What Is Hail and How Does It Form? Hail Basics and Size Chart

Hail is a common feature of severe thunderstorms typically seen in the spring, summer, and fall months. While for the most part, hail is only a nuisance, in severe storms, hailstones can grow to many inches in size, damaging homes and cars, and it can even be deadly to people and livestock.

What Is Hail?

Hail is a form of precipitation that is made up of solid ice. Hail starts as a raindrop, which is too small and light to fall to the ground. These smaller raindrops are pushed back up into colder areas of the cloud during thunderstorms by a current of air known as an updraft. Here, the droplets freeze and form into a hailstone.

How Is Hail Formed?

As mentioned previously, tiny rain droplets are pushed upwards by thunderstorm updrafts to extremely cold areas of the atmosphere where water can freeze. The hailstone then grows as it collides with supercooled water drops (liquid drops surrounded by air that is below freezing) inside the cloud, which freeze onto the hailstone’s surface, adding another layer to the growing hailstone.

The supercooled drops will freeze on contact, and air bubbles will be trapped in the ice, making the hailstone cloudy. However, raindrops may also freeze more slowly, allowing air bubbles to escape making the hailstone clear. Hailstones often have varying layers of cloudy and clear ice, depending on the conditions inside the thunderstorm cloud.

The strength of the updraft and winds inside the storm also play a crucial role in hailstone development. Hail doesn’t necessarily grow from alternating up and down cycles within a storm. Winds inside a thunderstorm can also blow horizontally. Strong horizontal winds from the surrounding environment can blow smaller hailstones away from the updraft, allowing gravity to take over. A lesser horizontal wind component or a wind pattern that keeps the stone over the updraft inside the storm (like the rotating winds in a supercell thunderstorm) creates the largest hailstones.

Hailstones eventually fall to the ground when they become too heavy to be supported by the thunderstorm’s updraft or if an updraft suddenly weakens.

Where do Hailstorms Occur?

Here in the United States, hail is most common across areas of the central Plains states. The region where Nebraska, Colorado, and Wyoming meet is known to meteorologists as “Hail Alley.” People in this area see an average of seven to nine days of hail each year.

However, nearly the entire central U.S. is at risk of severe hail during the year, as are those living in the southern Appalachians from Georgia northward through southwest Virginia. Other parts of the world that regularly see hailstorms include Australia, Canada, China, northern Italy, India, and Russia.

What is the Largest Hailstone Ever Recorded?

According to the National Severe Storms Laboratory (NSSL), the current record for the largest hailstone ever recovered in the United States has a diameter of 8 inches, a circumference of 18.62 inches, and it weighed 1lb. 15oz. The hailstone fell in Vivian, South Dakota, on June 23, 2010.

Measuring the circumference of the largest hailstone ever recovered in the United States. Photo credit: NWS Aberdeen

Hail Size Chart

The National Weather Service does not report hail sizes by their measurement but by their size relative to an everyday object. For example, the term “pea-sized hail” describes hail a quarter inch in diameter, while “softball-sized hail” describes a large hailstone four inches in diameter. It’s not unusual to see hailstones of varying sizes in a thunderstorm.

While the chart above is suitable to estimate hail size, using a ruler or a caliper is the best way to measure hail. Just make sure to measure hailstones after the storm has ended to avoid injury.

What Damage Can Hailstones Do?

It’s important to mention here that the vast majority of hailstorms do no damage at all. That’s because when hail does fall, it’s usually tiny—typically the size of a pea (0.25-inch diameter) or a mothball (0.5-inch diameter). At this size, all the damage you’ll notice is to foliage and small twigs, if anything.

However, it doesn’t take much for a hailstorm to be destructive. Hail that is the size of a quarter (1 inch) can damage roof shingles, golf ball size hail (1.75 inches) can dent your car, while hailstones the size of a tennis ball (2.5 inches) or larger can smash through car windows and destroy crops.

Almost all hailstorms will fall into one of these categories. However, on the rare occasions that hail exceeds tennis ball size, these hailstones have been known to do severe damage, including puncturing holes in roofs, injuring people, and in rare cases causing fatal injuries.

Each year, hailstorms in the U.S. cause approximately $1 billion in property and crop damage. However, a single hailstorm can inflict that much damage alone, if not much more. The costliest hailstorm in U.S. history hit the Phoenix metro area on October 5, 2010. Hailstones up to three inches in diameter were recorded in the West Chandler area. It’s estimated that property damage in this storm exceeded $2.8 billion.

What Can You Do to Avoid Hail Damage and Injury?

Unfortunately, it’s impossible to avoid hail damage in some areas of the country. However, there are some things you can do to limit the risk of damage and injury in a severe hailstorm.

Using higher quality materials on your siding and shingles can make your roof more hail damage-resistant. Parking vehicles in the garage when severe weather threatens will protect your car from hail damage.

To avoid being injured by hail, take shelter immediately when thunderstorms threaten and stay away from glass windows and skylights. Close your blinds and curtains to keep broken glass and hailstones from entering your home. It’s also important to tune in to a weather radio or another news source when severe weather occurs, so you can stay up to date with the latest information to keep your family and property safe.

If you are caught outside in a severe hailstorm and cannot find shelter, protect your head, and stay away from low-lying areas that may flood, and stay away from trees as they can lose branches and attract lightning in a storm.

If you’re in a car, pull over immediately, preferably in a place with a shelter, like a gas station or a parking garage. If you cannot find shelter, safely pull off to the side of the road and stay in your car. Lie down with your face away from the windows and if you have a blanket or coat, use it to protect your face and head from potential flying glass. The car will protect you from being hit by hailstones, but breaking glass may still cause injury.

Be Prepared and Check the Forecast

The best way to protect your family and yourself from a severe storm is by being prepared. Ensure you have a disaster preparedness plan for your family, so you all know where to go and how to contact each other after an emergency. Ensure your emergency storm kit is stocked up and accessible, and lastly, always plan ahead of time by checking the forecast outside with a weather radar app.