Tropopause - A distinct layer in the earth's atmosphere which marks the uppermost reaches of the troposphere. The tropopause exists at varying heights in the atmosphere, depending on your point of reference from the earth's surface. The tropopause is actually made up of several overlapping layers, through which the troposphere and stratosphere exchange materials and energy. Most of the water in the atmosphere exists below the tropopause. Moisture usually only penetrates the tropopause in cases of severe thunderstorms in the troposphere.
Winds are mostly absent here, except for some high-velocity winds just above the tropopause.
It's most noteworthy trait is its marked decline in temperature with increasing altitude. The highest part of the mesosphere, just under the mesopause, marks the upper boundary of the homosphere.