Hailstorm Safety

Take shelter if hail is possible during thunderstorms.

| July/August 2017

On July 23, 2010, a hailstone fell out of a thunderstorm near Vivian, South Dakota. It had a diameter of 8 inches, a circumference of 18.62 inches, and weighed nearly 2 pounds. It was the largest hailstone ever recorded.

Hailstorms are far more common than tornadoes or hurricanes. In 2015, there were more than 5,000 reports of large (more than 1-inch-diameter) hail in the United States, and hail has occurred in all 50 states. Hail can cover the ground and actually form drifts up to 4 feet deep. Snow plows have been needed to clear roads. It is estimated that hail does $1 billion worth of damage to property and crops in the U.S. every year.

What Is Hail?

Hail is officially defined by the National Weather Service (NWS) as “showery precipitation in the form of irregular pellets or balls of ice more than 5 millimeters in diameter, falling from a cumulonimbus cloud.” The size of hailstones varies greatly, and precipitation often only lasts for a few minutes at most. Pea-size hail (about 1/4-inch) is fairly common and usually does little damage. Marble-size (about 1⁄2-inch) is typically the smallest size still capable of crop damage. One-inch hail (quarter-size) is considered the minimum for property damage to occur. Hail the size of tennis balls (2-1⁄2 inches) or even softballs (4-1⁄2 inches) is not uncommon. The giant hailstones, like the record holder described above, are probably aggregates of several stones that have frozen together. Hail smaller than 1 inch can damage tender vegetation. While small hail is not specifically forecasted, local NWS forecasts and meteorology reports typically will mention the possibility of any hail.

A couple of points to make here: Sleet and hail are not the same thing. Sleet, which only occurs in the winter when temperatures near the ground are below freezing, starts out as rain, then freezes into small ice pellets before reaching the ground. There is also something meteorologists call “graupel.” These are small ice pellets similar to sleet, but they start out frozen rather than liquid. They typically occur in showers during colder seasons (late fall and early spring, sometimes into summer). Graupel is also sometimes called snow pellets or soft hail.

How Does Hail Form?

Cumulonimbus clouds, or thunderheads, extend well up into the atmosphere, as high as 5 to 10 miles. At these heights, even in summer, temperatures are well below freezing. At these temperatures, you can still have liquid water (“super-cooled water”) as well as ice crystals. A small crystal of ice can become coated with super-cooled water, and as the water freezes around the ice particle, it grows. This process can continue until the weight of the hailstone causes it to fall out of the cloud. When this occurs depends on the strength (speed) of the updraft, which is the rising current of air that is the heart of the storm. An updraft of 25 mph can uphold pea-size hail. The strongest thunderstorms, known as “supercells,” can have updrafts of 100 mph and produce the largest hail. The stronger the updraft, the better chance of producing larger hail, because it will have longer to collect more water around the hailstone.

If hail is mixed with rain, it indicates that the thunderstorm updraft isn’t that strong, and smaller hail can be expected. If only hail is falling, this indicates a stronger updraft (the lighter rain drops are upheld in the cloud), and larger hail can be expected.

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