Overview

The atmosphere is a relatively thin layer of gaseous fluid that envelops the earth. At sealevel, atmospheric pressure varies but has an average value of about 1013.2 hPa. Surface pressureis higher within anticyclones and lower with cyclones. The density of the atmosphere at sea levelis about 1.2 Kg/m ³. Both pressure and density decrease with altitude. At12 km (approximately the cruising altitude of jet aircraft) the pressure is only about 200 hPa andthe density is 0.31 Kg/m ³. Also, on average, temperature decreases fromabout 15 C at the surface to about 55 C at 12 km altitude.

The principal chemical constituents of the atmosphere are nitrogen (78.09 percent), oxygen(20.95 percent), argon (0.93 percent) and carbon dioxide (0.03 percent). Water vapour is also asignificant component whose concentration varies with temperature and atmospheric pressure. Thesaturation vapour pressure of water vapour (that is, the maximum concentration before condensationbegins) varies at sea level from about 6 hPa near 0 C to about 42 hPa near 30 C. Over tropicaloceans it is not unusual to have temperatures near 30 C and humidity near 80 percent so that theactual water vapour concentration is near 0.023 percent. Both carbon dioxide and water vapour arenaturally occurring greenhouse gases.

For most climate considerations the important layers of the atmosphere are the boundarylayer, the troposphere and the stratosphere (see Figure 10). The boundary layer is next to theearth s surface and may be only a few tens of metres thick under a night-time inversion or severalkilometres thick over a hot desert. The characteristic...