The broadly horizontal layering of the atmosphere, the layers being distinguished by differences in the rate of change of temperature with height, which either favour or discourage the development of vertical exchanges (convection). From the surface of the Earth upwards the layers are: (a) the troposphere, in which convection is often prominent, especially over warm regions, extending to the tropopause at a somewhat variable height, generally about 8 km over the poles, 11 km in middle latitudes, and 16 km over the equator; (b) the stratosphere, in which there is much less vertical motion, and which extends from the tropopause to about 50 km at the stratopause; (c) the mesosphere, in which there is once again more convection, extending from the stratopause to a height of about 80 km at the mesopause; and (d) the thermosphere, extending from the mesopause to the effective limit of the atmosphere, at about 200 km.