How does hail form? The short answer
Hail forms inside strong thunderstorms. A thunderstorm has a powerful rising wind called an updraft. The updraft carries raindrops up into parts of the cloud that are far below freezing, and they freeze.14 That little seed of ice, called a hail embryo, then collides with supercooled water droplets: drops that are colder than 0 °C (32 °F) but still liquid. They freeze onto it, layer by layer, and the ice ball grows.13 The hailstone keeps growing for as long as the updraft can hold it up. It falls when it gets too heavy, or when the updraft weakens.1
So hail is not frozen rain that simply dropped out of a cold cloud. It is ice that has been built while it was kept floating inside a storm.
What causes hail? Three ingredients
- A strong updraft. Warm, moist air rises quickly inside a thunderstorm. This rising air is what holds hailstones up while they grow. The stronger the updraft, the bigger the hail it can make.35
- Air far below freezing, high in the cloud. Even on a hot summer day, the middle and upper parts of a thunderstorm cloud are very cold.79 Most hail growth happens where the air is about −10 to −25 °C (14 to −13 °F).7
- Lots of supercooled water. Water droplets in a storm cloud often stay liquid below freezing. They freeze the moment they touch a piece of ice, and that is what the hailstone is made of.3
You can feel the idea of an updraft at home: point a hair dryer at the ceiling and put a ping-pong ball in the air stream. It floats. Put in something heavier and it drops. A storm's updraft works the same way, just much bigger and stronger.
How is hail formed, step by step?
- An embryo forms. Updrafts carry raindrops high above the freezing level, where they freeze. Hail embryos can be frozen drops or graupel, soft snow pellets made when droplets freeze onto a snow crystal.12
- It collects ice. The embryo rides through the part of the cloud full of supercooled droplets. Each droplet it hits freezes onto its surface.13
- It may go round again. Winds inside a storm don't only blow up and down. A stone can be blown out of the main updraft and start to sink. If the updraft is strong enough, it can be carried back into the cloud to grow some more, and this can happen several times.13
- It falls. When the stone is too heavy for the updraft to support, gravity wins and it falls. Small stones may melt into rain in the warm air near the ground. Only stones that grew big enough reach the ground as hail.15
One surprise: hailstones don't grow best at the very top of the storm. Up there it's colder than −40 °F, so all the water has already frozen, and a hailstone needs liquid water to grow.1
How strong does the updraft need to be? (hail size table)
Hail size is usually described by comparing it with everyday objects. Hail 1 inch across (quarter size) or bigger counts as severe.1 The National Weather Service gives these rough updraft speeds for each size:3
That's why the biggest hail comes from supercells, long-lived storms with a strong, steady, rotating updraft.3
Why do hailstones have rings?
Cut a large hailstone in half and you'll often see rings, like the layers of an onion. Some rings are milky white and some are clear.35
- Cloudy ice = dry growth. In very cold air, droplets freeze the instant they hit the stone. Tiny air bubbles get trapped, and trapped bubbles look white.13
- Clear ice = wet growth. In air that is below freezing but not as cold, the water spreads over the tumbling stone and freezes slowly. The bubbles have time to escape, so the ice is clear.13
The rings are a record of the different temperatures and amounts of water the stone passed through. They are not simply a count of trips up and down the storm.1 Big hailstones are also rarely perfect balls: they can be lumpy, bumpy and even spiky, sometimes because smaller stones stuck together.25
Why does it hail in summer, when it's hot?
Hail needs the strong updrafts of big thunderstorms, and those grow when the air near the ground is warm and moist. In the U.S., hail becomes more likely through spring, peaks in the High Plains in summer, and fades after September.5 It can be hot at ground level while the upper cloud is far below freezing.9 Winter storms have weaker updrafts, so any hail is small.7
That also answers a common question: why can it hail but not snow? Hail only needs cold air high in the cloud. The stones are big enough to fall through the warm air below before they melt. Snowflakes are light and fragile, and they melt on the way down.
What's the difference between hail and sleet?
Sources: NOAA NSSL2, NWS Aberdeen6.
Where does hail happen most?
Florida has the most thunderstorms in the U.S., with 80 to more than 105 thunder days a year.8 But its freezing level is so high that hail often melts before it lands.3 The most hailstorms usually hit Nebraska, Colorado and Wyoming. Where those three states meet, called hail alley, there are seven to nine hail days a year on average.1 The Great Plains sit high up, so the freezing level is closer to the ground.5 Other places with damaging hail include China, Russia, India and northern Italy.1
How big can hail get?
