Weather Imagery

Hail forms in thunderstorms where the updraft is strong enough to carry water droplets and ice condensing nuclei high into the atmosphere where they interact and freeze turning into ice. When the weight of the ice is too heavy for the updraft to keep it suspended in the cloud or when the hail falls outside the updraft and is allowed to fall back to the ground, a hailstone is created.

Hail Formation

The dynamics behind what creates the hail is somewhat complex. The basic ingredients are ice condensing nuclei, super cooled water droplets, latent heat, and powerful updrafts. In order for ice to form, super cooled water must come in contact with an ice condensing particle such as dust, pollen, or insects. By supercooled, I mean the temperature of the droplet is less than 32°F yet remains in liquid form. If these ice condensing nuclei aren’t present, water droplets can become super cooled and remain unfrozen. An updraft from a thunderstorm however will carry with it dust, pollen, and insects which will collide with the super cooled water droplets. Once the water droplets touch an ice condensing particle, the super cooled water instantly freezes and turns to ice (same principal behind ice forming on an aircraft wings).

Hail Growth

At this point, the hail is very small and is easily moved by air currents. As the small hail makes contact with other super cooled water droplets, more ice forms around the hail until it grows large enough that it begins to fall from the sky due to its increased weight. The updraft of the thunderstorm is what keeps the hail suspended in the air. The longer the hail remains in the air, the larger the hailstones become. Once the hail grows too large and gains weight, the wind can no longer keep it aloft. It then falls the the ground as hail.

Hail Size

Although small hail can form anytime there is super cooled water, ice condensing nuclei and an updraft, large hail can only form when the updraft is powerful and tilted or spiraling. As the hail gets carried high into the thunderstorm it gets shot out the top like a popcorn machine. If the updraft is vertical in nature, the hail will fall outside the updraft and eventually hit the ground. However, if the updraft is tilted, some of the hail will re-enter the updraft as it’s falling and go through the growth cycle all over again. If the updraft is powerful enough, it can lift some pretty heavy hailstones and keep them suspended for hours until they become so large that the updraft is no longer capable of lifting them high into the cloud anymore.

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.)

The largest hailstone ever to fall in the United States (as seen above) had a 7 inch diameter, 18.75 inch circumference and fell in Aurora, Nebraska on June 22nd, 2003. What’s even more remarkable is, NOAA officials think 40% of the stone broke off when it hit the gutter of the house and another percentage melted before they actually got to it. Yet after all that, it still broke the record!

Why Hail Looks the Way it Does

If you have ever been in a hailstorm, you most likely noticed there are different sized hailstones. Some large and some small. As mentioned above, some hail will re-enter the updraft (larger hailstones) while others will miss it (smaller hailstones). The larger hailstones look knobby or clumpy because as they re-enter the updraft, the hailstones will collide and freeze together. This gives them a very irregular and clumpy look. The smaller hailstones on the other hand, may look more smooth but may still be irregular in shape. These hailstones probably didn’t make too many round trips in the updraft because they didn’t get a chance to collide and clump together.