Precipitation

Precipitation serves an effective cleansing process of pollutants in the atmosphere as follows:

Noon

FIG. 5.6.10 Frequency of stagnating high-pressure cells over the eastern United States. The contours give the number of periods of four or more successive days between 1936-1965.

FIG. 5.6.10 Frequency of stagnating high-pressure cells over the eastern United States. The contours give the number of periods of four or more successive days between 1936-1965.

The washing out or scavenging of large particles by falling raindrops or snowflakes (washout) The accumulation of small particles in the formation of raindrops or snowflakes in clouds (rainout) The removal of gaseous pollutants by dissolution or absorption

The efficiencies of these processes depend on complex relationships between the properties of the pollutants and the characteristics of precipitation. The most effective and prevalent process is the washout of large particles in the lower layer of the atmosphere where most pollutants are released.

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