How does hail form?

Of all the different types of weather we can see in Eastern Carolina, I think hail is the most intriguing. Sure, hurricanes are amazing and tornadoes are the ultimate example of the power of nature, but the notion of a baseball sized piece of ice suspended inside a thunderstorm when it’s 90 degrees outside is almost hard to believe—unless your car has ever been hit by one!

But how does it happen?

Well, obviously you have to have cold air somewhere for ice to form. As you probably know, the atmosphere gets colder the higher up you go. The best illustration of this is driving up the side of a mountain. Even in the summer when it’s 90+ down here at the surface, temperatures can be below zero just a few miles up in the atmosphere. That’s where hail forms—in the very tall cloud tops of thunderstorms--typically between 10,000 and 50,000 feet up.

Now back to this baseball size piece of ice. How does that happen? The answer to that in one word is updrafts.

Updrafts are how thunderstorms sustain themselves. In fact, damaging straight line winds often result when a thunderstorm’s updraft collapses causing cold, heavy air from the top of the thunderstorm to come crashing down to the surface as wind. Updrafts can be caused by a variety of factors such as converging winds (wind coming together at one point that is forced upward by the ground) and wind shear, but most hail here in South Carolina is caused by hot air on a summer afternoon rising into subfreezing air directly above. This is called instability.

The best example of how this works is a hot air balloon. Hot air balloons get their lift from heating up the air inside the balloon to a point that it is much warmer than the air outside the balloon. This hot air rises and pulls upward on the balloon. The hotter the air inside the balloon gets, the stronger the lift of the balloon and the more weight it can carry. The same goes for a thunderstorm.

The hotter the air at the surface relative to the air in the top of the thunderstorm, the stronger the updraft and the larger hail stone it can support. Hail gets its start as raindrops on their way to the ground are captured in updrafts and pushed high into the cold part of the thunderstorm where they freeze into small chunks of ice. As this small chunk of ice continues to climb, another layer of ice forms onto it causing it to get bigger. Eventually, the hail stone is heavy enough that the updraft can no longer support it. As it makes its trek back to the surface, it may be caught by another even stronger updraft which pushes it once again into the cold top of the thunderstorm where more ice forms onto it. When it’s too big for the updraft to support, it falls again. This process can happen dozens, if not hundreds of times, for very large hail.

In fact, here’s an experiment you can do—but be very careful and have an adult supervise you if you’re a child. Next time, you see a hail stone, cut it in half and see if you can see rings similar to those found inside a tree. Those rings separate the layers of ice and tell how many trips the hail stone made up and down inside the thunderstorm before it finally crashed to the ground.

Have any questions about hail or other phenomenon you’d like me to answer. Let me know by commenting below and then check back in a few days and I’ll have an answer for you!

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