Earth's Atmosphere
Constituent Molecular Content
Weight (fraction of total molecules)
Nitrogen (N2) 28.02 0.7808 (75.51% by mass)
Oxygen (O2) 32.00 0.2095 (23.14% by mass)
Argon (A) 39.94 0.0093 ( 1.28% by mass)
Water vapor (H2O) 18.02 0 - 0.04
Carbon dioxide (CO2) 44.01 325 parts per million
Neon (Ne) 20.18 18 parts per million
Helium (He) 4.00 5 parts per million
Krypton (Kr) 83.70 1 part per million
Hydrogen (H) 2.02 0.5 parts per million
Ozone (O3) 48.00 0-12 parts per million
(from J. M. Wallace and P. V. Hobbs, Atmospheric Science,
An Introductory Survey, Academic Press, 1977, page 5)
Above the homosphere, lies the heterosphere, where the gases are no longer uniformly mixed. Relatively more of heavy gas molecules such as N2 and O2 are found near the bottom of this region, and relatively more of the lighter gases such as hydrogen and helium are found hear the top.
The atmosphere is also divided into 4 regions according to temperature trends. In order of increasing height these are:the troposphere, the stratosphere, the mesosphere, and the thermosphere. Figure 1 shows typical temperature trends in these regions. In the troposphere, as altitude increases air temperature decreases at a rate of about 3.5ø per 1000 ft. This is known as the observed or normal lapse rate, which varies around the Earth. The temperature stops dropping at the tropopause, which is the boundary between the troposphere and the stratosphere. The tropopause is found at about 5 miles above the surface of the Earth at the poles, and at about 11 miles at the equator. Note the temperature trends in the stratosphere and mesosphere. In the upper stratosphere is found the ozone layer (O3 molecules), which absorbs harmful ultraviolet radiation from the sun. The absorption of this solar energy explains the increase in temperature in this region. Figure 2 shows how the temperature in the thermosphere depends on the amount of solar activity.
Part of the upper atmosphere is composed of charged particles (ions and electrons). This region is known as the ionosphere. Figure 3 shows the heights of the different ionospheric layers or regions (D, E, and F) which are capable of reflecting high frequency radio waves. Notice that the electron concentration of the ionosphere is much greater during the day than at night, and in fact the D region disappears at night. This is because the ionospheric layers are produced by the action of solar extreme ultraviolet radiation, which is not present at night.
Above the ionosphere is found the magnetosphere (Figure 4), a vast region of charged particles formed by the interaction between the solar wind and the Earth's magnetic field. The magnetosphere begins at about 600 miles (1000 km) above the Earth's surface. It extends to a distance of about 40000 miles on the side facing the sun, and to much greater distances on the side of the Earth turned away from the sun.
If there were no atmosphere, the Earth's surface would be continually bombarded with ultraviolet and other energetic solar radiation, and the temperature at the surface would vary from about 180øF during the day to about -220øF at the same place at night.
The sun radiates a tremendous amount of energy, but only one two-billionth of the total solar radiation reaches the vicinity of the Earth. Of this amount only about 20% is absorbed in the Earth's atmosphere. This heats the atmosphere. Most of the absorption of solar energy occurs in the ozone layer and in the ionosphere.
30 to 40% of incoming solar radiation may be reflected back into space from clouds, air molecules, dust particles, and the surface of the Earth itself. This radiation is not available for heating the lower atmosphere or the surface of the Earth. Thus only 40-50% of incoming solar radiation is available to be absorbed by the surface of the Earth. These percentages are averages, and vary from place to place depending on the amount of cloud cover and the nature of the surface of the Earth that the solar radiation reaches. For example, ice and snow will reflect much more radiation than plants.
The energy absorbed by the Earth's surface is radiated back into the atmosphere as heat, or infrared radiation. This heat is absorbed by water vapor and carbon dioxide in the air. This is how the troposphere is heated.
The amount of heating of the atmosphere depends on the nature of the surface that is heating the air above it. Surfaces that are good absorbers of radiation (e.g. dirt) are also good radiators of heat. A surface that is a good reflector of radiation (e.g. snow) is a poor absorber and therefore a poor radiator of heat. Land is a better absorber of radiation than water. During the day land warms up more quickly than water. At night it loses heat more quickly than water. For this reason, places near the seashore have more moderate climates (less extremes of high and low temperatures) than places well inland. During the daytime mountainsides warm up more quickly than the valley floor. At night the mountain slopes cool more quickly.
WORKSHEET - The Earth's Atmosphere
- What is the force that holds the atmosphere to the Earth as it travels through space?
- At what point in the atmosphere does the temperature reach its coldest value?
- Which temperature regions of the atmosphere lie entirely within the homosphere?
- What causes the temperature to increase in the upper stratosphere?
- Extreme ultraviolet radiation, which would be harmful to humans, does not reach the Earth's surface. Where are these rays absorbed and what atmospheric layers are produced as a result?
- Why does the Earth's magnetosphere extend to a much greater distance above the side of the Earth turned away from the Sun than above the side facing the Sun?
- The planet Mercury, the planet closest to the Sun, has essentially no atmosphere. Is the temperature on Mercury during the day similar to the temperature there at night?
- What type of radiation heats the troposphere and how is it produced?
- Why is the temperature difference between day and night much greater in the interior of a continent than on the seacoast?
- Problem: Show that the fraction of solar radiation reaching the Earth is 0.00000000045 = 4.5x10^-10, or approximately one divided by 2 billion. This is a good test for your new calculators. Data: radius of Earth = 6370 km distance from Earth to Sun = 150,000,000 km = 1.5x10^8 km.
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