Water and Weather

Water is the only substance that can exist as a liquid (water), solid (ice), and gas (water vapor) at normal temperatures and pressures. The familiar forms of water are rain, hail and snow, and fog.

The amount of moisture in the air is known as the humidity. When the humidity reaches a certain level, the water in the air condenses into a mass of tiny droplets.

Near the ground, this causes fog or mist.

Higher up, the water droplets form clouds of all shapes and sizes.

As long as its tiny droplets or ice crystals stay separate, a cloud will simply drift along, perhaps joining with others to make vast layers of moisture in the air.

Often, however, larger droplets gather smaller ones and begin to grow.

The same thing happens if a piece of dust or ice falls through the cloud - it gathers droplets around it and grows.

Eventually it becomes so large and heavy, perhaps a million times larger than the original droplet, that it falls out of the cloud as rain, snow, or hail.

This is precipitation.

Precipitation is also formed when a cloud is forced to rise, perhaps to pass over a range of mountains. As it rises, it cools and cannot hold as much moisture.

Cool air can hold less water vapor than warm air. It therefore unloads its excess moisture by condensation into rain, snow or hail.

Fronts also cause precipitation. These are zones where warm (less dense) air masses meet cold (dense) air masses. The warmer air rises over the cold air. This is called a warm front. The upward movement causes cooling and thus precipitation.