Atmospheric stratification is the division of the
atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
into distinct layers, each with specific characteristics such as temperature or composition.
In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as
balloon soundingsA weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, and humidity by means of a small, expendable measuring device called a radiosonde...
, temperature provides a useful metric to distinguish between atmospheric layers.
The lowest layer is the
troposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols....
, in which temperature decreases at what is known as the the
atmospheric lapse rateThe lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and...
.
Atmospheric stratification is the division of the
atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
into distinct layers, each with specific characteristics such as temperature or composition.
In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as
balloon soundingsA weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, and humidity by means of a small, expendable measuring device called a radiosonde...
, temperature provides a useful metric to distinguish between atmospheric layers.
The lowest layer is the
troposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols....
, in which temperature decreases at what is known as the the
atmospheric lapse rateThe lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and...
. The farther that air is from Earth's surface, the less heat it receives radiated from the ground. This continues until the
tropopauseThe tropopause is the atmospheric boundary between the troposphere and the stratosphere. Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry. More formally, it is the region of the atmosphere where the lapse rate changes from...
is reached. The tropopause ranges in altitude depending on latitude and weather patters, but is typically around 9 km at the
polesA geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...
, and 17 km at the equator.
Above the tropopause, the temperature begins to rise due to increased absorption of ultraviolet radiation by the
ozone layerThe ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 93-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on earth. Over 91% of the ozone in Earth's atmosphere is present here...
. This region is known as the
stratosphereThe stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
. While the temperature may be -60°
CelsiusCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
at the troposphere, the top of the stratosphere is much warmer, and may be near freezing temperature. A temperature gradient that warms with altitude, as in the stratosphere, is known as an
inversionIn meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs...
.
The stratosphere is capped by the
stratopauseThe stratopause is the level of the atmosphere which is the boundary between two layers, stratosphere and the mesosphere...
, above which is the
mesosphereThe mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located about 50 to 85 kilometers above the Earth's surface.The stratosphere and mesosphere are referred to as the middle atmosphere...
. The temperature again falls with height in the mesosphere, and may drop to -100° Celsius at the top of this layer in the
mesopauseThe mesopause is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong cooling by radiative emission from CO2, the mesopause is the coldest place on Earth with temperatures as low as -100°C...
.
The next layer is the
thermosphereThe thermosphere is biggest of all the layers of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the upper part of the thermosphere, between...
, where there is once again a temperature inversion. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The absorption of solar radiation by such a thin atmosphere causes the temperature to rise tremendously, to an average of 80°C at 120 km altitude. The air is so rarified, that an individual molecule (of
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
, for example) travels an average of 1 kilometer between collisions with other molecules. Because of the relative infrequency of collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposhpere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the
turbopauseThe turbopause marks the altitude in the Earth's atmosphere below which turbulent mixing dominates. The region below the turbopause is known as the homosphere, where the chemical constituents are well mixed and display identical height distributions; in other words, the chemical composition of the...
. The region below is the homosphere, and the region above is the heterosphere.
The last layer is termed the
exosphereThe exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule can escape to space if it is moving fast enough to attain escape velocity; otherwise it will be pulled back to the celestial body by gravity...
, and it is here that a quickly moving particle can easily escape Earth's gravity and leave the atmosphere without colliding with another molecule.