Severe storms batter US: How hail forms and why it's destructive

Severe weather has rattled much of the country this year, and May saw one of the most destructive stretches, according to National Oceanic and Atmospheric Administration's U.S. monthly climate report.

Two major disasters cost the country at least $1 billion in May. The first was the tornado outbreak on May 6-9. The second was a derecho − a fast-moving storm system with powerful straight-line winds − that killed seven people as it moved from Texas to Louisiana.

On May 28, The National Weather Service issued its first warning of large hail – 5 inches in diameter – in Hockley County, Texas. It was the largest hail the service had ever forecast. And on May 30, Commerce City, Colorado, experienced baseball-sized hail – almost 3 inches in diameter –  its largest reported in 35 years.

Meteorologists have reported 3,816 hailstorms this year, as of June 16:

Unable to view our graphics? Click here to see them. 

Where hail has hit the most this year

Nebraska, Colorado, and Wyoming routinely rank highest among all other states in number of hailstorms, according to the NOAA. In the "hail alley" area where these three states meet, seven to nine hail days are reported every year. Texas, however, is suffering most this year.

Storm chasers near Vigo Park, Texas, about 50 miles southeast of Amarillo, discovered an enormous hailstone – about 7 inches in diameter – on June 2, the size and shape of a pineapple, according to a report from AccuWeather. The National Weather Service in Texas said the hailstone could be a new state record.

How hail forms

Hail forms inside cumulonimbus clouds, which also produce thunder and lightning.

Hailstones form through a cycle that circulates raindrops throughout a cloud. Gravity pulls the water particles toward the bottom of the cloud, but strong air drafts pulls them back up. It's in the cloud's upper region where the particles encounter supercooled water. With each cycle, more water freezes onto the ice particles and adds layers to the developing hailstone.

The cycle can be repeated several times, creating very large hailstones. After a while, the hailstone's weight prevents it from being drawn back up, and it will fall to the ground.

Hailstones have an onion-like structure because of the way they form. The biggest hailstones develop in storm clouds with extremely powerful updrafts.

In the U.S. every year, hailstorms pose a threat to animals and humans. According to the NOAA, 24 people are injured on average every year, and some suffer injuries serious enough to require hospitalization.

How fast can hail fall?

Small hailstones, less than an inch in diameter, typically fall at 9 mph to 25 mph. With every inch in diameter, hailstones fall faster:

• 1 to 2 inches: 25 to 40 mph
• 2 to 4 inches: 44 to 72 mph
• Over 4 inches: Can exceed 100 mph

Researchers at the Journal of the Atmospheric Sciences looked into how the shape of a hailstone affects its trajectory and speed. They used 3D-scanning technologies of real hailstones to determine the wide variety of shapes.

How to protect yourself from hail

Hail is dangerous and can inflict billions of dollars in damage to homes and other property in one fell swoop. Here are ways to protect yourself and your home:

• Monitor the weather in your area, especially if there is a chance for severe thunderstorms, because they can produce hail.
• When severe thunderstorms threaten, seek shelter, such as a small interior room on the lowest level of your home, school or business. And avoid standing near windows.
• Cover any windows with curtains or shut blinds to avoid flying glass.
• If in a vehicle, avoid driving through a thunderstorm. Consider pulling over in a safe area or delaying your travel if possible.
• If the thunderstorm is still miles away, move your vehicle into the garage or under a carport if possible.
• Be sure pets are brought inside.

SOURCE NOAA Storm Prediction Center, AccuWeather, National Geographic, Journal of the Atmosphere Sciences, USA TODAY research