How Does Hail Form?

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 aloft, or when the hail escapes the updraft, it falls to the ground as a hailstone.

Hail Formation

hail_formationThe 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, supercooled water must come in contact with an ice condensing particle such as dust, pollen, or even insects. By supercooled, we 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 supercooled and remain unfrozen. An updraft from a thunderstorm however will carry with it small particulates like dust, pollen, and insects which will collide with the supercooled water droplets. Once the water droplets touch an ice condensing particle, the supercooled water instantly freezes and turns to ice (same principal behind ice forming on an aircraft wing).

Hail Growth

When the super-cooled water first freezes, the hail is very small and is easily moved by air currents. As the small hail makes contact with more super cooled water droplets and other hailstones, more ice forms around the hail. Some stones may freeze together forming a larger stone. Layer upon layer will be added until the hailstone. If you cut a hailstone in half, you’ll see it’s made of layers, much like the rings of a chopped down tree trunk.

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. Hail from these storms usually isn’t much bigger than the size of a pea. However, if the updraft is tilted (or leaning over), 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, hailstones can make several round trips adding layer upon layer. They can stay aloft for an hour or more. When this happens, hail can grow to the size of baseballs, or even a softballs!

  • 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

hailstoneIf you have ever been in a hailstorm, you may have noticed there are different sized hailstones. Some large and some small. Some round, others clumpy. 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, other hailstones collided and froze together. The smaller hail missed the updraft and didn’t get a chance to go through the entire process a multiple times, thus it is much smaller in size.

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