From Pea To Grapefruit: Understanding Hail Sizes

Tornadoes may be the most talked-about severe weather threat, but hailstorms can give them a run for their money when it comes to damage and cost.

Here are some facts about hail that you should know:

1. Hail Is A Form Of Precipitation

Similar to rain or snow, hail is a type of precipitation — with its defining characteristic being that is made of solid ice.

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But, this is not the same as freezing rain.

Freezing rain falls as liquid water and freezes once it makes contact with the ground. Meanwhile, hail falls as a solid, known as a hailstone.

2. Hailstones Are Formed In Updrafts

During thunderstorms, rain droplets are carried upwards by a current of air, which is known as an updraft.

As the rain gets lofted higher into the atmosphere, it freezes, and becomes hail.

Hailstones then grow in size as the frozen moisture droplets collide with surrounding water vapor, causing that water to freeze on the hailstone’s surface in layers.

(MORE: What Are Supercell Thunderstorms And How Do They Form?)

And the stronger the thunderstorm, the larger the hail can get.

This is because a stronger storm will have a stronger updraft, allowing the hailstone to continue to be sustained within the cloud by the updraft for longer — colliding with more and more droplets.

When gravity finally wins out over the strength of the updraft, the hailstone will fall to the ground.

3. Hail Size Is Often Estimated Using Comparison

What do peas, quarters, softballs and grapefruits all have in common?

They are all on the hail size chart, which is composed of typical household items.

This size chart was created to help standardize reports, allow emergency managers to assess risk quickly and give the public a simple, visual reference for comparison.

Though it should be noted that the National Weather Service emphasizes that actually measuring hailstones is best when reporting.

Most hailstones are pea-size, but once they reach quarter-size — or 1-inch in diameter — that is when it is considered a severe threat by the National Weather Service,

The largest hailstone ever recovered in the U.S. fell in Vivian, South Dakota on June 23, 2010 — coming in at a whopping 8 inches in diameter and nearly two pounds.

4. Where And When Hailstorms Are Most Common

Hail can form any time of year as long as there are strong enough thunderstorms.

But there are times of the year where it is more common — particularly the spring and summer.

The Southeast and south-central U.S. see the threat for hail increase as winter gives way to spring, with the peak month being May.

However, on a national-scale, the peak hail numbers happen in June as thunderstorms produce the most severe hail in the Plains.

(MORE: Severe Weather Safety 101)

In terms of where hail is the most common, it is most likely from Dakotas to Texas throughout the year.

The area where Nebraska, Colorado and Wyoming meet is known as “hail alley” and it averages seven to nine hail days per year, according to NSSL.

Abroad, China, Russia, India and northern Italy get frequent hail storms too.

5. Hailstorms Can Cause Significant Damage

Hailstones can cause a lot of damage to buildings, vehicles, crops and livestock.

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.

And as you might imagine, the cost of these damages add up very quickly.

In a few cases, we have seen a single hailstorm become a destructive, billion-dollar event.

The costliest hailstorm in U.S. history hit the Phoenix metro area on October 5, 2010, causing an estimated $2.8 billion in damages. Hailstones up to 3 inches in diameter were measured in the area, setting the state record.

And while reported human deaths from being struck by hail are somewhat rare in North America, they do happen.

In 2000 a man in Fort Worth, Texas was killed when he was struck by softball-sized hailstone.

Even hail storms that produce a lot of small hail can be dangerous because all those hailstones can completely cover roads. If these hail piles are deep enough, they can prevent car tires from touching the road at all. This makes driving conditions similar to icy winters.

Caitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.