The vertical temperature profile
The sun’s role in atmospheric temperature
The temperature of Earth’s atmosphere varies with the distance from the equator (latitude) and height above the surface (altitude). It also changes with time, varying from season to season, and from day to night, as well as irregularly due to passing weather systems. If local variations are averaged out on a global basis, however, a pattern of global average temperatures emerges. Vertically, the atmosphere is divided into four layers: the troposphere, the stratosphere, the mesosphere, and the thermosphere.
The vertical temperature profile
Averaging atmospheric temperatures over all latitudes and across an entire year gives us the average vertical temperature profile that is known as a standard atmosphere. The average vertical temperature profile suggests four distinct layers (Figure 1). In the first layer, known as the troposphere, average atmospheric temperature drops steadily from its value at the surface, about 290K (63°F; 17°C) and reaches of minimum of around 220K (–64°F;–53°C) at an altitude of about 6.2 mi (10 km). This level, known as the tropo-pause, is just above the cruising altitude of commercial jet aircraft. The decrease in temperature with height, called the lapse rate, is nearly steady throughout the troposphere at 43.7°F(6.5°C) per 0.6 mi (1 km). At the tropopause, the lapse rate abruptly decreases. Atmospheric temperature is nearly constant over the next 12 mi (20 km), then begins to rise with increasing altitude up to about 31 mi (50 km). This region of increasing temperatures is the stratosphere. At the top of the layer, called the stratopause, temperatures are nearly as warm as the surface values. Between about 31–50 mi (50–80 km) lies the mesosphere, where atmospheric temperature resumes its decrease with altitude and reaches a minimum of 180K (–136°F;–93°C) at the top of the layer (the mesopause), around 50 mi (80 km). Above the mesopause is the thermosphere that, as its name implies, is a zone of high gas temperatures. In the very high thermosphere (about 311 mi (500 km) above
Earth’s surface) gas temperatures can reach from 500–2,000K (441–3, 141°F; 227–1, 727°C). Temperature is a measure of the energy of the gas molecules’ motion. Although they have high energy, the molecules in the thermosphere are present in very low numbers, less than one millionth of the amount present on average at Earth’s surface.
Atmospheric temperature can also be plotted as a function of both latitude and altitude. Figures 2 and 3 show such plots, with latitude as the x coordinate and altitude as the y.
The sun’s role in atmospheric temperature
Most solar radiation is emitted as visible light, with smaller portions at shorter wavelengths (ultraviolet radiation) and longer wavelengths (infrared radiation, or heat). Little of the visible light is absorbed by the atmosphere (although some is reflected back into space by clouds), so most of this energy is absorbed by Earth’s surface. The Earth is warmed in the process and radiates heat (infrared radiation) back upward. This warms the atmosphere, and, just as one will be warmer when standing closer to a fire, the layers of air closest to the surface are the warmest.
According to this explanation, the temperature should continually decrease with altitude. Figure 1, however, shows that temperature increaseS with altitude in the stratosphere. The stratosphere contains nearly all the atmosphere’s ozone. Ozone (O3) and molecular oxygen (O2) absorb most of the sun’s short wavelength ultraviolet radiation. In the process they are broken apart and reform continuously. The net result is that the ozone molecules transform the ultraviolet radiation to heat energy, heating up the layer and causing the increasing temperature profile observed in the stratosphere.
The mesosphere resumes the temperature decrease with altitude. The thermosphere, however, is subject to very high energy, short wavelength ultraviolet and x-ray solar radiation. As the atoms or molecules present at this level absorb some of this energy, they are ionized
(have an electron removed) or dissociated (molecules are split into their component atoms). The gas layer is strongly heated by this energy bombardment, especially during periods when the sun is emitting elevated amounts of short wavelength radiation.
The greenhouse effect
Solar energy is not the only determinant of atmospheric temperature. As noted above, Earth’s surface, after absorbing solar radiation in the visible region,
Greenhouse effect —The warming of Earth’s atmosphere as a result of the capture of heat re-radiated from Earth by certain gases present in the atmosphere.
Infrared radiation —Radiation similar to visible light but of slightly longer wavelength.
Lapse rate —The rate at which the atmosphere cools with increasing altitude, given in units of degrees C per kilometer.
Mesosphere —The third layer of the atmosphere, lying between about 50 and 80 kilometers in height and characterized by a small lapse rate.
Stratosphere —A layer of the upper atmosphere above an altitude of 5–10.6 mi (8–17 km) and extending to about 31 mi (50 km), depending on season and latitude. Within the stratosphere, air temperature changes little with altitude, and there are few convective air currents.
Thermosphere —The top layer of the atmosphere, starting at about 50 mi (80 km) and stretching up hundreds of miles or kilometers in to space. Due to bombardment by very energetic solar radiation, this layer can possess very high gas temperatures.
Troposphere —The layer of air up to 15 mi (24 km) above the surface of Earth, also known as the lower atmosphere.
Ultraviolet radiation —Radiation similar to visible light but of shorter wavelength, and thus higher energy.
X-ray radiation —Light radiation with wavelengths shorter than the shortest ultraviolet; very energetic and harmful to living organisms.
emits infrared radiation back to space. Several atmospheric gases absorb this heat radiation and re-radiate it in all directions, including back toward the surface. These so-called greenhouse gases thus trap infrared radiation within the atmosphere, raising its temperature. Important greenhouse gases include water vapor (H2 O), carbon dioxide (CO2), and methane (CH4). It is estimated that Earth’s surface temperature would average about 32°C (90°F) cooler in the absence of greenhouse gases. Because this temperature is well below the freezing point of water, the planet would be much less hos-pitable to life in the absence of the greenhouse effect.
While greenhouse gases are essential to life on the planet, more is not necessarily better. Since the beginning of the industrial revolution in the mid-nineteenth century, humans have released increasing amounts of carbon dioxide to the atmosphere by burning fossil fuels. The level of carbon dioxide measured in the remote atmosphere has shown a continuous increase since record keeping began in 1958. If this increase translates into a corresponding rise in atmospheric temperature, the results might include melting polar ice caps and swelling seas, resulting in coastal cities being covered by the ocean; shifts in climate perhaps leading to extinctions; and unpredictable changes in wind and weather patterns, posing significant challenges for agriculture. Predicting the changes that increased levels of greenhouse gases may bring is complicated. The interaction of the atmosphere, the oceans, the continents, and the ice caps is not completely understood. While it is known that some of the emitted carbon dioxide is absorbed by the oceans and eventually deposited as carbonate rock (such as limestone), it is not known if this is a steady process or if it can keep pace with current levels of carbon dioxide production.
Ahrens, Donald C. Meteorology Today. Pacific Grove, Calif.: Brooks Cole, 2006.
Palmer, Tim, and Renate Hagedorn, editors. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.