III.

Effects of Temperature

Temperature plays an important part in determining the conditions in which living matter can exist. Thus, birds and mammals demand a very narrow range of body temperatures for survival and must be protected against extreme heat or cold (see Body Temperature). Aquatic species can exist only within a narrow temperature range of the water, which differs for various species. Thus, for example, the increase in temperature of river water by only a few degrees as a result of heat discharged from power plants may kill most of the native fish. See Water Pollution.

The properties of all materials are also markedly affected by temperature changes. At arctic temperatures, for example, steel becomes very brittle and breaks easily, and liquids either solidify or become very viscous, offering high frictional resistance to flow (see Viscosity). At temperatures near absolute zero, many materials exhibit strikingly different characteristics (see Cryogenics). At high temperatures, solid materials liquefy or become gaseous; chemical compounds may break up into their constituents.

The temperature of the atmosphere is greatly influenced by both the land and the sea areas. In January, for example, the great landmasses of the northern hemisphere are much colder than the oceans at the same latitude, and in July the situation is reversed. At low elevations the air temperature is also determined largely by the surface temperature of the earth. The periodic temperature changes are due mainly to the sun's radiant heating of the land areas of the earth, which in turn convect heat to the overlying air. As a result of this phenomenon, the temperature decreases with altitude, from a standard reference value of 15.5° C (60° F) at sea level (in temperate latitudes), to about -55° C (about -67° F) at about 11,000 m (about 36,000 ft). Above this altitude, the temperature remains nearly constant up to about 33,500 m (about 110,000 ft). For the temperature-humidity index, see Humidity.