Human influence on the weather and, ultimately, climate. This can be either intentional, as with cloud seeding to clear fog from airports or to increase precipitation, or unintentional, as with air pollution, which increases aerosol concentrations and reduces sunlight. Weather is considered to be the day-to-day variations of the environment—temperature, cloudiness, relative humidity, wind-speed, visibility, and precipitation. Climate, on the other hand, reflects the average and extremes of these variables, changing on a seasonal basis. Weather change may lead to climate change, which is assessed over a period of years. Air pollution Climate history Cloud physics

Specific processes of weather modification are as follows: (1) Change of precipitation intensity and distribution result from changes in the colloidal stability of clouds. For example, seeding of supercooled water clouds with dry ice (solid carbon dioxide, CO₂) or silver iodide (AgI) leads to ice crystal growth and fall-out; layer clouds may dissipate, convective clouds may grow. (2) Radiation change results from changes of aerosol or clouds (deliberately with a smoke screen, or unintentionally with air pollution from combustion), from changes in the gaseous constituents of the atmosphere (as with carbon dioxide from fossil fuel combustion), and from changes in the ability of surfaces to reflect or scatter back sunlight (as replacing farmland by houses.) (3) Change of wind regime results from change in surface roughness and heat input, for example, replacing forests with farmland. Precipitation (meteorology)