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 troposphere is divided into six zonal flow regions, called cells. These are responsible for Atmospheric circulation, and produce the prevailing winds.
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 the poles, the troposphere is thinner and the temperature sinks only to -45 °C, while at the Equator the temperature can reach -75 °C.
The reason for the temperature variations in the troposphere is because the temperature is determined by the radiation from the land back into the air. As we move away from the earth's surface, convective heating has a smaller effect and the air cools.
For every 1000 meter increase in altitude, the temperature goes down by approximately 6.4°C. this is because the higher the altitude, the less atmospheric particles there are to trap the heat, therefore resulting in the heat escaping.
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.