Pollution causes more problems than initially thought

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  • Rain falls at Lord's Cricket Ground, London. Image:Anthony Devlin/PA Wire/Press Association Images

    Pollution in London last April. Image: Philip Toscano/PA Wire/Press Association Images

    Drops and condensation nuclei size. Image: http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter07/

  • Pollution causes more problems than initially thought
    26.07.2014 14:38

    The shortage of drinkable water in some countries is due to the scarcity of rainfall and the increase of consumption. Some researchers have been carrying out investigations in how to increase precipitation by seeding clouds. However, there has also been a growing interest in whether seeding could also be a factor that prevents rainfall.

    Some researchers believe that industrial air pollution can stifle rain and snowfall because the pollution particles prevent cloud water from condensing into raindrops and snowflakes. The theory is that water vapor always needs a particle (condensation nuclei, CCN) to help form the initial step of a cloud droplet, just of a 10µm (micrometer). Some nuclei are very hygroscopic and condensation begins on these particles at relative humidity less than 75%. Without this particle, there would need to be a relative humidity of 300%, which is unlikely in our atmosphere.

    The theory says that when a cloud is forming droplets in warm rain processes and they pass through a contaminated area the updraught of the pollution adds an excessive number of CCN, creating a lot of little drops but inhibiting further development.

    Approximately one million small droplets must collide and coalesce in order to make a precipitation-sized droplets, i.e, one large enough to fall below the cloud base and reach the ground before evaporating. In polluted clouds, there are too many small droplets and not enough larger ones.

    Industry and flights release a huge quantity of pollutants that are ingested into the clouds. This suppresses the coalescence by multiplying the quantity of cloud droplets to an excess that avoids forming rain drops heavy enough to fall.

    Some scientists are looking at that problem closely, especially for the water supply in desert areas. For example, they have found that precipitation moving over topographical barriers downwind of major coastal urban areas, such as California and in parts of Israel, decreases after moving over a polluted area. They have quantified a decrease between 15%–25% of the annual precipitation.

    A better understanding of these processes could make a big difference. If we can reduce pollution in the big cities we can both improve our health and obtain more water from rainfall without seeding the clouds that could produce unwanted side-effects.

    By: Mario Cuellar