Atmospheric Layers

Troposphere The troposphere is the lowest layer of Earth’s atmosphere. In this layer, the temperature of the air is generally highest near Earth’s surface and decreases with altitude. Throughout the troposphere, the temperature falls by an average of 6.5 °C per 1000 meters of increase in altitude. Earth’s surface is the major direct source of heat for the troposphere, but nearly all of that heat comes originally from the sun. Earth’s surface absorbs sunlight and radiates it back into the atmosphere as heat. The temperature is also higher near the surface because of the greater density of gases there.

Because of the temperature gradient in the troposphere, warmer air is located below cooler air. This condition is unstable. Warmer air near the surface rises because it is less dense, and cooler air higher in the troposphere sinks because it is denser. As a result, air in the troposphere does a lot of mixing, which causes the temperature gradient to vary with time and place. The rising and sinking of air in the troposphere also causes weather, all of which takes place in the troposphere.

Under certain conditions, a temperature inversion may occur in the troposphere. This happens when warmer air sits over colder air, which is at the surface. Temperature inversions often occur during the winter when the ground is very cold at night. The cold ground cools the air directly above it, making this layer of air denser than the air above it. Temperature inversions may also occur near a coast where cold seawater cools the air above it. The cooler, denser air moves inland and slides beneath warmer air over the land. A temperature inversion is very stable, so it may last for several days or even weeks.

At the top of the troposphere is a thin layer of air in which the temperature does not change with altitude. This layer, called the tropopause, traps the cooler, denser air of the troposphere beneath the warmer, less dense air of the stratosphere. This prevents air in the troposphere and stratosphere from mixing