Precipitation

The term precipitation includes all forms of con­densation - dew, mist, fog, smog, frost, rain, hail, sleet and snow. In the turbulent warm air of tropi­cal regions clouds may consist almost entirely of water droplets, and raindrops heavy enough to overcome air resistance form as the water drop­lets merge. But in temperate regions the tempera­ture in the clouds is often well below freezing point. In this case, the supercooled droplets freeze on contact with the ice crystals in the clouds. The ice crystals eventually become so heavy that they fall towards the ground. If the air near the ground is warmer than about 4°C, the ice crystals melt and become raindrops; if the air is colder, some crystals may melt to form sleet, or none may melt and all the crystals reach the ground as snow.

Artificial rainmaking is carried out by "seeding" a cloud from above with ice or certain other crys­tals (silver iodide, for example). Like naturally-formed ice crystals, the introduced crystals also become larger when they collide with super­cooled droplets in a cloud, and so rain may be arti­ficially induced.

Precipitation occurs in three main ways. Con-vectional rain is produced when intense heating of the air near the ground causes fast-moving, warm and moisture-laden air to rise in strong cur­rents, only to sink again after it has cooled, so set­ting up convection currents. In tropical regions, this may occur in a daily cycle. Near large expanses of water convection currents are often set up in the morning, cumulonimbus clouds form in the late morning and afternoon, and thunder­storms occur in the late afternoon.

Orographic rain occurs when moist winds from the oceans are forced to rise up mountain slopes. This air cools as it rises, resulting in pre­cipitation on the windward slopes.

Cyclonic rain occurs when warm air rises above cold air along fronts in the low-pressure regions that form in the middle latitudes.

Every year approximately 45,000 cubic kilometers of ocean water evaporates, and about 11% of this eventually falls on the continents as rain or snow. The water cycle, of which this is a part, enables life-forms to live on land by supplying a constant source of water.