What makes hail? Well, do you like to play catch? If you've ever played catch alone, you know that you throw a ball up into the air and then try to catch it so it doesn't hit the ground. Imagine you're playing catch right now. But instead of using a regular ball, you have a magic ball. This magic ball is really small...no bigger than a pea. You throw the ball up into the air. As the ball goes higher and higher, it grows bigger and bigger. When it starts to fall back toward you, it shrinks just a little bit. You've just caught the ball. When you look at the magic ball, it's no longer the size of a pea. It's the size of a ping-pong ball! Pretty cool, huh?
You throw the ball up in the air again. The ball goes higher and higher...it gets bigger and bigger. It's starting to fall back down. You caught it again! Wow! You're really good at this! You look at the magic ball again...now it's the size of a softball.
You throw the magic ball up in the air many more times. Each time, the ball gets bigger. It grows to a volleyball, then a basketball, then a kickball...it's now so big that you can't possibly catch it! The ball finally hits the ground. Game over!
That magic ball is a lot like a piece of hail. A piece of hail is called a "hailstone." Hailstones start out as really tiny balls of ice in a thunderstorm. The winds of a thunderstorm are just like your throwing arm. They throw the hailstone up high into the thunderstorm cloud, just like you threw the magic ball up into the air. As the hailstone climbs higher and higher, water in the cloud grabs onto it. This water freezes onto the hailstone, making it bigger. When the hailstone starts to fall, the winds "catch" it and throw it up into the cloud again. More water freezes onto the hailstone and makes it even bigger. This keeps happening until the hailstone becomes too heavy for the winds to handle. The hailstone then falls to the ground as a clump of ice.
Hail only occurs during severe summer thunderstorms. Why only in the summer? You don't like to play catch when it's really cold outside, do you? The winds in a thunderstorm, are the same way. They need warm air to be strong enough to throw hail up into the air!
What makes hail? Well, do you like to play catch? If you've ever played catch alone, you know that you throw a ball up into the air and then try to catch it so it doesn't hit the ground. Imagine you are inside of a thunderstorm cloud. You've got a baseball mitt in your hand and you are ready to play catch! Instead of using a regular ball, you're using a tiny piece of ice called a hailstone. It's so small that you can barely see it. You throw this hailstone high up toward the top of the thunderstorm cloud. As it flies up toward the top of the cloud, little water particles from the cloud itself grab onto it. These water particles freeze, making the hailstone larger. The hailstone starts falling back toward you. You catch it and notice that it has gotten bigger. You throw it back up to the top of the cloud. More water freezes onto the hailstone, making it even larger.
You keep catching the hailstone and throwing it back up to the top of the cloud. Meanwhile the hailstone keeps growing bigger and bigger until it becomes too big for you to catch. You let the hailstone drop out of the cloud and onto the ground.
What you've just imagined is similar to what happens to hail in a thunderstorm. Instead of you catching and throwing the hailstone, violent winds inside the thunderstorm are doing it. Violent upward winds blow pieces of ice high up into the thunderstorm cloud where they grow in size. When they become too heavy for the wind, they fall out of the cloud and onto the ground as hail.
In order for hailstones to grow large enough, they must stay in the cloud for a long time. The cloud must have strong upward winds inside of it. These winds only form in a warm environment. Therefore, hail only occurs in the summertime during severe thunderstorms.
Hail develops in a cumulonimbus cloud. Graupel (a kind of mushy ice), frozen raindrops, or dust particles can be the nucleus of a hailstone. Supercooled (really cold, but not frozen) liquid droplets in the upper part of the cloud "accrete" onto the nucleus. Accretion means slowly growing in size by the addition of particles. The supercooled droplets stick onto the hailstone's nucleus and freeze, allowing the hailstone to grow.
A hailstone needs over ten billion supercooled droplets to grow to the size of a golfball! For comparison, a typical raindrop needs about one million droplets. In order for a hailstone to gather all those droplets, it must stay in the cloud for 5 to 10 minutes.
How does a hailstone stay in a cloud for that long? Well, hail needs a severe thunderstorm and the heat of the summer to do it. Violent winds that can exceed 50 mph exist in a severe thunderstorm. The thunderstorm consists of powerful updrafts and downdrafts. The updrafts play a kind of tug-of-war with the downdrafts. These two opposing forces exist because summer heat create them. You can learn more about the development of a thunderstorm here.
As a hailstone starts to form, it becomes too heavy for the top of the cloud. It starts to fall through the cloud gathering more and more supercooled droplets as it falls. At the same time, the hailstone starts to evaporate a little as it reaches the warmer midsection of the cloud. Due to the evaporation, the young hailstone isn't heavy enough for the powerful updraft. So the updraft pushes the hailstone back up into the upper recesses of the cloud. The hailstone once again gathers more droplets and grows even bigger. It starts to fall again. The hailstone is still not heavy enough, so it gets thrown up into the cloud again. This cycle continues until the hailstone is heavy enough to overcome the updraft. At this point the hailstone falls out of the cloud and onto the ground.
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