The Sun’s radiant energy drives the atmosphere into the patterns of everyday wind and weather. Earth’s atmosphere is a mixture of many gases but is comprised mostly of nitrogen and oxygen (78% and 21%, respectively, by volume). The atmosphere gets thinner with altitude; there is no definite upper limit. Almost 99% of the atmosphere lies within 19 miles of Earth’s surface.
Earth’s first atmosphere was likely hydrogen and helium and hydrogen compounds. Outgassing (the outpouring of gases from the hot interior) created Earth’s second atmosphere and supplied water vapor, which formed into clouds. Molecular oxygen was created when energetic rays from the Sun split water vapor into hydrogen and oxygen through photodissociation.
Condensation is the changing of water vapor into liquid water. Evaporation is the process of liquid water becoming water vapor. Precipitation is falling rain or snow. Water vapor that changes into liquid water or ice releases large amounts of latent heat, which is an important source of atmospheric energy. Water vapor is also a potent greenhouse gas.
Carbon dioxide, another greenhouse gas, is added to the atmosphere mainly from the decay of vegetation and is removed from the atmosphere by processes such as photosynthesis and chemical weathering. Carbon dioxide is currently entering the atmosphere at a greater rate that it is being removed. The increase is caused mainly by burning fossil fuels but also by deforestation.
Other greenhouse gases are methane, nitrous oxide, and chlorofluorocarbons (CFCs). CFCs also play a part in destroying the ozone in the stratosphere. The ozone shields life from harmful ultraviolet rays.
Aerosols are solid or liquid particles of various composition suspended in the air. Some of these impurities are beneficial but most are pollutants. Acid rain is a major environmental problem.
There are as many molecules in five breaths of air as there are stars in the universe.
Normal atmospheric pressure near sea level is 14.7 pounds per square inch. Atmospheric pressure is the amount of force exerted over an area of surface. Atmospheric pressure decreases with increasing altitude. Air temperature normally decreases up to an altitude of 36,000 feet. The lapse rate is the rate at which the air temperature decreases with altitude. Temperature inversion is the condition in which the air temperature increases with altitude.
The atmosphere may be divided into layers according to its vertical profile of temperature. The troposphere is the atmospheric layer nearest the surface (and which contains all of the weather with which we are familiar). Above the troposphere is the stratosphere. The boundary separating the troposphere and the stratosphere is the tropopause. Above the stratosphere is the mesosphere. The boundary separating the stratosphere and the mesosphere is the stratopause. Above the mesosphere is the thermosphere. The boundary separating the mesosphere and the thermosphere is the mesopause. The top of the thermosphere is the exosphere. The warmest layer is the thermosphere. Most of the ozone is in the stratosphere.
The atmosphere may also be divided into layers according to its gaseous composition or its electrical properties. The homosphere is the layer where the composition of the air is fairly uniform (by being well mixed); it begins at the surface and extends to the lower part of the thermosphere. The heterosphere is the layer where diffusion separates the gases; it lies above the homosphere. The ionosphere (not a true layer) is the electrified region in the upper atmosphere where large concentrations of ions and free electrons; it extends from about 37 miles above the surface to the top of the atmosphere (and therefore the bulk of it is in the thermosphere).
Weather is the condition of the atmosphere at any particular time and place. It includes the elements of air temperature, air pressure, humidity, clouds, precipitation, visibility, and wind. Climate is the measured and observed weather elements over a specified (long) interval of time.
Meteorology is the study of the atmosphere and its phenomena.
—July 2020
Lance Jones 