According to the National Severe Storms Laboratory, hail is a “form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into balls of ice.
Hailstones grow by colliding with supercooled water drops. Supercooled water will freeze on contact with ice crystals, frozen raindrops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm’s updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft, the larger the hailstone can grow.”
Hailstones develop layers like an onion when they travel up and down in an updraft, or they can have few or no layers if they are balanced in an updraft.
So each hailstone holds the secret of its formation: you can tell how many times a hailstone traveled to the top of the storm by counting its layers.
Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Strong winds can even blow hail at an angle. This is why sometimes only one side of a structure sustains hail damage.
Hail is usually pea-sized to marble-sized, but big thunderstorms produce big hail. According to the Storms Laboratory, the largest hailstone recovered in the U.S. “fell in Vivian, SD on June 23, 2010. It had a diameter of 8 inches and a circumference of 18.62 inches. It weighed 1 lb 15 oz.”
Florida has the most thunderstorms, and Nebraska, Colorado, and Wyoming usually have the most hail storms. The area where these three states meet is termed “hail alley,” and it averages seven to nine hail days per year.
This area gets so much hail because the freezing levels in the high plains are much closer to the ground than they are at sea level, where hail has plenty of time to melt before reaching the ground. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy. One highly localized event in Europe caused a billion dollars worth of damage in 15 minutes. Hail falls in paths known as hail swaths. They can range in size from covering just a few acres to areas 10 miles wide and 100 miles long. Some hail swaths are so deep snow plows are used for removal.
Hail size is estimated by comparing it to a known object. Most hail storms are made up of a mix of sizes, and only the very largest hailstones pose serious risk to people caught in the open.
Pea = 1/4 inch diameter
Marble/mothball = 1/2 inch diameter
Dime/Penny = 3/4 inch diameter
Nickel = 7/8 inch
Quarter = 1 inch — hail quarter size or larger is considered severe
Ping-Pong Ball = 1 1/2 inch
Golf Ball = 1 3/4 inches
Tennis Ball = 2 1/2 inches
Baseball = 2 3/4 inches
Tea cup = 3 inches
Grapefruit = 4 inches
Softball = 4 1/2 inches
The National Oceanic and Atmospheric Administration encourages the use of traditional object-to-size conversion for reporting severe hail events. Measure, don’t estimate.