precipitation

The boundaries between two air masses of different temperature and humidity. A warm front occurs when warm air displaces cold air; if cold air replaces warm air, it is a cold front.

In meteorology, water that falls to the Earth from the atmosphere. It is part of the water (hydrological) cycle. Forms of precipitation include rain, snow, sleet, hail, dew, and frost.

The amount of precipitation in any one area depends on climate, weather, and phenomena like trade winds and ocean currents. The cyclical change in the Peruvian Current off the coasts of Ecuador and Peru, known as El Niño, causes dramatic shifts in the amount of precipitation in South and Central America and throughout the Pacific region.

Precipitation can also be influenced by people. In urban areas dust, smoke, and other particulate pollution that comprise condensation nuclei, cause water in the air to condense more readily. Fog is one example. Precipitation also can react chemically with airborne pollutants to produce acid rain.