Troposphere, << TROH puh sfihr, >> is the layer of the atmosphere closest to Earth’s surface. People live in the troposphere, and nearly all of Earth’s weather—including most clouds, rain, and snow-occurs there. The troposphere contains about 80 percent of the atmosphere’s mass and about 99 percent of its water. Its upper boundary, called the tropopause, separates it from the stratosphere, the next layer of Earth’s atmosphere. The tropopause varies in altitude, lying an average of about 12 miles (19 kilometers) above the equator and about 6 miles (10 kilometers) above the North and South poles.
Within the troposphere, the air generally grows colder as altitude increases. On average, the air temperature drops about 3.5 Fahrenheit degrees every 1,000 feet (6.5 Celsius degrees every 1,000 meters). The troposphere’s temperature averages about 59 °F (15 °C) near Earth’s surface and about -60 °F (-51 °C) at 6 miles (10 kilometers) above the surface. The troposphere’s temperature varies with latitude. At the equator, the tropopause can be as cold as -112 °F (-80 °C).
Air temperature variations within the troposphere play an important role in creating weather. Colder, denser air sinks, forcing warmer, less dense air to rise. Rising air expands and cools. As air cools, water vapor forms clouds and precipitation may fall. Sinking air is compressed and warms. In warmer air, clouds either vaporize or fail to develop, resulting in fair weather.
The troposphere helps maintain moderate temperatures at Earth’s surface. It allows most sunlight to pass through it and heat the surface. The surface and the atmosphere radiate heat, which eventually escapes into space. Some of the heat, however, is absorbed by certain atmospheric gases—mainly water vapor and carbon dioxide—and radiated back to Earth’s surface. This warming, called the greenhouse effect, raises Earth’s average surface temperature by 59 Fahrenheit degrees (33 Celsius degrees).
