The coldest place in the Earth's lower atmosphere is usually not over the North or South Poles, as might be expected, but over the Equator. The tropopause, the boundary between the two lowest layers of the atmosphere, varies in height from an average of only 9 kilometres above the two poles to 18 kilometres over the Equator. Air temperature steadily decreases up to the top of the tropopause. So, temperatures often fall as low as −80°C high over the equator, whereas over the poles they rarely fall below −55°C.