Water is the only substance that can exist as a liquid (water), a solid (ice), or a gas (water vapor) on Earth at normal temperatures and pressures.
The amount of water vapor in the air is known as the humidity. When the humidity in the air reaches a certain level, the water in the air condenses into a mass of tiny droplets.
Fog occurs when water condenses close to the ground.
Higher up, water droplets and ice crystals forms clouds.
Precipitation - rain, snow, sleet and hail - occurs when water or ice from clouds falls to the ground.
Clouds come in two main shapes.
Cumuliform, or heap, clouds have considerable vertical depth. They form in quickly rising air. The highest cumuliform cloud, the cumulonimbus or thunderstorm cloud, may measure more than 13,000 feet (4,000 meters) between its dark, heavy base and its often anvil-shaped top.
Stratiform or layer clouds look like thin sheets spread across the sky. They generally form when air rises slowly and at relatively gradual gradients.
In tropical regions, clouds may consist almost entirely of water droplets. In temperate regions, however, the temperature of a cloud is often well below freezing, and a cloud may contain supercooled water droplets, which remain liquid in temperatures as low as -40 degrees Fahrenheit (40 degrees Celsius) as well as ice crystals.
As long the water droplets or ice crystals stay separate, the cloud will simply drift along, perhaps joining with others to make vast layers of moisture in the air.
Often, however, larger droplets will gather around a smaller one, and the droplets will coalesce.
The same thing happens if a piece of dust or ice falls through the cloud. Droplets will gather around it, creating a larger mass.
Eventually, a mass perhaps a million times larger than the original droplet is created. It is now so heavy that falls out of the clouds.
Every year approximately 45,000 cubic kilometers of ocean water evaporates, and about 11% of this eventually falls on the continents as precipitation.
Rain can be created artificially by seeding a cloud from above with ice or other crystals, such as silver iodide. These introduced crystals also become larger when they collide with the droplets in the cloud.
Precipitation also occurs when a cloud is forced to rise, for example to pass over a range of mountains. As the cloud rises, it cools. Cool air cannot hold as much moisture as warm air, so the water in the cloud precipitates.
Rain that occurs when air from the ocean rises above mountain slopes and then cools is known as orographic rain.
Precipitation also takes place at fronts, zones where cold air and warm air meet.
In a cold front, cold air replaces warm air. The cold air lifts the warm air out of the way. As the warm air rises, it cools and precipitation forms.
Warm air rises up and moves over cold air in a warm front. As the warm air rises and cools, precipitation occurs.
Cyclonic rain occurs when warm air rises above cold air along fronts in low pressure regions that form in the middle latitudes.
Convectional rain takes place where there is warm air near the ground. Fast-moving, warm, moist air rises and then sinks after it cools. This movement of air is known as a convection current. In tropical regions, this cycle of rising and cooling air may take place daily. 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.
If air near the ground is warmer than about 39 degrees Fahrenheit (4 degrees Celsius) when precipitation occurs, 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.
Hail occurs during a thunderstorm. A water droplet is picked up by an updraft of warm air. As it rises, it cools and freezes. It is then carried downward by a draft of cold air. As it moves toward the ground, it begins to thaw.
However, it may then be picked up by another warm updraft. It moves upward again, and freezes again.
This can happen repeatedly. Each time the droplet refreezes, another layer of ice is added to it and the droplet becomes heavier.
Eventually, the droplet becomes so heavy that it falls to the ground as hail.