Ask the Meteorologist: How Does Hail Form?
You’re probably familiar with hail, which falls in all 50 states. But, have you ever wondered what causes hail to form?
Hail only occurs in thunderstorms. Hailstones form when wind currents from a thunderstorm carry raindrops up into cooler air, where they freeze. When the wind current, also called an updraft, can no longer support the weight of the hailstone, gravity pulls it down.
In most of the country, thunderstorms with hail only occur in about six months of the year, during the warmer seasons. However, in warm climates (like south Florida, for example), thunderstorms with hail can occur at any time during the year.
What Determines the Size of Hail?
Hailstones can continue to grow as they are carried up and down in the cloud by updrafts and downdrafts. Hailstones grow bigger by colliding and fusing with supercooled water droplets. A supercooled water droplet has a temperature below freezing, but it won’t freeze into ice without something to stick to, such as a hailstone. As a result, hailstones can have layers. In fact, if you split a hailstone in half, you can count the layers inside to see how many times the hailstone traveled up and down.
Like snowflakes, hail comes in many different forms. Hailstones that melt and refreeze together take on very unique shapes and sizes. Most hailstones are the size of a pea or marble, but the National Oceanic and Atmospheric Administration says a severe thunderstorm with very strong updrafts can produce hail as large as golf balls and even grapefruit.
How Can There Be Hail on a Warm Day?
Have you ever wondered how hail occurs during the warmer months? Even though temperatures may be warm at ground level, temperatures are typically cooler higher in the air. If the cloud tops are high enough, temperatures could very well be below freezing in part of the cloud, making it possible for raindrops to freeze into balls of ice. During a severe thunderstorm, intense updrafts carry falling raindrops back up into the higher, colder part of the cloud, where raindrops freeze into hail.
Is Sleet the Same as Hail?
Sleet is not the same as hail. While hail forms from the updrafts and downdrafts of thunderstorms, sleet occurs during colder months when falling snowflakes melt into raindrops and then refreeze before reaching the ground.
Unlike hail, sleet only falls once and as a result, is much smaller in size. Because sleet pellets only fall once, they do not have rings of ice inside like hailstones do.
While you can’t prevent hail from falling, the next time it hits your neighborhood, you may have a better understanding of the factors that caused it.