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The relationship between pollution and cloudiness

The particulate produced by the human activity, especially in industrialized areas, has a direct effect on the rain

It is well known that human activities has deep effects on climate and on the change of cloud characteristics and on their ability to provoke rains, especially in industrialized areas. However, the relationship was not been clear until now: in some cases pollution seems to increase rains, in some others do not. This relationship is now clear, thanks to an article on Science by Sandro Fuzzi, with the Atmosphere and Climate Institute of the CNR Bologna, and colleagues.

This research is very important, since clouds and rains have a pivotal role in the Earth climate determination, and their role is one of the main uncertainties for the creation of reliable climate models.

“The small particles in the atmosphere”, Sandro Fuzzi says, “are necessary for the creation of clouds, since these particles work as “condensation nuclei” for the small drops that form the cloud. Without these particles, clouds would not exist. In particular, the characteristics of such particles determine the structure of clouds and their ability to produce rains. As a matter of fact, just a small percentage of clouds give rise to precipitations. Most of them evaporate and free in the atmosphere steam and the condensation nuclei”.

Researchers were able to single out two contrasting effects of aerosol onto precipitations: a radiative effect, that tends to lower precipitations, and a microphysical effect, that tends to rise them. “Until now these effect were take into consideration independently”, Fuzzi explains. “Now, we demonstrate that their combined effect gives rise to an increment of precipitations for small rises in the aerosol concentration produced by the human activity, and to a decrement of rains for strong increments in the aerosol concentrations. In the latter case, the radiative effect prevails”.

In brief, the emission of pollution by industrial processes, traffic and other human activities, increases rains until a critical level. Beyond such a limit, precipitations decrease. “Given the central role of water for humanity, the scientific community studies from years the relationships between industrialization and rains, necessary for the water supply”, the researcher says. “Notwithstanding the number of observations regarding the relationship between pollution and precipitations, in some cases this relationship is positive, in some other is negative. In any case, it is very difficult to understand the causal connection between these two phenomena.”

The new approach exposed in the article, which is based on a wide set of experimental recordings in situ and from satellite, is therefore able to reconciles the apparently opposed observations about the link between pollution and rains, and it could be the basis for more accurate aerosol-cloud-precipitation based climate models. Finally it could also be important for the studies on the artificial changes of precipitations.

by T N
04 may 2009, Technical Area > Science News