An interesting phenomenon in the atmosphere is that of thermal inversionwhen the action of air pollutants can be greatly aggravated.
It works like this: Normally, the air near the ground surface is in constant vertical movement due to the convective process (convection currents). Solar radiation heats the surface of the ground, which in turn heats the air that bathes it; This hot air is less dense than cold air, so hot air rises (upward vertical movement) and denser cold air goes down (vertical downward movement).
This cold air that touches the ground surface, receiving heat from it, heats up, becomes less dense, rises, giving way to a new downward movement of cold air.
And the cycle repeats itself. It is normal, therefore, to have warm air in a near-ground layer, cold air in a layer just above it, and even colder air in higher layers, but in constant exchange for convection currents. This normal air situation contributes to the dispersion of local pollution.
In thermal inversion, unfavorable conditions may, however, cause a change in the arrangement of the layers in the atmosphere. Usually in winter there may be rapid cooling of the soil or rapid warming of the upper atmospheric layers.. When this happens, the warm air above the cold air layer acts as a block, not allowing vertical convection movements: the cold air near the ground does not rise because it is the densest and the warm air that comes to it. it is on the top it does not go down because it is the least dense. If so, the fumes and gases produced by chimneys and vehicles will not be dispersed by vertical currents. The smoke coils of the chimneys assume a horizontal position, being close to the ground. The city is shrouded in fog and consequently the concentration of toxic substances increases greatly.
The phenomenon is common in winter in cities such as New York, Sao Paulo and Tokyo, aggravated by the high concentration of toxic pollutants daily discharged into the atmosphere.