From Academic Kids
The troposphere starts at the earth's surface and extends to an altitude of 16-18 km over tropical regions, decreasing to less than 10 km over the poles. This layer contains approximately 80% of the atmosphere's total mass. Generally, jets fly near the top of this layer. The troposphere is directly below the stratosphere.
The word troposphere stems from the Greek "tropos" for "turning" or "mixing". This region, constantly in motion, is the densest layer of the Earth's atmosphere. Nitrogen and oxygen are the primary gases present in this region. The lapse rate, which is the change of temperature with respect to height, is larger than in other layers, the temperature decreasing at middle latitudes from approx. +17°C at sea level to approx. -52°C at the beginning of the tropopause. At poles, troposphere is thinner and the temperature sinks only to -45 °C, while at the equator temperature can reach -75 °C.
The tropopause marks the limit of the troposphere and the beginning of the stratosphere. The temperature above the tropopause increases slowly with height up to about 50 km.
- Earth's Atmosphere (http://liftoff.msfc.nasa.gov/academy/space/atmosphere.html) according to NASA
- Composition of the Atmosphere (http://csep10.phys.utk.edu/astr161/lect/earth/atmosphere.html), from the University of Tennessee Physics dept.cs:Troposféra