All About Hail: A Severe Weather Guide

Hail is just ice, right? Wrong!

Do you know how it forms? Or what size it has to be?

Keep reading this guide to explore everything there is to know about hail. Use the buttons below to skip to the section you’re most interested in, or give the whole guide a read. It’s up to you!

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What is Hail?

Red cloud icon with hail falling out of it in blue circleHail is precipitation in the form of small ice balls or pieces with a diameter of 5 mm (about 0.2 inch) to over 15 cm (about 6 inches).

This precipitation often accompanies thunderstorms. It can damage and destroy buildings, crops, machinery, and living things.

The National Weather Service defines severe hail as hail that is 1 inch or greater in diameter.

Hail vs. Sleet / Ice Pellets vs. Hail

Many confuse hail with sleet and ice pellets, but there is a difference based on size.

How big is sleet? Sleet refers to ice pellets with a diameter of less than 5 mm.

Ice pellets are raindrops that freeze before they hit the ground. You can use sleet and ice pellets interchangeably. These often accompany freezing rain.

The most important thing for you to remember is that sleet and ice pellets are smaller than 5 mm, while hail is 5 mm or larger.

Now that you have a better understanding of the size of hail, and the differences between it and ice pellets/sleet, it’s time we dive into the more intricate topics about this precipitation.

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How Does Hail Form?

It’s time to get into hailstorm science and answer a pressing weather question: “How does hail form?”

You can keep reading, or listen to our Senior Meteorologist, Chad Merrill, explain.

Hail formation starts with thunderstorms. The first two things that must be present are strong updrafts of warm air and downdrafts of cold air.

These strong currents of air are typical in Cumulonimbus clouds.

Anvil-shaped Cumulonimbus cloud

These clouds are the massive anvil or mushroom shaped clouds you see during thunderstorms. They can be over 65,000 feet tall!

In the Clouds…

the hail formation process | how hail forms | graphicThe updrafts can pick up water droplets and carry them high into the clouds. The higher the water droplets go, the colder the temperature becomes.

When it gets below freezing level (below 32F), these water droplets will turn into ice particles.

Once our water droplet-turned-ice particles make it to high altitudes within the clouds, they meet up with high concentrations of super-cooled water droplets.

Hailstone Layers

These super-cooled droplets then latch onto an ice particle’s surface. This forms multiple ice layers around it, creating larger hailstones.

The higher a water droplet travels into a cloud, the more super-cooled particles will latch on. A hailstone can also make multiple trips up and down a cloud. With each trip above freezing level, the hailstone will also grow with another ice layer.

As the hailstone gets larger and larger, it also gets heavier. At some point, gravity causes the stone to fall back down towards the Earth.

Layers of Ice

The way hail forms dictates what a hailstone looks like. Did you know that larger hail has alternating layers of clear and opaque ice?

This pattern happens thanks to irregular rates of freezing when the super-cooled water particles layer on. As the hailstone moves up and down in a cloud’s updrafts and downdrafts, it gathers different layers of ice.

When freezing occurs slowly in warmer temperatures, trapped air can escape. This produces clear ice.

When freezing occurs quickly in colder temperatures, the process traps the air and produces a layer of white ice.

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red ruler icon in dark blue circle

Hail Size

As we’ve discussed, hail comes in different sizes. While it has to have a diameter larger than 5 mm, it can get much bigger.

In this section, we’ll go over:

A dark blue "1" in a red circle Hail sizes

A dark blue "2" in a red circle How we classify size

Some of the biggest hailstones on record

While technically there is no such thing as “normal” or “abnormal” hail, most is less than 2 inches in diameter. When estimated hail is less than 2 inches, it can be anywhere in between the size of a pea and a golf ball.

Estimating Size

It’s often difficult to get an accurate measurement of hail diameter since it melts.

This is especially true when it’s falling, since no one wants to get hit with hail!

The National Weather Service uses a table to estimate hail sizes between 0.25 inch diameter and 4.5 inch diameter. This helps people estimate hail by sight rather than putting themselves in harm’s way.

NWS Size List

The National Weather Service uses the following object to help estimate hail size:

A red check mark with a transparent background Pea: 1/4 inch diameter

A red check mark with a transparent background Marble/Mothball: 1/2 inch diameter

A red check mark with a transparent background Penny/Dime: 3/4 inch diameter

A red check mark with a transparent background Nickel: 7/8 inch diameter

A red check mark with a transparent background Quarter: 1 inch diameter

A red check mark with a transparent background Ping Pong Ball: 1 1/2 inch diameter

A red check mark with a transparent background Golf Ball: 1 3/4 inch diameter

A red check mark with a transparent background  Tennis Ball: 2 1/2 inch diameter

A red check mark with a transparent background Baseball: 2 3/4 inch diameter

A red check mark with a transparent background Tea Cup: 3 inch diameter

A red check mark with a transparent background Grapefruit: 4 inch diameter

A red check mark with a transparent background Softball: 4 1/2 inch diameter

If the diameter of hail is an inch or larger, we consider it severe.

