Precipitation

Rain

Rain starts to form from condensed water in clouds. Usually, water condenses in the air in tiny droplets about 1/100 of a millimeter in diameter, so small they remain aloft as clouds. Often, there are small solid particles suspended in the air when the water condenses. After that, these tiny droplets begin to expand by colliding and combining with other droplets. Sometimes, the tiny droplets bounce off larger droplets or are pushed aside by them. Eventually, once the droplets get large enough, we can see them falling to the ground as rain.

Snow

Snow begins in the atmosphere as water condenses into tiny droplets. As more and more water vapor condenses onto its surface, the droplet grows. Cold air then freezes this water into an ice crystal. When the ice crystals get heavy enough, the begin to fall. Ice crystals grow by capturing other ice crystals and water droplets in their path.

Sleet

In order for sleet to form, the upper region of the atmosphere must be cooler than the air below it. Sleet forms in advance of a warm front in a narrow band, usually between an area of snow and an area of rain. Then, rain being produced in a warm layer falls into a cold air layer below. Eventually, sleet freezes into pellets before bouncing off the ground.

Freezing Rain

Freezing rain often forms north of a warm front. It starts as snow near the cloud, melts into the liquid form in that warm layer a few thousand feet up. However, the warm layer extends almost all the way to the ground so the rain doesn’t have a chance to freeze into ice pellets in the air. Instead, it freezes on contact with the earth’s surface and becomes "freezing rain".