The biggest hailstone ever found in the United States fell in Vivian, South Dakota, in 2010. It measured 8 inches across and 18.6 inches around, and weighed almost 2 pounds (1 lb 15 oz).61 The National Weather Service estimated that the storm's updraft reached 160–180 mph.6
Is hail dangerous? Can hail kill you?
Yes, big hail is dangerous. It damages cars, roofs, crops and aircraft, and it can injure or kill people and animals.1 In 2000, a person in Fort Worth, Texas, was killed by grapefruit-size hail.3 Hail falls fast: stones under 1 inch fall at about 9–25 mph, 2–4 inch stones at about 44–72 mph, and the very largest can pass 100 mph.1 Baseball-size hail can hit the ground at around 70 mph.5
Staying safe: when a severe thunderstorm warning is issued, get inside a sturdy building.10 Stay away from windows, doors and skylights, and park cars under cover if you can do so safely beforehand.7 Large hail often falls near the part of a storm where tornadoes form, so treat a big hailstorm as a sign a tornado could be close.11 Want to know more about storms? See what causes lightning, how tornadoes form and how snow forms.
Common misconceptions
Myth: hail is frozen rain. Frozen raindrops are sleet. Hail grows layer by layer inside a thunderstorm and can reach the size of a grapefruit.
2
Myth: hail needs cold weather. Hail is mostly a warm-season event, because it needs the strong updrafts of spring and summer storms.
5
Myth: each ring is one trip up and down. Rings record changes in temperature and water around the stone, not simply up-and-down cycles.
1
Myth: hail grows at the top of the storm. The top is so cold that no liquid water is left, and hail needs liquid water to grow.
1
Myth: hailstones are smooth balls. Many are lumpy, bumpy or even spiky.
2
FAQ
How does hail form, in simple words?
Strong rising winds inside a thunderstorm, called updrafts, carry raindrops up to where the air is far below freezing, and they freeze. The little ball of ice then bumps into supercooled water droplets (liquid water colder than 0 °C), which freeze onto it in layers. It keeps growing while the updraft holds it up, and falls when it gets too heavy.
What causes hail?
Hail needs a strong thunderstorm updraft, air far below freezing high in the cloud, and plenty of supercooled water. The stronger the updraft, the bigger the hailstones it can hold up while they grow.
How does it hail when it's hot outside?
Even on a hot day, the upper part of a tall thunderstorm is far below freezing. Hot, humid weather actually helps, because it powers the strong updrafts that hail needs. Big stones fall fast enough to reach the ground before they melt.
Why can it hail but not snow?
Hail only needs freezing air high in the storm cloud. Hailstones are dense and heavy, so they can fall through warm air near the ground before melting. Light, fragile snowflakes would melt on the way down.
What is the difference between hail and sleet?
Sleet is a raindrop, or a melted snowflake, that refreezes into a small ice pellet as it falls, usually in winter storms. Hail grows layer by layer inside a thunderstorm updraft, mostly in spring and summer, and can reach the size of a grapefruit or bigger.
What state gets the most hail?
Nebraska, Colorado and Wyoming usually have the most hailstorms. Where the three states meet, called hail alley, there are about seven to nine hail days a year on average.
Does hail hurt when it hits you?
Yes. Even small hail can sting, and large hail can cause serious injuries and has killed people. Stones 2 to 4 inches across fall at about 44 to 72 mph. If hail starts, go inside a sturdy building and stay away from windows.
Sources
- NOAA National Severe Storms Laboratory, Severe Weather 101: Hail Basics
- NOAA National Severe Storms Laboratory, Severe Weather 101: Hail Types
- NOAA National Weather Service JetStream, Thunderstorm Hazards: Hail
- NASA Global Precipitation Measurement, How does hail form?
- Midwestern Regional Climate Center (Purdue University), Hail
- National Weather Service Aberdeen, Record hailstone, Vivian, South Dakota
- Australian Bureau of Meteorology, Explainer: How does hail form?
- NOAA National Weather Service JetStream, Introduction to Thunderstorms
- UCAR Center for Science Education, Why Does Hail Happen?
- National Weather Service, Severe Thunderstorm Watch vs. Warning
- NOAA National Severe Storms Laboratory, Hail FAQ