The atmosphere studied here is limited to the troposphere, the lower layer of the Earth's atmosphere. It is characterized by a regular decrease in temperature as altitude increases, at the average lapse rate of —5 to —6°K per km. This is where most meteorological phenomena, including the formation of clouds, develop. The altitude of the upper limit of the troposphere varies between 8 km at the poles and 18 km at the equator depending on the geographical latitude, seasons, and meteorological conditions. In the region immediately above the troposphere, known as the tropopause, the temperature varies between 190°K at the equator and 220°K at the poles. The tropopause constitutes an upper limit for most clouds.