When it doubt, don’t go outside! Wait until the thunderstorm moves away before going outside to measure the size of hail.

Remember to trust all-clear signals from trusted outdoor alerting systems or organizations like the National Weather Service.

Biggest Hailstone on Record

So what is the biggest hail ever recorded?

According to the U.S. National Weather Service, the largest all-around hailstone in the U.S. fell on July 23, 2010. Just how large was it?

A red check mark with a transparent background 8 inch diameter

A red check mark with a transparent background 18.62 inch circumference

A red check mark with a transparent background 1 lb and 15 oz

That’s nearly the size of a volleyball! This monster hailstone fell in Vivian, South Dakota. You can look at the hail in the image below from our friends at the NOAA Photo Library, NOAA Central Library.

The largest recorded hailstone fell on July 23, 2010 in Vivian South Dakota. This image shows the resident who found it measuring the circumference with a tape measure. The circumference was over 18 inches long!
Credit: NOAA Photo Library, NOAA Central Library

The heaviest hailstone recorded worldwide fell in Gopalganj District, Bangladesh. The hailstone weighed in at 2.25 lbs and fell on April 14m, 1986.

The largest hail circumference ever officially measured was 18.74 inches. This hailstone fell in Aurora, Nebraska on June 22, 2003.

Hard-Hitting Hail Facts

What else do you need to know about hail before we jump into hailstorm prediction? We think the following hail facts are pretty interesting.

A red check mark with a transparent background Did you know that baseball-sized hail can fall at speeds of 120 mph? Imagine getting hit by that! On average, the velocity of a falling hailstone is a little less than that at approximately 106 mph.

A red check mark with a transparent background Hail causes $1 billion in damages to crops and property each year in the U.S.

A red check mark with a transparent background Hailstorms are most common in the Midwest United States and usually last about 15 minutes. You’re most likely to experience a hailstorm during the mid-to-late afternoon.

A red check mark with a transparent background The U.S. averages 5,000 reports of large hail each year. That’s five times the amount of tornado reports!

Hail Map

Interested in learning where and when hail typically happens?

The National Oceanic and Atmospheric Administration (NOAA) has a hail climatology map. This shows the probabilities of seeing hail in thunderstorms across the U.S. as the year unfolds.

You can also check out our weather map, Sferic Maps, to see where major storms are that could contain hail. This weather visualization tool also shows National Weather Service alerts and reports of hail.

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Hail Forecasting

The more we talk about hail and how large it can be, the more we think about the damage it can cause.

A blue car hood with ping pong ball sized dents in it from hail

Hail of any size can cause damage, but severe hail (hail 1 inch diameter or greater) typically causes the most.

Another factor that determines how much damage hail can cause is the wind. Higher wind speeds mean more potential damage as the hail moves faster. Hail can damage cars, windows, aircrafts, crops, roofs, and more.

Since plants are very sensitive, even the smallest hailstones can cause a lot of damage to crops.

hail falling at a farm to show that hail can have a large impact on crops and agriculture

Another dangerous weather hazard is hail accumulation. Accumulating hailstorms can cover the ground in over 2 inches of hail. This can cause power outages by weighing down power lines and trees.

Flash flooding and mudslides are also a hazard in areas with steep terrain.

As we mentioned before, hailstorms cause over $1 billion in damages annually in the United States. The best ways to prevent damage are through hailstorm forecasting and hail suppression programs.

Detecting Hail With Radarblue and red radar icon

Forecasters have been using weather radar to observe and measure hail since the end of WWII in 1945.

The current generation of weather radar technology does not directly measure precipitation. Instead, it measures the energy reflected to the radar from whatever the radar is looking at. This can be hail, but also rain, snow, or even insects.

Forecasters then infer precipitation by the amount of returned energy to the radar and the location of the strong radar echoes in storms. Radar echoes are particularly important for detecting hail.

Forecasters look for areas of high reflectivity in the middle to upper levels of storms as a sign of large hail formation. They also look for signs of sustained updrafts that could keep hailstones aloft long enough to become large and cause damage once they fall to the ground.

