Atmosphere

ATMOSPHERE, the air envelope surrounding the Earth (see Air). Its density is greatest at the Earth's surface and gradually decreases with distance from it. Therefore, one cannot speak of any sharply defined boundary of the atmosphere. In any case, at an altitude of 100 km above the Earth, light clouds that glow faintly at night are still observed; at altitudes from 100 to 200 km, the density of the atmosphere is still sufficient for 'shooting stars' to burn up in it. At even greater altitudes, the aurora borealis is observed. Atmospheric pressure was discovered by the Italian physicist Torricelli in the 17th century. The particles of gases that make up the air are attracted by the Earth, like all material bodies, and therefore each layer of air presses on the layers below it. The greatest pressure should therefore be at sea level, and as one moves upward from it, the pressure should decrease. Experiments show that at sea level, under normal conditions, atmospheric pressure fluctuates around 1 kg per square centimeter of surface. To measure atmospheric pressure, barometers (see) are used, and it is expressed in millimeters of mercury column that could produce the same pressure. Thus, normal atmospheric pressure corresponds to a mercury column 760 mm high. When rising above sea level, pressure decreases by 1 mm of mercury for every 10.5 meters*. This value is called the barometric step; it varies depending on the temperature and pressure in a given location. Atmospheric pressure at a specific point on the Earth is subject to fluctuations, both periodic and random. The latter can be extremely sharp. The lowest pressure (at sea level) was observed in 1891 in the Chinese Sea: it reached 686.3 millimeters. The highest atmospheric pressure (also at sea level) was recorded in 1900 in Barnaul, where it reached 808.7 millimeters.