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What is Hail? - Definition, Formation & Causes

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  • 0:00 Introduction to Hail
  • 0:44 Definition of Hail
  • 2:31 Causes of Hail
  • 3:40 Formation of Hail
  • 4:13 Lesson Summary
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Lesson Transcript
Instructor: Megan Wahl

Megan has taught middle school science and developed curriculum for k-higher ed. She has a master's degree in Educational Technology.

Hail is one of four types of precipitation that falls from the sky. It's also the most dangerous, damaging type, occurring during severe storms. Read more to learn the definition of hail, how hail forms, and what causes hail.

Introduction to Hail

A summertime thunderstorm is incredibly dramatic to watch. You can sense the temperatures shifting from warm to cool and feel rain on your skin. The winds change direction, and thunder rumbles in the distance. Lightning streaks across the sky. In a very severe thunderstorm, pieces of ice fall to the ground in the form of hail.

After a storm that includes hail, people often come outside and are bewildered to see balls of ice covering outdoor surfaces. They might also be shocked to see damage from hail, which sometimes results in cracked car windshields and dents in the siding of houses. According to the National Oceanic and Atmospheric Administration, hail causes over one billion dollars of damage in the United States each year.

Definition of Hail

So now you know what hail can do, but what exactly is it? Hail is a form of precipitation that falls as ice pellets during a severe thunderstorm. If hail measuring larger than 0.75 inches in diameter falls during a thunderstorm, it is classified as severe weather. Sometimes damaging winds accompany this type of storm as well.

Meteorologists like to use common objects to estimate the size of hail in their forecasts. Hail can fall in small pieces, such as the size of a pea (0.25 inches) or marble (0.5 inches). It can also form in larger chunks, like the size of a quarter (1 inch), golf ball (1.75 inches), baseball (2.75 inches), or even a grapefruit (4 inches).

On July 23, 2010, in Vivian, South Dakota, a record-breaking hailstone fell to the ground. An observer saw a huge ball of ice that had fallen during the storm and rushed to freeze it. At 8 inches in diameter and nearly 2 pounds, this is the largest hailstone ever recorded in the United States!

But hail doesn't have to be large to be damaging. Small, pea-sized hail can completely wipe out a young crop growing in fields and be a costly loss for the farmer. Large, softball-sized hail can be fatal for livestock and humans, as well as shatter glass in homes and businesses and total vehicles. Though human fatalities are rare, in May of 1986, 100 people were killed and over 9,000 people were injured in China by a major hailstorm. The parts of the United States most affected by hail and severe storms include Wyoming, Colorado, and Nebraska. Areas of the world where hail is common includes Northern Italy, and parts of India, China, and Russia.

Causes of Hail

Severe storms develop in cumulonimbus clouds. This type of cloud forms tall, tower-like shapes that go high into the atmosphere. Cumulonimbus clouds can intensify into supercells, which produce severe thunderstorms. A supercell can produce hail, along with lightning, tornadoes, and high winds.

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