Whither Rain
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Hi, Iím Dave Thurlow for the Weather Notebook. Air pollution has never been attributed to the suppression of rain and ice precipitation. However, recent studies indicate that pollution particles in clouds prevent water from condensing. This means less rain and snowfall because of major urban areas and pollution sources, such as oil refineries and powerplants.

Pollution particles in urban and industrial areas condense moisture, making smaller than normal droplets. It takes around 1 million small droplets just to make a raindrop or snowflake big enough to reach the ground.

Also, there are too many small droplets and not enough large ones. These small droplets have a low probability of bumping into each other and forming raindrops. Smaller droplets also freeze into ice crystals more slowly. This results in less sleet and snowfall.

The global climate system is determined by the amount, location and timing of precipitation. Tropical rain provides much of the energy needed for worldwide movement of air in the atmosphere. Any change in precipitation in the tropics affects the global climate. Rainfall in the tropics has decreased despite predictions that global warming would make the area receive more rain.

Air pollution stifles rain from short-lived tropical clouds than from longer-lasting clouds in northern latitudes. However, air pollution also decreases the precipitation from these colder, more vertically developed clouds. This could substantially affect the Earthís climate.

Aerosol pollution makes clouds shiny, and this may counteract some of the greenhouse effect by reflecting sunlight away from Earth. However, aerosol pollution carries health hazards, and you canít beat the greenhouse effect with more pollution.

The Weather Notebook is a production of the Mount Washington Observatory and is funded by Subaru: The beauty of all wheel drive.