How Does Hail Form?

Those big thunderstorms that produce tornadoes, powerful downdrafts and torrential rains are almost always accompanied by hail. Some of these storms can drop so much hail, it looks as if it’s snowed in the middle of summer.

Others can drop hail the size of softballs, large enough to kill livestock and even unsuspecting people caught outdoors.

These storms cause billions of dollar in damage to crops and property each year. For example, one storm that ravaged the Denver Metro area on May 8th, 2017 caused $1.4 billion in damages with an estimated 200,000 insurance claims.

So how and why does hail form? Why the different sizes and shapes?

Hail Formation

supercell thunderstorm

When rain gets caught in a thunderstorm’s updraft, it’s lifted into the colder air above. It’s not uncommon to have super-cooled water in liquid form high in the atmosphere.

Despite temperatures being well below freezing, the water never freezes unless it comes in contact with something like pollen, dust, insects or other physical debris. When it does, it instantly freezes around the particle.

Hail Growth


At first, the hail is very small and is easily moved by air currents. If the thunderstorm’s updraft isn’t strong enough, of the hail fails to get caught in the updraft, it will fall to the ground an usually be small in size.

As the hail falls through the thunderstorm, it’s possible it will again get caught in the updraft. If the updraft is strong enough, it will lift the hailstone back into the super-cooled water high above. As before, another layer of ice will coat and freeze to the hailstone, making it bigger, but also heavier. The more trips the hail takes into the super-cooled water, the bigger it will get, and the harder it is to stay aloft.

Large supercell thunderstorms have tilted updrafts and can last for hours. Their updrafts also tend to be larger and much more powerful, often exceeding 100 mph. Hail in these storms can make multiple round-trips into the super-cooled water above and grow very large in size.

Why Hail Looks the Way it Does

Hailstones cut to show the growth rings

If you have ever been in a hailstorm, you may have noticed there are different size, shape, and textures to hailstones. Size is determined by how much time the hailstone spent in the storm. Cloudy looking hail forms when the super-cooled water freezes quickly trapping air bubbles in the ice. Clear looking hail forms when the super-cooled water freezes slowly, allowing the air to escape. Large hailstones will sometimes freeze together, forming a clumpy looking mass.

If you cut a hailstone in half, you’ll see layers, much like the rings of a chopped down tree trunk. Each layer represents a trip back into the super-cooled water.

Hail Size

It’s hard to believe, but hail can grow to be the size of baseballs, or even a softballs! The largest hailstone ever officially recorded fell outside Vivian, South Dakota on July 23, 2010. It measured 8.0 inch diameter, 18.5 inch circumference, and weighed in at 1.94 pounds! This obliterated the previous record which measured in with a 7 inch diameter, 18.75 inch circumference which fell in Aurora, Nebraska on June 22nd, 2003.

  • Pea (0.25 in.)
  • Half-inch (0.50 in.)
  • Dime (0.75 in.)
  • Nickel (0.88 in.)
  • Quarter (1.00 in.)
  • Half Dollar (1.25 in.)
  • Ping Pong Ball (1.50 in.)
  • Golf Ball (1.75 in.)
  • Hen Egg (2.00 in.)
  • Tennis Ball (2.50 in.)
  • Baseball (2.75 in.)
  • Tea Cup (3.00 in.)
  • Grapefruit (4.00 in.)
  • Softball (4.50 in.)

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