Weather modification is the act of intentionally manipulating or altering the weather. Weather modification has been used in an attempt to increase rainfall and snowfall, to suppress hail, to clear fogs, to modify tropical cyclones and to suppress lightning. The term "weather modification" is not normally used to describe inadvertent changes in our weather. For example, heat and aerosols released from major city centres affect the temperature and precipitation pattern around those cities. Weather modification also does not refer to climate change caused by industrial and natural sources of aerosols and gases such as carbon dioxide.

The most common form of weather modification is cloud seeding to increase rain or snow, usually for the purpose of increasing the local water supply. Weather modification can also have the goal of preventing damaging weather, such as hail or hurricanes, from occurring; or of provoking damaging weather against an enemy or rival, as a tactic of military or economic warfare.

Humans have long sought to purposefully alter such atmospheric phenomena as clouds, rain, snow, hail, lightning, thunderstorms, tornadoes, hurricanes, and cyclones. Magical and religious practices to control the weather are attested in a variety of cultures, thousands of years before the modern era.

Techniques

Most weather modification projects attempt to change the rate at which clouds form precipitation through the injection of ice nucleating chemicals. These chemicals cause supercooled cloud droplets (those below 0°C) to freeze into ice crystals, and these ice crystals grow much more rapidly than the droplets. The ice crystals then begin to fall and sweep out more cloud droplets and ice crystals. Precipitation then falls in the form of snow or rain, depending on whether they fall into regions warmer than 0°C. 

Hail suppression techniques attempt to create many smaller hail embryos and thus increase the possibility the hail will melt before it hits the ground. For supercooled fog dispersal, ice crystals are generated that deplete the small droplets and thus increase visibility. Some projects attempt to totally freeze all the liquid in the cloud, thus quickly releasing the latent heat of freezing, which increases the buoyancy of the cloud, and results in larger and more active clouds. It is also possible to modify clouds or fogs that are entirely warmer than 0°C. This is done through the use of efficient condensation nuclei that create larger and fewer droplets in the cloud, which accelerates the precipitation formation process.

A few experiments have attempted to reduce lightning by using rockets to fire thin metallic wires into growing clouds to trigger and channel premature lightning strokes. In short, a variety of weather modification techniques have been used around the world.