How tiny ice crystals can grow into large hail

In my last column, we looked at why some thunderstorms produce hail and why the Canadian Prairies, especially Alberta, are particularly vulnerable.

That naturally leads to another question: how does a tiny piece of ice become a hailstone large enough to smash windows, shred crops or leave golf-ball-sized dents in vehicles?

The answer lies inside the powerful updraft of a thunderstorm.

Every hailstone begins as a tiny ice particle high within the cloud.

Strong rising air carries that particle upward into temperatures well below freezing, where liquid water freezes onto its surface. If the hailstone simply continued upward and then fell to the ground, it would remain relatively small.

Instead, severe thunderstorms often send the developing hailstone on a remarkable journey.

Powerful updrafts repeatedly lift it into the cold part of the storm, while gravity pulls it back into the warmer portion, where it collects a fresh coating of liquid water.

Each time it rises again, that new layer freezes solid. After several trips through the storm, the hailstone grows larger and larger.

A useful way to picture this process is to imagine an old-fashioned bingo cage with the balls constantly tumbling around inside.

The hailstones are doing much the same thing, bouncing up and down within the thunderstorm until they eventually become too heavy for the updraft to support.

If you were to slice open a large hailstone, you would often see a series of distinct layers, each representing another trip through the storm.

You may also notice that large hailstones are occasionally made up of several smaller hailstones frozen together.

How strong can updrafts be?

The strongest thunderstorms contain impressive updrafts that can exceed 100 km/h. Those rising winds are more than capable of suspending surprisingly large hailstones.

In fact, it is often not the strength of the updraft that limits hail size, but rather its width and how long it lasts. The larger and longer-lived the updraft, the more time hailstones have to continue growing.

When hail finally falls, its size determines the amount of damage it can cause.

Pea-sized hail is usually little more than a nuisance.

Once hail reaches golf-ball size, however, the potential for serious crop damage, broken windows and vehicle damage increases dramatically.

In the most intense storms, hailstones can even approach the size of baseballs or softballs.

That is also why meteorologists often describe hail using familiar objects rather than measurements.

Saying “golf-ball-sized hail” paints a much clearer picture than simply reporting hailstones measuring 45 millimetres in diametre.

Fortunately, truly destructive hailstorms are relatively uncommon, even across the Prairies.

However, when all the atmospheric ingredients come together, they can develop quickly and leave behind millions of dollars in damage.

In our next look at severe summer weather, we’ll turn our attention from falling ice to another thunderstorm hazard that is often underestimated — wind.