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?
Hail 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.
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.
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 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.
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.
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:
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.
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:
Pea: 1/4 inch diameter
Marble/Mothball: 1/2 inch diameter
Penny/Dime: 3/4 inch diameter
Nickel: 7/8 inch diameter
Quarter: 1 inch diameter
Ping Pong Ball: 1 1/2 inch diameter
Golf Ball: 1 3/4 inch diameter
Tennis Ball: 2 1/2 inch diameter
Baseball: 2 3/4 inch diameter
Tea Cup: 3 inch diameter
Grapefruit: 4 inch diameter
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?
8 inch diameter
18.62 inch circumference
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 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.
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.
Hail causes $1 billion in damages to crops and property each year in the U.S.
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.
The U.S. averages 5,000 reports of large hail each year. That’s five times the amount of tornado reports!
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.
The more we talk about hail and how large it can be, the more we think about the damage it can cause.
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.
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 Radar
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
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.
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:
Adequate updraft to keep the hailstone aloft for an appropriate amount of time
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.
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
Seek shelter immediately.
If you can’t get to shelter, at least find something to protect your head.
Stay out of culverts and lowland areas that might fill suddenly with water or hail accumulations.
Keep away from trees as hail can cause branches to fall. Lightning can also strike trees.
IF YOU ARE INDOORS
Stay inside until the hail stops.
Stay away from skylights and windows.
Account for all coworkers, family members, building occupants, pets, etc.
Do 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
The 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.
Do not stop under bridges or overpasses as this can create dangerous traffic jams and puts you at risk in case of a tornado.
Stay in your vehicle until it stops hailing.
Keep away from car windows and cover your eyes with something, like a piece of clothing.
If possible, get to the floor face down. You can also lay down on the seat with your back to the windows.
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.
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.