Deliberate or inadvertent alteration of atmospheric conditions by human activity, sufficient to modify the weather on a local or regional scale.
Deliberate alterations include covering plants to keep them warm at night, seeding clouds to induce or augment precipitation, and firing silver-iodide particles into clouds to suppress or mitigate hail and to reduce fog at airports. Inadvertent alterations are the result of industrialization and urbanization, which have added billions of tons of carbon dioxide and other gases to the atmosphere (see acid rain , global warming , greenhouse effect ).