become heated or cooled rapidly. Heat is also distributed by motions within the water, whereas land is immobile. The climate on the western shores of continents in the temperate zones is generally more equable than that on eastern shores due to the eflfect of the prevailing winds from off the oceans. The prevailing winds 250
CLIMATE AND WEATHER 251 vary because of seasonal changes and differ from place to place, but in general they blow from westerly directions in the temperate zones because of the effects of the rotation of the earth on the circulation of the atmosphere. Winds blowing toward the equator from higher latitudes are deflected westward; these prevailing northeasterly and southeasterly winds of the tropical regions are called the trade winds. There are belts of comparative calm at about 30° latitude north and south, between the trade winds and the prevailing westerlies. Here the atmosphere forms high-pressure belts, particularly over the North Pole '/ / / NortKeaat'Trades / / / \ Doldrums \\\ \\^ WW/ I \ \ \ Southeast Trades \ \ \ / Hi?h pressure 30° N. calm. High pressure "S. calm South Pole Fig. 75. The most important prevailing winds on the earth. oceans. These latitudes are called horse latitudes because sailing vessels carrying horses from New England to the West Indies were obliged to throw a part of their cargo overboard when water became scarce because of slow progress due to the lack of winds. In these latitudes one would expect to find the chief desert regions of the world. The prevailing westerly winds are especially well developed in the southern hemisphere, where, free to blow with great violence, they are known as the roaring jorties. The area of equatorial calm called the doldrums shifts north and south with the seasons. Near the poles the atmosphere is cooled and flows away from the poles. The prevailing winds tend to move the water at the surface of the oceans by friction in the direction in which they are blowing. The waters are thus blown toward the equator by the winds from the north.