The National Weather Service looks at dual-polarization radar technology to tell the difference between precipitation types and even determine hailstone size.

How to Forecast Hail with Radar

Hail often appears much larger than rain on a weather radar. Typical raindrops are 1-3 mm in diameter. Hail can be over 10 times that size!

Large hail also returns a lot of energy back to the radar.

Forecasters use computer algorithms to examine the radar data and calculate statistical forecasts of the probability of hail and if it is severe. They also find the probability of maximum size for the hailstones.

You can dive deeper into observing precipitation with NEXRAD radar on the Community Collaborative rain, hail, & snow resource center.

How to Forecast Hail with Total Lightning

Dangeorus Thunderstorm Alerts over Russia

Hail forms in thunderstorms, so total lightning detection is a good way to forecast hail.

When a thunderstorm happens, there is lightning. But not all lightning hits the ground. In fact, a majority of all lightning strikes are in-cloud pulses.

Total lightning is the combination of in-cloud and cloud-to-ground lightning strikes and is a precursor to hail and other dangerous weather like tornadoes and torrential rains.

Supercell thunderstorms, in particular, have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the freezing air at the top of the cloud.

When we issue one of our Dangerous Thunderstorm Alerts (powered by our total lightning network) there is a good chance that large hail is present in the storm.

Forecasting Hail: The Bottom Line

Ok, so we got a little technical there.

So when meteorologists say that hail is in the forecast, what are they really talking about? They’re saying that they’ve observed deep moist convection along with these three basic ingredients:

A dark blue "1" in a red circle Adequate updraft to keep the hailstone aloft for an appropriate amount of time

A dark blue "2" in a red circle Sufficient super-cooled water near the hailstone to enable growth as it travels

A piece of ice, snow, or rain to grow upon

Basically, there is no distinction between storms that do and do not produce hailstones, but forecasters can use tools like radar to detect hail and algorithms and total lightning detection to anticipate it.

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Hail Alert

If you know where severe weather is and when it’s coming you can protect yourself from damages and possible bodily harm. Can you imagine being outside when a 2 lb hailstone comes falling out of the sky?

Safety During Hailstorms

What can you do to stay safe during a hailstorm? It really depends where you are.

IF YOU ARE OUTDOORS

orange and blue horizontal arrow pointing rightSeek shelter immediately.

orange and blue horizontal arrow pointing rightIf you can’t get to shelter, at least find something to protect your head.

orange and blue horizontal arrow pointing rightStay out of culverts and lowland areas that might fill suddenly with water or hail accumulations.

orange and blue horizontal arrow pointing rightKeep away from trees as hail can cause branches to fall. Lightning can also strike trees.

IF YOU ARE INDOORS

 orange and blue horizontal arrow pointing rightStay inside until the hail stops.

orange and blue horizontal arrow pointing rightStay away from skylights and windows.

orange and blue horizontal arrow pointing rightAccount for all coworkers, family members, building occupants, pets, etc.

orange and blue horizontal arrow pointing rightDo not go outside for any reason. Large hail can cause serious or even fatal injuries. Observe other thunderstorm safety tips, like staying away from electrical appliances and plumbing.

IF YOU ARE IN A CAR

orange and blue horizontal arrow pointing rightThe first thing you should do is stop driving. If you can see a safe place like a garage or service station awning, get there safely.

orange and blue horizontal arrow pointing rightDo not stop under bridges or overpasses as this can create dangerous traffic jams and puts you at risk in case of a tornado.

orange and blue horizontal arrow pointing rightStay in your vehicle until it stops hailing.

orange and blue horizontal arrow pointing rightKeep away from car windows and cover your eyes with something, like a piece of clothing.

orange and blue horizontal arrow pointing rightIf possible, get to the floor face down. You can also lay down on the seat with your back to the windows.

Hail Suppression

Some countries and organizations rely on hail suppression to minimize damages to crops and infrastructure. Cloud seeding is a popular form of hail suppression.

Cloud seeding is using flares to create smaller particles of hail. The first step is to seed thunderstorms with billions of silver iodide smoke particles given off by the flares.

Then the flares act as artificial ice crystals that freeze up the super-cooled water drops in the storm’s updraft.

The end result is that the storm produces a greater amount of small, pebble-sized hailstones as opposed to a lesser amount of large, extremely damaging hailstones. These smaller hailstones cause less damage than the larger ones.

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Learn More About Severe Weather

After reading this guide, you’re practically a hail expert!

There are more dangerous forms of weather other than hail, though. It’s time to dive into another severe weather guide to help increase your knowledge and overall preparedness.

Head over to our Weather 101 page to explore another topic.

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