The temperature gradient in each layer is determined by the heat source of the layer Figure below. The four main layers of the atmosphere have different temperature gradients, creating the thermal structure of the atmosphere. The layers of the atmosphere appear as different colors in this image from the International Space Station. Most of the important processes of the atmosphere take place in the lowest two layers: the troposphere and the stratosphere.
The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6. What is the source of heat for the troposphere? The temperature is also higher near the surface because of the greater density of gases. The higher gravity causes the temperature to rise. Notice that in the troposphere warmer air is beneath cooler air. What do you think the consequence of this is? This condition is unstable.
The warm air near the surface rises and cool air higher in the troposphere sinks. So air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. Sometimes there is a temperature inversion , air temperature in the troposphere increases with altitude and warm air sits over cold air. Inversions are very stable and may last for several days or even weeks. Inversions form:. Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities Figure below.
Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air. At the top of the troposphere is a thin layer in which the temperature does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere.
Air from the troposphere and stratosphere rarely mix. Ash and gas from a large volcanic eruption may burst into the stratosphere , the layer above the troposphere.
Once in the stratosphere, it remains suspended there for many years because there is so little mixing between the two layers. Pilots like to fly in the lower portions of the stratosphere because there is little air turbulence.
In the stratosphere, temperature increases with altitude. Between about 53 miles 85 km and miles km lies the thermosphere.
This layer is known as the upper atmosphere. While still extremely thin, the gases of the thermosphere become increasingly denser as one descends toward the earth. As such, incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase. Because of this absorption, the temperature increases with height. However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin due to the very thin atmosphere.
The high temperature indicates the amount of the energy absorbed by the molecules but with so few in this layer, the total number of molecules is not enough to heat our skin. This layer extends from around 31 miles 50 km above the Earth's surface to 53 miles 85 km. The gases, including the oxygen molecules, continue to become denser as one descends.
The gases in the mesosphere are now thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Both the stratosphere next layer down and the mesosphere are considered the middle atmosphere. The transition boundary which separates the mesosphere from the stratosphere is called the stratopause. The Stratosphere extends around 31 miles 50 km down to anywhere from 4 to 12 miles 6 to 20 km above the Earth's surface.
Another important feature of the stratosphere is the cold pool of air that forms at high latitudes during the winter. This cold air is centered in the lower stratosphere at about 25 km.
During the Southern Hemisphere winter air can reach temperatures colder than C near the South Pole. In the Northern Hemisphere, the lowest temperatures reach about C. As a result, a zone of strong westerly winds or vortex forms and surrounds each pole. Because the temperature contrast is greatest in the vicinity of the South Pole, the vortex that forms there during the Southern Hemisphere winter is considerably stronger than the vortex that forms during the Northern Hemisphere winter.
One consequence of the very cold temperatures of the stratosphere near the South Pole is the formation of two types of polar stratospheric clouds PSC. One consists of pure water ice.
Although the air contains very little moisture, at very low temperatures even these small amounts can produce ice crystal-containing clouds through the process of sublimation deposition. The other and more common type is composed of a hydrated form of nitric acid HNO 3 : the nitric acid molecules are attached to water molecules. Reactions in these clouds convert stable forms of chlorine to Cl 2 which readily dissociates under the influence of sunlight and destroys ozone.
These reactions also remove gaseous HNO 3. The result is almost total destruction of ozone within the lower stratosphere at altitudes of about 14 to 19 km. The NO 2 then reacts with ClO, removing it from reactions with ozone. In the Southern Hemisphere, they are a yearly occurrence, although their spatial extent and temporal duration vary.
This figure shows where the PSCs can be expected.
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