The various regions of the atmosphere are described in the article on Atmosphere. The mass of the atmosphere is about 5.15 × 1015 tonne. The Earth’s atmosphere is a mixture of gases and vapor (air), and also of some amount of aerosols (dust, smoke, condensation products of vapor). The percentage ratio of main gases of a dry atmosphere (see Figure 1) changes slightly up to an altitude of about 100 km (in homosphere). At an altitude of 20-25 km an ozonal layer is situated which prevents living beings on the Earth from a harmful shortwave radiation. A share of light gases rises above 100 km (in heterosphere) and at very high altitudes helium and hydrogen prevail; a part of molecules decay into atoms and ions thus forming an ionosphere.
Figure 1. Variation of molecular weight (M) and % of various gases as a function of distance above the ground (h).
The vapor content is subject to variations to a greater extent than others. One of the most vital characteristics of the climate is the air humidity, the content of vapor in it. “Absolute” humidity can either be defined as mass of water vapor per unit volume or as mass of water vapor per unit mass of dry air. “Relative” humidity is the ratio of the absolute humidity to that for saturation. The highest mean value of humidity at the Earth’s surface is 30 g/m3 (absolute) (or 3% relative) and is characteristic of the equator area, the lowest (2 × 10−5% relative) is in Antarctica. The atmospheric formations (clouds) are the aggregation of water drops and ice crystals suspended in the atmosphere. The diameters of cloud drops are of the order of several mm. The enlarged drops fall out as rain, snow, hail. The size of rain drops varies from 0.5 to 6-7 mm; with a smaller size of drops the rainfall is called drizzle. The cooling of air below 0°C brings about snow fallout.
A typical distribution of absolute humidity with height in a rain cloud is represented in Figure 2.
The atmospheric pressure is the pressure of air on the objects in it and on the earth surface. The pressure at each point of the atmosphere is equal to the weight of the air column lying above it; it is measured in Pascals (1 Pa = 1 N/m2).
The international standard atmosphere to be used in calculation and designing the flying vehicles, in reducing the results of flight testings to the same conditions, in graduating the altimeters and in solving other technical and thermophysical problems is accepted, in which the distribution of parameters with height is calculated from the mean sea level (see Table 1) with certain suppositions of temperature distribution along the vertical.
The variation of temperature of the atmosphere with distance from the earth is shown in Figure 3
Heat & Mass Transfer, and Fluids Engineering