How is Rain Formed?

Water in the Air

• Rain is a form of precipitation in which water falls back to earth as a liquid. Precipitation that occurs in a solid form is snow or hail. There is always some water in the air, but the amount of water the air can hold depends mostly on temperature. Very warm air can contain more water than cooler air. Sudden drops in temperature can force water to fall, or precipitate, out of the air. In fact, this happens every morning when the cooler night air loses some of its moisture as dew.

The Water Cycle

• Water moves on the earth in a great cycle. The water that falls as rain comes mainly from oceans, but also to a lesser extent from lakes and rivers. Heat from the sun causes water to evaporate, that is, to change phase and become a vapor. The water vapor rises and, as it does, reaches a level in the atmosphere where the temperature is cool enough for it to change back into a liquid. The change of phase from vapor to liquid is called condensation -- the exact same process that causes steam from a hot shower to fog up a cold mirror.

  Water condenses in the air in tiny droplets about 1/100 of a millimeter in diameter, so small they remain aloft as clouds. Usually for rain drops to form there must also be small solid particles suspended in the air. If there air, the water droplets collect around these particles, growing larger and larger. When they are too heavy to remain in the sky, they fall back to earth as rain. The water then runs into lakes, rivers and ultimately the ocean, completing the water cycle.

Other Rain

• Rain can also be produced by a process called seeding. Frozen particles of silver iodide, carbon dioxide or salt are released into the air, not only cooling it, but providing a substrate on which water droplets will condense. Cloud seeding is used in the United States to bring rain to drought-stricken areas, to prevent the formation of large hailstones, and to reduce fog around airports. The People's Republic of China has the largest cloud seeding program in the world. Prior to the 2008 Beijing Olympics, China launched rockets with silver iodide over Beijing to induce rain storms which cleared some of the city's notorious air pollution.

  High levels of pollution in the atmosphere, especially sulfur and nitrogen, react to form acidic compounds. These can combine with water in clouds to create rain that falls to earth with unusually low pH, a phenomenon called acid rain. Acid rain has harmful effects on plants and animals, including humans, and can damage infrastructure and buildings.