Atmospheric Layers

The atmosphere is classified into five different layers,These layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer differs in composition and temperature profile.

• Troposphere The layer closest to Earth’s surface, the troposphere, contains most of the mass of the atmosphere. Weather occurs in the troposphere. In the troposphere, air temperature decreases as altitude increases. The altitude at which the temperature stops decreasing is called the tropopause. The height of the tropopause varies from about 16 km above Earth’s surface in the tropics to about 9 km above it at the poles. Temperatures at the tropopause can be as low as –60°C.

• Stratosphere Above the tropopause is the stratosphere, a layer in which the air temperature mainly increases with altitude and contains the ozone layer. In the lower stratosphere below the ozone layer, the temperature stays constant with altitude. However, starting at the bottom of the ozone layer, the temperature in the stratosphere increases as altitude increases. This heating is caused by ozone molecules, which absorb ultraviolet radiation from the Sun. At the stratopause, air temperature stops increasing with altitude. The stratopause is about 48 km above Earth’s surface. About 99.9 percent of the mass of Earth’s atmosphere is below the stratopause.

• Mesosphere Above the stratopause is the mesosphere, which is about 50 km to 100 km above Earth’s surface. In the mesosphere, air temperature decreases with altitude. This temperature decrease occurs because very little solar radiation is absorbed in this layer. The top of the mesosphere, where temperatures stop decreasing with altitude, is called the mesopause.

• Thermosphere The thermosphere is the layer between about 100 km and 500 km above Earth’s surface. In this layer, the extremely low density of air causes the temperature to rise. Temperatures in this layer can be more than 1000°C. The ionosphere, which is made of electrically charged particles, is part of the thermosphere.

• Exosphere The exosphere is the outermost layer of Earth’s atmosphere. The exosphere extends from about 500 km to more than 10,000 km above Earth’s surface. There is no clear boundary at the top of the exosphere. Instead, the exosphere can be thought of as the transitional region between Earth’s atmosphere and outer space. The number of atoms and molecules in the exosphere becomes very small as altitude increases.

In the exosphere, atoms and molecules are so far apart that they rarely collide with each other. In this layer, some atoms and molecules are moving fast enough that they are able to escape into outer space.