The troposphere is a layer of the Earth’s atmosphere located between the ground and about 14 km altitude. It is the part of the atmosphere where the temperature decreases rapidly with increasing altitude.
In the figure, the lower part of the temperature profile, from the ground to 14 km altitude, air temperature decreases with altitude.
Warmer air masses in the troposphere tend to rise towards colder, less dense air masses above it and cool down, while colder air masses have the tendency to descend and warm up. This leads to the air constantly being mixed at all altitudes in the troposphere.