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What causes hail to form?

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Earlier this week, we saw reports and photos of baseball sized hail from central Mississippi. We know it’s associated with severe thunderstorms, but what is the process that causes it to form?

All thunderstorms start with an updraft, which is basically warm, moist air flowing upward away from the ground. Hail starts as rain drops that get caught in the updraft and carried higher into the cloud. After meeting with supercooled air (air below freezing temperatures), they turn into little balls of ice.

With non-severe storms, hail usually falls to the earth’s surface not long after formation when it becomes too heavy to remain in the updraft. It’s usually pea to marble sized at that point. However, when the updraft is very strong, the hail remains up there long enough to grow additional layers of ice. Eventually, it will become too heavy as well, and fall to the ground.

Hail that is one inch in diameter or larger is large enough for the storm that produces it to be considered severe. While most hail is smaller than that, hail can grow very large, especially in the type of supercell storms often seen in the central region of the country known as Tornado Alley. For example, a record-breaking hailstone fell on July 30th, 2010, in Vivian, South Dakota. It was 8 inches in diameter and weighed 1 pound, 15 ounces – very close to the size of a soccer or volley ball. Despite comparison to various game balls, hail that large is rarely smooth and round. If you do an internet search for photos of large hail, you’ll see images of oddly shaped, spiky chunks of ice.

As you can imagine, larger hailstones can do quite a bit of property damage, and on rare occasion, they have been reported to cause death to people and livestock. For this reason, it’s very important to be indoors and away from windows during severe thunderstorms when the warning includes hail. Of course, it’s always a good idea to stay away from windows during severe weather anyway.

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About the blogger

Like most meteorologists, Niki Morock has been interested in weather since she was a child. After earning a bachelor’s degree in meteorology from N.C. State University in 2007, Niki moved to Minnesota and worked at Weather Eye Radio Network as a broadcast meteorologist, doing daily, live call-ins to morning radio show hosts across the Upper Midwest and Northern Plains and covering severe weather as it happened. While there, she also volunteered as a Skywarn storm spotter trainer, teaching civilians and first responders how to identify parts of severe thunderstorms and tornadoes and how to call in storm reports to the National Weather Service. Niki is now the vice president of the Central North Carolina Chapter of the American Meteorological Society and a member of the national American Meteorological Society.