In some cities, it is sometimes possible to see pollution concentrated on the horizon and right above this layer of brownish air, a cloudless, blue sky. The pollution is so different that there seems to be a fine transparent line dividing the atmosphere. This concentration of pollutants generally occurs when there is thermal inversion. It happens when the humidity of the air is low, the sky is practically cloudless and there is no wind.
It is commonest during the winter in Brazil’s South, Southeast and Midwest. In the Northeast it happens practically the whole year long.
When there is thermal inversion the cold air (denser) is trapped close to the ground, pressed down by a layer of warm air (lighter). The lack of wind and humidity also prevents dispersal of the air. Therefore, the pollutants emitted by vehicles and industries accumulate between about one and three kilometers above the earth’s surface.
The phenomenon was named thermal inversion because the air close to the ground is, generally speaking, warm and not cold. When there is no thermal inversion the air moves cyclically in the earth’s atmosphere: the cold air descends, heats up close to the ground, gets lighter and rises because it is warm. The winds and clouds help with this movement and in this way the pollutants become diluted in the atmosphere rather than trapped close to the city.
Maria Assunção Faus da Silva Dias
University of São Paulo (USP)