If you've learned about clouds, you know that they're filled with tiny droplets of water and ice. That's where all precipitation — rain, sleet, snow — begins.
What Causes Rain?
When clouds become heavier and collect enough water droplets, the droplets bang together and form even bigger drops. When the drops get heavy enough, gravity makes them fall to the earth. That's when you see and feel rain.
What is Hail?
Hail is formed in thunderstorms with strong updrafts — winds that blow upwards through the clouds. Water droplets are lifted higher and higher into the sky until they freeze into ice. Once these tiny hailstones become heavy, they start to fall. If they get caught in the updraft again, another layer of water is splashed on them. The water layer freezes, the hailstones get bigger and fall. This happens over and over again until the hailstone is too heavy and falls to the ground.
In strong updrafts, the hail has time to form many layers of ice, so it's bigger. In weak updrafts, the hail doesn't have to get as big before it is able to fall to the ground. Sometimes the wind is so strong that the hailstones grow larger than softballs.
The largest single hailstone ever recorded in the United States was 7" in diameter, 18.75" around, and weighed in at just under 1 pound. Imagine a large cantaloupe falling from the sky at nearly 100 miles per hour! That hailstone fell during a storm in Aurora, Nebraska. The heaviest hailstone in the U.S. was 1.67 pounds and fell in Coffeyville, Kansas in 1970.
What is Snow?
If the air is cold, water vapor crystallizes around a speck of ice or dust and falls to the earth as snow. If there is no dust for the water vapor to crystallize on, it will stay in the air as a cloud, even if it gets as cold as 40 degrees below zero.
What Causes Sleet?
Sleet starts out as ice crystals in the clouds. The crystals melt if the air is warmer than 32 degrees F, and turn into drops of water. But if there is a layer of freezing air below the warmer air, the water freezes into ice before it hits the ground. Sleet looks different from snow because it has lost its crystal shape.
