Vertical Structure of Earth's Atmosphere

Earth is surrounded by a gaseous envelope called the atmosphere. The vertical structure of the earth's atmosphere is described by its composition, and the distribution of mass (pressure) and temperature.

Composition of Atmosphere

The atmosphere is a mechanical mixture of gases called air. The dry air component of earth's present-day atmosphere is mostly composed of nitrogen (78.08% by volume), oxygen (20.95%), and argon (0.93%). Although on geologic time scales (over millions of years) the ratio of these gases change, on the human time scale they may be considered as constant. The remaining 0.04% of dry air consists of so-called trace gases which are measured in parts per million by volume (ppmv). Several of these trace gases influence the radiative balance of the planet (greenhouse effect). Of primary importance are carbon dioxide (355 ppmv and increasing), ozone (variable), and methane (1.7 ppmv). In addition to dry air, the atmosphere contains highly variable amounts (in both space and time) of water vapor. The amount of water vapor in the atmosphere is temperature dependent; warm air can hold more water vapor than cold air. Water vapor comprises up to 4% of air by volume near the surface, but only a few ppmv above 10 to 12 km. Water vapor is the most important contributor to the greenhouse effect and thus plays a major role in determining the temperature of the lower atmosphere.

Distribution of mass

The density (mass per unit volume) of a gas at constant temperature is proportional to pressure (Boyle's Law) and at constant pressure is inversely proportional to temperature (Charles' Law). Combining these laws leads to the relationship between pressure (p), density (r), and temperature (T) known as the equation of state, p = rRT, where R =287 J kg^-1K^-1 is the gas constant for dry air. Due to the compressibility of air, lower layers of the atmosphere are much more dense than those above. Figure 1 (left panel) shows the density of air as a function of height from the surface to 100 km. Fifty percent of the total mass of the atmosphere lies in the lower 5 km, eighty percent below 10 km. The average density of air is 1.2 kg m^-3 at the surface, 0.7 kg m^-3 at 5 km, and 0.4 kg m^-3 at 10 km. The density stratification (decrease in density with altitude) is maintained by a balance between the pressure gradient force (which is directed upward) and the pull of gravity (which is directed downward). This force balance is expressed by the hydrostatic equation, dp/dz = -rg, where dp/dz is the rate of change of pressure with height, g is the acceleration of gravity, and r is density. The average pressure (the force per unit area exerted by the weight of the atmosphere) at sea level is equal to the mass of the atmosphere (5.14X10¹⁸ kg) multiplied by the acceleration of gravity divided by the surface area of the earth, or, slightly more than 1000 millibars (mb), where 1 mb = 100 Nm^-2. The global mean value for sea level pressure is 1013.25 mb. From the hydrostatic equation and the equation of state, it follows that pressure in the earth's atmosphere decreases nearly exponentially with height. This relationship between pressure and height is shown in figure 1 (right panel).

Distribution of temperature

The vertical temperature distribution of the earth's atmosphere (shown in figure 1) is divided into four layers, the troposphere, stratosphere, mesosphere, and thermosphere. The boundaries at the top of each of these layers are called the tropopause, stratopause, mesopause, and thermopause, respectively. The height of these boundaries vary in both time and space (primarily with latitude and season). The heights shown in figure 1 are for the so-called standard atmosphere which is representative of typical (average) mid latitude conditions.

The globally averaged surface temperature for earth is approximately 15°C due to the greenhouse effect. Incoming solar radiation entering earth's atmosphere is predominantly short wave. Radiation leaving the earth is long wave, or infra-red. The earth's atmosphere absorbs infra-red radiation due to the properties of water vapor, carbon dioxide, methane, and other trace gases. While incoming and outgoing radiation tend to remain nearly in balance, the net effect of the earth's opacity to infra-red radiation is to warm the surface of the planet. If not for the presence of an atmosphere, the surface temperature on earth would be well below the freezing point of water (less than -20°C).

In the troposphere, temperature decreases with altitude from the surface up to the tropopause (approximately 8 km in polar regions, 16 km in the tropics). When gas expands, its temperature decreases; when gas is compressed, its temperature increases. This change in temperature of a gas due to expansion or compression is called an adiabatic temperature change. Thus when air in the atmosphere rises it expands and cools adiabatically; when it descends it is compressed and heats adiabatically. The decrease in temperature with increasing altitude is called the lapse rate. The dry adiabatic lapse rate on earth is approximately 10°C/km. When air temperature reaches the dew point (i.e., the temperature at which air becomes saturated with water vapor), continued cooling results in results in condensation of water (cloud formation) and a release of latent heat to the atmosphere. The heat released slows the cooling with altitude to around 6°C/km (the saturated adiabatic lapse rate). The temperature at the tropopause is approximately -55°C in mid to high latitudes, and -75°C in the tropics.

In the lower stratosphere, temperature remains nearly constant (isothermal) from the tropopause up to around 20 km. In the upper stratosphere, extending to 50 km, temperature increases by up to 4°C/km rising to near 0°C at the stratopause. This statospheric temperature increase is caused by the presence of ozone. In the layer between 20-60 km ozone is generated by photochemical reactions. The long-term flux of ozone is downward from source region in the stratosphere to sink at the surface. The maximum concentration of ozone is between 15 and 30 km. Ozone absorbs ultraviolet radiation from the sun and consequently, the stratosphere is heated.

Above the stratopause, in the mesosphere, temperature again decreases with height up the mesopause at 80 km. The temperature at the mesopause approaches -90°C.