Composition Atmosphere and Structure

The atmosphere can be divided into layers based on the atmospheric pressure and temperature profiles (the way these quantities change with height). Atmospheric temperature drops steadily from its value at the surface, about 290K (63°F; 17°C), until it reaches a minimum of around 220K ( −64°F; −53°C) at 6 mi (10 km) above the surface. This first layer is called the troposphere, and ranges in pressure from over 1,000 millibars at sea level to 100 millibars at the top of the layer, the tropopause. Above the tropopause, the temperature rises with increasing altitude up to about 27 mi (45 km). This region of increasing temperatures is the stratosphere, spanning a pressure range from 100 millibars at its base to about 10 millibars at the stratopause, the top of the layer. Above 30 mi (50 km), the temperature resumes its drop with altitude, reaching a very cold minimum of 180K ( −135°F; −93°C) at around 48 mi (80 km). This layer is the mesosphere, which at its top (the mesopause) has an atmospheric pressure of only 0.01 millibars (that is, only 1/100,000th of the surface pressure). Above the mesosphere lies the thermosphere, extending hundreds of miles upward toward the vacuum of space. It is not possible to place an exact "top" of the atmosphere; air molecules simply become scarcer and more rarefied until the atmosphere blends with the material found in space.