Wx Savvy | Storms

How Does Hail Form?

Hail only forms in thunderstorms. The reason hail is always formed in T-Storms is because those are the storms that have the strong updrafts that allow hail to grow. Let me explain this further.

In a T-Storm there are strong winds going up (called updrafts) and strong winds coming down (downdrafts). Although these winds can be next to each other, if they were to be acting against each other it would weaken and likely dissipate the T-Storm. This is actually one way a storm can fizzle out quickly. These updrafts and downdrafts can reach speeds of 100 Mph or faster! You often feel the downdrafts out ahead of a storm since the wind hits the ground and spreads out as a gust of wind ahead of the storm.

Hail can start off as something tiny like an ice crystal and grow to become larger than a grapefruit. How does this happen? Well supercooled water drops are the key (for more on supercooled water drops, click here). These liquid drops suspended in the cloud will freeze on contact with an object, particularly an ice crystal. So, our little ice crystal turns into a ball of ice. Now that it is heavier, it starts to fall to earth. As it falls through the cloud it collects more of these supercooled water drops which freeze on contact and make our hailstone grow larger and larger. But before out hailstone can fall to earth it gets caught in the updrafts and shoots high into the atmosphere...yet again collecting more supercooled water drops and growing larger. Then, it falls again to earth and perhaps gets caught in the updraft again.

So the hail bobs up and down in the updrafts and downdrafts of a T-Storm growing larger and larger. Actually, I would assume the hailstone grows and shrinks because some evaporation/sublimation may occur particularly in the higher and drier levels of the atmosphere. Also, there would likely be some frictional effect on the hail from air and ice crystals that may chip away at the size of our hailstone. But there is a net growth of the hailstone to the point where it is heavy enough to fall through the updraft or it gets into an area of the storm where it no longer is caught in the updraft and falls to earth.

Notice in the picture above that the hail is actually comprised of several hailstones that froze together. And in the picture below, hail split in half reveals rings like tree rings. For more on why there are different colors in a hailstone, click here.

All Pictures on this page courtesy of NOAA photo library- The NSSL Photo Album
http://www.photolib.noaa.gov/nssl/hail1.html