| Weather conditions have a significant impact on the levels of pollution observed on the ground. High pressure periods characterized by calm weather, with weak wind and temperature inversion, lead to a rapid increase in the concentration of pollutants at ground level. |
| THE INVERSION OF TEMPERATURE: | |
| Air temperature decreases with altitude. Hot air containing pollutants at ground level disperses vertically. In a temperature inversion condition, the ground cooled significantly during the night (nocturnal terrestrial radiation). | |
| The temperature at an altitude of a few hundred meters is therefore higher than that measured at ground level, and the pollutants are then trapped under an inversion layer which acts as a thermal cover. If, at the same time, there is no wind, the pollutants increase in significant proportions. | |
| THE FALL IN PANACHE: | |
| In the presence of fairly strong winds, the plumes of pollutants emitted by the tall chimneys may only fall several hundred meters or even several kilometers from their sources. Under these fallouts, the pollution levels are sometimes very high. These are acute, brief and localized phenomena. | |
| THE GREENHOUSE EFFECT | |
| Only a small part of solar radiation is absorbed directly by the atmosphere; another is diffused in all directions, finally a third part reaches the ground. The latter in turn sends this energy back to space in the form of infrared radiant heat, heat which is in part retained, trapped by the greenhouse gases present in the atmosphere (carbon dioxide, methane, CFCs, protoxide nitrogen, ozone, etc.). The result is an increase in the temperature of the air in the lower atmosphere. It is this heating of the atmosphere by the absorption of infrared rays emitted by the earth that we call the greenhouse effect. In itself, this phenomenon is natural, even welcome: without the greenhouse effect, the earth would have an average temperature of –18 ° C against + 15 ° C currently. But today, due to the increase in greenhouse gas concentrations in the atmosphere, this phenomenon is intensifying. In the coming decades, it could lead to an increase in temperatures that is all the more serious because nothing would be done to reverse the trend. ATmolor |