radiation, an essential feature of the Earth's atmosphere for the continuing sustenance of life.
The so-called 'mixing layer' of the atmosphere is generally turbulent, varying in depth with meteorological conditions, ranging from a few hundred metres (the height of the Eiffel tower) to about 2 km. Pollutants released into this layer are thus effectively dispersed into a larger volume of air within a few hours, including some transfer to higher levels of the troposphere. If turbulence in the mixing layer is impeded, pollutants accumulate in an even smaller volume, giving rise to high concentrations ('smog'). This occurs when wind speeds are low and when the Earth's surface temperature is lower than the air above (a temperature inversion). Contrary to the troposphere, where on average the temperature decreases with height, the inversion layer (typically 100 to 300 m in depth) is characterised by a temporary increase of temperature with height, with weak wind and slow mixing, and practically no exchange with the overlying background troposphere.