The Earth’s atmosphere (excluding the exosphere) extends for several hundred kilometers above the surface of the Earth and consists of a number of distinct layers. From the surface up, these layers are the troposphere, the stratosphere, the mesosphere, and the thermosphere.
The Troposphere
The troposphere is small but mighty. Although it is the thinnest layer of the atmosphere, the troposphere contains 80 % of its total mass, supports all life on Earth, and is where the weather happens. The height of the troposphere varies with temperature, so it changes with latitude and the season. The troposphere is only about 7 km deep at the poles during winter and can reach as high as 20 km over the tropics.
The lowest part of the troposphere, where the air is influenced by friction from the Earth’s surface, is known as the planetary boundary layer, while the top of the troposphere, where it meets the stratosphere, is known as the tropopause. From the Earth’s surface to the tropopause, the air cools with height. Temperatures at the tropopause are about -70 ºC at the equator and -50 ºC t the poles. Using weather balloons and radiosondes, the tropopause is easy to locate because temperatures above the tropopause begin to increase with height. This phenomenon creates an inversion at the tropopause which separates the turbulent troposphere from the stable stratosphere and inhibits mixing between the two layers.
The tropopause is responsible for the classic anvil shape observed at the top of cumulonimbus clouds. Violent convection causes these clouds to increase in height until they reach the inversion layer at the tropopause. The inversion limits mixing into the stratosphere, causing the cloud to spread laterally and forming an anvil top. In some cases, cumulonimbus clouds have a dome shaped cloud above the anvil top, known as an overshooting top. An overshooting top indicates that the vertical motion of the cloud was strong enough to cause mixing above the tropopause, and indicates a particularly severe cumulonimbus. These are commonly seen in supercells and hurricanes. The tropopause also determines the height of the subtropical and polar jet streams.
The Stratosphere
The stratosphere is immediately above the troposphere and extends to approximately 50 km above the Earth’s surface. Temperatures in the stratosphere warm with height, from -70 ºC at the tropopause to about -15 ºC at the stratopause. This limits vertical mixing and makes the stratosphere very stable.
One of the most important characteristics of the stratosphere is the ozone layer. Stratospheric ozone is formed by ultraviolet radiation which splits apart oxygen molecules. These highly reactive solitary oxygen atoms bond with intact oxygen molecules to form ozone. Stratospheric ozone is responsible for absorbing most of the high energy incoming ultraviolet (shortwave) radiation from the sun. In fact, the ozone layer absorbs so much radiation that most life on Earth could not exist without it.
Horizontal winds in the stratosphere are strong, particularly at the higher latitudes. Stratospheric polar winds are known as a polar vortices, which cause chemical contaminants that reach the stratosphere to accumulate in polar regions. When intrusions into the stratosphere do occur, due to supercell thunderstorms or volcanic eruptions for example, water vapour and dust can be transported very rapidly around the world and tend to remain in the stratosphere for a long time. In very cold conditions clouds of super cooled water droplets can form in the stratosphere above the polar regions. These are known as polar stratospheric clouds or nacreous clouds and, while beautiful and iridescent in appearance, their chemical composition is commonly associated with the destruction of the ozone layer.
Although most of our weather occurs in the troposphere, conditions in the stratosphere can impact the surface. The best example of this is a phenomenon known as sudden stratospheric warming. Occasionally, temperatures in the stratosphere increase by as much as 50 ºC over the course of a few days. This impacts, and in some cases reverses, the west to east flow of winds in the stratosphere. These wind changes cause the polar vortex to expand and oscillate, which in turn influences the position of the polar jet stream and conditions at the surface. Sudden stratospheric warming events can increase the likelihood of extreme snowfall and storms in both Europe and North America.
The Mesosphere
At around 40 km above the Earth, the stratopause separates the stratosphere from the mesosphere. The mesosphere is approximately 50 km thick, and cools with height. It is the coldest layer of the atmosphere, reaching temperatures below -150 ºC at the mesopause. Because it is too high for airplane flight and too low for space flight, less is known about the mesosphere compared to other regions of the atmosphere.
However, we do know that the mesosphere contains atmospheric disturbances including gravity waves, planetary waves, and atmospheric tides. These features act to transfer energy around the planet and it is believed that they help to drive the general circulation of the lower atmosphere.
Due to the lack of water vapour in the mesosphere, clouds at this height are uncommon. Occasionally, noctilucent clouds form in the mesosphere. Because of their extreme height (80 km or 260,000 feet), noctilucent clouds are only visible before astronomical twilight and appear as glowing white clouds in the night sky. Speculation abounds in terms of the formation mechanisms for these clouds and they have been attributed to volcanic eruptions, space flight, and meteor dust.
The Thermosphere
At the boundary between space and the Earth's atmosphere lies the thermosphere. The thermosphere warms with height, has a very high diurnal temperature range, and an extremely low molecular density. Incoming solar radiation tends to split any molecules that find their way to the thermosphere and the movement of these ionized particles produces phenomena such as the aurorae, which are often seen in polar regions.
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