The Earth’s atmosphere is a complex gaseous envelope held in place by gravity. Its physical properties change dramatically with increasing distance from the surface. Scientists classify this vast region into distinct layers to better understand phenomena, from weather patterns to space weather effects. Classification depends on the specific property being studied, such as energy transfer, gas mixing, or solar radiation interaction. Using different criteria based on altitude provides a clearer framework for analysis.
Classification Based on Temperature
The most common method of dividing the atmosphere is based on how the temperature changes vertically, resulting in alternating temperature gradients. The Troposphere begins at the surface and extends up to an average altitude of about 12 kilometers, varying between 9 km at the poles and 17 km at the equator. This layer contains approximately 75% of the atmosphere’s total mass and is where nearly all weather occurs. Temperature generally decreases with height at a rate of about 6.5°C per kilometer because the air expands and cools as it rises away from the warm surface.
The Tropopause marks the upper boundary of the troposphere, where the temperature decrease stops. Above this boundary, the Stratosphere reaches an altitude of approximately 50 kilometers. Within the stratosphere, the temperature increases with height, rising from about -60°C at the tropopause to a maximum of about -15°C at its upper limit. This warming is caused by the ozone layer, which absorbs energetic ultraviolet radiation from the sun and converts it into heat.
The temperature profile reverses at the Stratopause, the boundary above the stratosphere. The Mesosphere extends from the stratopause up to about 80 to 85 kilometers. In this region, the temperature decreases with altitude, reaching the coldest temperatures in the entire atmosphere, averaging around -85°C at the top. These cold temperatures result from the declining absorption of solar radiation and the loss of heat to space.
The Mesopause serves as the transition point into the final layer, where temperatures begin to rise dramatically. The Thermosphere extends from the mesopause up to about 700 kilometers, with temperatures reaching as high as 2,000°C near the top. This intense heating is due to the absorption of high-energy ultraviolet and X-ray radiation by sparse oxygen and nitrogen molecules. Despite the high temperature values, the air feels cold because the molecules are so far apart that very little heat energy can be transferred. Beyond the thermosphere is the Exosphere, where the atmosphere gradually merges with the vacuum of outer space.
Classification Based on Chemical Composition
A different classification system focuses on the relative abundance and mixing of atmospheric gases, dividing the atmosphere into two broad regions. The lower portion is the Homosphere, extending from the Earth’s surface up to roughly 80 to 100 kilometers. Within this region, the air is constantly mixed by turbulent eddy diffusion, driven by winds and air currents. Because of this vigorous mixing, the chemical composition remains relatively uniform, consisting mostly of 78% nitrogen and 21% oxygen.
Above the homosphere lies the Heterosphere, which extends to the outer edge of the atmosphere. The boundary between these two regions is known as the Turbopause (or homopause), situated at about 100 kilometers. In the heterosphere, the air is too thin for turbulent mixing to be effective, allowing gases to separate based on their molecular weight. This process, known as gravitational separation, causes lighter gases like hydrogen and helium to rise to the highest levels, while heavier gases remain lower. Consequently, the composition in the heterosphere is not uniform with altitude.
Classification Based on Electrical Charge
A third classification method is based on the presence of electrically charged particles, defining two regions that overlap with the temperature-based layers. The Ionosphere is a vast region where high-energy solar radiation, such as X-rays and extreme ultraviolet light, strips electrons from neutral atoms and molecules. This process, called ionization, creates a layer of plasma—a gas containing free ions and electrons—that spans from about 50 km up to 965 km, encompassing parts of the mesosphere and thermosphere.
The presence of this electrically charged layer has practical significance because it affects the propagation of radio waves. The ionosphere was historically known for reflecting certain radio frequencies back to Earth, allowing for long-distance communication. The Magnetosphere represents the outermost region, defined not by the gas itself, but by the influence of the Earth’s magnetic field. This magnetic bubble extends tens of thousands of kilometers into space, trapping charged particles from the solar wind and shielding the planet from harmful cosmic radiation. The interaction between these trapped particles and the upper atmosphere produces the spectacular aurora displays near the polar regions.