Normally, the temperature of the air fells with increase in height and warm flue gases rise and are gradually cooled by mixing with the air.
On clear nights, however, the temperature of the ground falls more quickly by radiation than does that of the air above and there develops a boundary layer above, where the temperature of the air again increases with height.
More than one inversion layer can develop. The mixture of air and flue gas from C, though still warm, cannot rise in the warmer air at the inversion boundary and, therefore,, accumulates below it.
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The flue gases from a issuing above the inversion flow smoothly away and, because of the temperature gradient, cannot diffuse down to the ground through the inversion layer. If the volume of flue gases is very large and the chimney reaches nearly up to the inversion boundary, they may penetrate it, as proved by observation from a loft.
Accumulation of flue gases below an inversion boundary can become most unpleasant and even dangerous in a steep-sided valley as proved by disasters at Donera (U.S.A.) and the Meuse Valley (Belgium).
When there is a development of water mist (fog), its mixture with smoke is called ‘smog’. The SO2 in the flue gases dissolves in the water droplets and is quickly oxidized to form dilute sulphuric acid, H2SO4; this hinders evaporation of the water and stabilizes the fog.
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Inversions frequently develop upwards from the ground to a height of about 40 m, and infrequently up to 100 m as during the London smog of 1952. It is highly desirable that, factory chimneys, with their large volumes of flue gas, compared with that from houses, be, at least, 40 m high.
At this height, turbulence and down-draughts from buildings do not disturb the flow of gases from most factories. Immediately after the London smog, the 1952 UK Government Committee on Air Pollution recommended a height for factory chimneys of 120 ft. (37 m), with discretion for small emissions. The complaints from certain small fertilizer factories ceased when the chimneys were raised to 37 m as reported in the United Kingdom.