The troposphere begins at the Earth's surface and extends up to 4–12 miles (6–20 km) high varying from the equator to the poles. At the equator it is around 11–12 miles (18–20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high. The troposphere is the most dense layer of the atmosphere due to it containing 75% of the atmosphere's mass. And is composed of 78% nitrogen, 21% oxygen, small traces of other gases, and almost all of the atmosphere's water vapor. As the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height. As you climb higher, the temperature drops from about 62 °F (17 °C) to -60 °F (-51 °C). However, sometimes the temperature does not decrease with height in the troposphere, but increases. Such a situation is known as a temperature inversion. Temperature inversions limit or prevent the vertical mixing of air. Such atmospheric stability can lead to air pollution episodes with air pollutants emitted at ground level becoming trapped underneath the temperature inversion.
The troposphere is the layer where most of the world's weather takes place. Since temperature decreases with altitude in the troposphere, warm air near the surface of the Earth can readily rise, being less dense than the colder air above it. In fact air molecules can travel to the top of the troposphere and back down again in a just a few days. Such vertical movement or convection of air generates clouds and ultimately rain from the moisture within the air, and gives rise to much of the weather which we experience. The troposphere is capped by the tropopause, a region of stable temperature. Air temperature then begins to rise in the stratosphere. Such a temperature increase prevents much air convection beyond the tropopause, and consequently most weather phenomena, including towering cumulonimbus thunderclouds, are confined to the troposphere.