Precipitation occurs in one of several forms, including rain, drizzle, hail, sleet, snow, and ice crystals. Which form of precipitation reaches ground level is dependent upon temperature and size of the water droplet.
What Is Precipitation?
Precipitation begins in the clouds overhead. Clouds are a visible collection of water vapor and condensation nuclei, such as dust or smoke.  The water vapor will continue to collect and condense until it acquires enough weight to exceed the cloud’s updraft speed at which time it will fall from the cloud as precipitation.  All forms of precipitation begin as ice crystals or snow due to the colder temperatures higher in the atmosphere at cloud level.  Which form of precipitation reaches ground level will depend upon the atmospheric temperature.
Rain is defined as droplets of water that are greater than .5 mm in diameter. Rain will occur if the temperature is above 32°F. Freezing rain occurs when water falls to the earth as rain and moves through a thin layer of freezing air prior to reaching the ground.  Most precipitation falls as rain. 
The fine droplets that make up drizzle may resemble fog. However, drizzle differs from fog in that the water droplets reach the ground instead of remaining suspended in the air. Drizzle is defined as water droplets that are less than .5 mm in diameter. 
Hail forms in thunderstorms that include strong updrafts. Water vapor condenses in a cloud and are then pulled up by the updraft, where they are frozen. The hailstones grow as additional water vapor is deposited on the surface. Eventually the hailstones acquire enough weight to escape the updraft and fall to earth. Pieces of ice that exceed 5 mm in diameter are considered hail; smaller pieces are called small hail or snow pellets. 
Sleet is defined as frozen raindrops that bounce on impact. Sleet may seem similar to small hail, but sleet is a winter precipitation that is formed by an entirely different set of atmospheric conditions than those that form hail. Sleet begins as snow or ice that begins to melt as it falls to Earth. It will then refreeze prior to reaching the ground. 
Snow is comprised of hexagonal ice crystals that fall to earth. Small flakes are made of individual crystals, while large flakes are clumps of smaller crystals. Temperatures must not exceed 32°F for precipitation to take the form of snow. 
Also known as “diamond dust,” ice crystals do not have a regular crystalline shape like snow. Ice crystals fall as small needles, rods or plates. 
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