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HAIL: Severe Weather 101

Massive thunderstorms in 1981 brought winds of over 100 mph, tornadoes, flash floods, and grapefruit-sized hailstones of over 4.5 inches in diameter to Texas and the surrounding region, including Oklahoma, Kansas, and Alabama.

There were 20 casualties. Total estimated damages were $1.2 billion.

It is not the first time that hail storms have been fatal. In fact, hailstorms have killed many people throughout history. As an example, on "Black Monday," around 1,000 English soldiers died in Chartres, France, during the Hundred Years' War. A bad hailstorm with orange-sized hail killed 246 people in Moradabad, India in 1888.

In the United States, hail storms are relatively common. It's important to be informed about hailstorms so that you can stay safe and avoid injury.

Here are 14 facts about them:

1. Hail is a form of precipitation - like rain or snow - made up of solid ice.

2. It differs from frozen rain. Frozen rain falls as water, then freezes as it gets near the ground. It becomes hailstones when it reaches the ground.

3. Hailstones are formed when raindrops are carried upward by a current of air, known as an updraft, during thunderstorms.

4. The stronger the thunderstorm, the bigger the hail can be. Hailstones grow in size when frozen moisture droplets collide with surrounding water vapor, causing layers of water to freeze on the surface.

Even in severe thunderstorms, winds inside a thunderstorm aren't just gone up and down. Often, there are horizontal winds, such as rotating updrafts in supercell thunderstorms, that can move hailstones and affect how they grow.

Eventually, the hail does fall to the ground. When updrafts can no longer support the weight of the hailstones.

5. Hailstones may be clear or cloudy. Depending on how the hailstone forms: If the hailstone collides with water droplets, and they freeze instantly, cloudy ice will result because air bubbles will be trapped inside. As the water freezes more slowly, air bubbles can escape and the ice will be clearer.

Additionally, due to the different conditions in a thunderstorm, the hailstone can have layers of clear and cloudy ice.

6. The size of the hail is often estimated by comparing it to a known object.

Hail that is 1 inch in diameter is called a pea, hail that is 1 inch in diameter is called a quarter, and hail that is 4 inches in diameter is called a softball. Hailstones are generally small, about the size of a pea. "The National Weather Service considers hail dangerous to life and property when the stones reach the size of quarters. We often see hail up to the size of softballs." It's important to keep in mind, however, that most hailstorms are composed of a variety of hail sizes.

7. The largest hailstone ever found in the United States measured eight inches in diameter and had a circumference of 18.62 inches.

8. Hail falls at different speeds depending on a number of factors. It depends on the size of the hailstone, friction between the hailstone and the surrounding air, local wind conditions, and whether the hailstone starts to melt.

According to NSSL, small hailstones under an inch usually fall at speeds between 9 and 25mph, whereas hailstones of an inch to 1.75-inches in diameter typically fall faster — between 25-40mph. The strongest supercells, which can produce hail between 2 and 4 inches in diameter, can cause hail to fall at speeds of 44-72mph.

9. Hail storms can occur at any time of year. As long as the thunderstorms are strong enough, hail can form at any time of year. Large hail is often associated with severe thunderstorms in the Plains and Southeast from February through June, but hail is also common along the West Coast in the cooler months as storm systems take advantage of the cold air in the winter.

10. There are some regions that experience more hailstorms than others, and it isn't always the ones with the most thunderstorms. Despite being prone to thunderstorms, Florida is not the place where hail storms are most common. During the year, hail is most likely to fall from the Dakotas to Texas. The strongest thunderstorms overlap with cold air aloft and fast winds in the jet stream at this location.

11. Hail can fall in paths called "hail swaths." These can be seen from the planes, and they occur when thunderstorms move while the hail falls.

12. Hail storms can cause considerable damage. Hailstones can cause significant damage to buildings, vehicles, crops, and livestock.

Hail causes approximately $1 billion in property and crop damage every year in the United States. While quarter-size hail will cause damage to shingles, golf-ball-size hail can cause dents on cars and baseball-sized hail can smash windshields. Softball-sized hail, meanwhile, can cause holes in roofs.

Human deaths resulting from hail are rare in North America, but they do happen. In 2000, a man in Fort Worth, Texas was killed by a hailstone the size of a softball.

Hail storms can also cause serious injuries. Each year, an estimated 24 people are injured by large hail, but sometimes, there are a lot of injuries from one storm. In May 1995, a hailstorm in Texas injured 400 people who were caught outside during Mayfest with little shelter available; 60 of them needed hospitalization.

Hailstorms that produce large amounts of small hail can still be dangerous because all those hailstones can cover the roads completely. When these hail piles are deep enough, they can prevent car tires from even touching the road. The driving conditions are similar to those during icy winters.

13. It's difficult to predict when a hailstorm will occur in advance. There are usually a few days ahead that conditions are ripe for hail, but we don't know that any community will have hail until an hour or so before it occurs.

14. Being prepared is the best way to prevent hailstorm damage, especially if you live in an area prone to hailstorms. It's also a good idea to make a disaster preparedness plan for your family, so you know where to go for safety and how to reach each other in case of an emergency. Listen to the radio or other news sources if severe weather occurs, such as a bad thunderstorm, to stay up to date on any immediate threats to your family or property.

15. If you are caught in a hailstorm outside, seek shelter indoors.

As soon as the hail stops, make sure you stay inside and stay away from skylights and windows. If you have drapes or curtains, close them to prevent broken glass and hailstones from entering your home. It's also a good idea to seek shelter at least one level below the roof.

If you are driving, pull over as soon as possible, preferably near a place with shelter, like a garage or awning at a gas station. Make sure you are completely off the highway. If you find yourself in a hail storm in your car without sturdy structures nearby, please stay inside and cover yourself if you can. Windows might break, but your head should be protected. Outside, if you can't find shelter, find something to at least protect your head. Stay out of ditches and low-lying areas because they could fill with water. During thunderstorms, trees can lose branches and isolated trees can attract lightning.

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