NIGHT RADIATION AND TEMPERATURE INVERSION At night, wind speeds tend to diminish; the laminar boundary layer then becomes thicker than by day and the turbulent boundary layer may become thinner, being reduced to perhaps only io to 15 metres in thickness.
These changes may be carried still farther if the sky is cloudless, thus allowing radiation from the ground to escape into space. Loss of heat by radiation cools the ground and this in turn cools the air lying nearest to the ground. Thus, instead of temperature decreasing with increasing height, a `temperature inversion' is set up : over the cold air near the ground lies air at a higher temperature—up to a certain height, the top of the inversion, above which the usual lapse rate is again encountered.
As the air is coldest and densest at the bottom of the inversion, gravity tends to prevent it from ascending and mixing with warmer air above. The air in the inversion becomes stratified according to temperature and remains very stable, in contrast to the instability that is apt to develop when the ground is heated. Such a layer of cold, heavy air may flow slowly downhill as a nearly laminar katabatic wind, filling hollows with cold air and aiding the formation of frost pockets (Geiger, 195o, p. 203).
In a temperature inversion, spores and dust particles tend to settle out, leaving the air relatively clean although the air above the inversion may continue to carry a normal spore-load.