In the last issue we literally took a look at tornadoes and as I promised, this issue we are going to examine hail – something Albertans know all about.
The first question when it comes to thunderstorms and hail is often “can it be too warm for hail?” The answer is yes. If the upper atmosphere is warm, then the freezing level is very high up. If a thunderstorm does develop and if hail forms in the storm, chances are the hail will melt well before it ever reaches the ground. So the key ingredient for hail to form is to have plenty of cold air aloft and to make sure it is not that high off the ground.
Let’s back up for just a second. I need to touch on one of my weather peeves which, you guessed it, has to do with hail, or rather, the improper use of the term hail. Hail refers to the falling of ice from a cumulonimbus (thunderstorm) cloud. Ice pellets, snow pellets, and graupel (a snowflake that has been coated in ice) are not hail and should not be called hail. These types of precipitation will often occur in the spring or late fall and are not associated with thunderstorms.
OK, now back to our thunderstorm with low-level freezing.
Most thunderstorms will produce hail. The question is whether the hail will grow large enough to make it to the ground without completely melting. As we have already discussed, a very low freezing level helps this happen, because the hailstone only has a short distance to fall through the relatively warm air. Another way to get a hailstone from melting before it hits the ground is to start off with a really big hailstone! This is one of the main reasons Alberta sees so much hail (compared to everyone else in Canada). The topography of Alberta is such that while ground temperatures can be really warm, the freezing layer is not that high up relative to what it might be in other Prairie provinces.
How to make big ones
Now, here is where a second common misconception about thunderstorms and hail lies. To get really big hailstones you do not necessarily need a really tall (or high) thunderstorm.
Hail forms when a particle passes from the warm (liquid) part of the cloud into the cold (freezing) part of the cloud. When this occurs, any water on the particle freezes and you now have a small hailstone. If that hailstone just kept going up towards the top of the thunderstorm it wouldn’t accumulate much more ice and therefore it would remain small.
For hailstones to get really big they must go back into the warm (liquid) section of the storm, pick up more water, then go back up into the cold section of the cloud so the water can freeze. Repeat this cycle a number of times and you can get some really big hailstones.
The biggest hailstone ever recorded fell in Nebraska in 2003 and measured seven inches in diameter (a softball is about 4.5 inches in diameter)! The heaviest hailstone ever recorded fell in Kansas and weighed in at 1.67 lbs. On a more global scale, in 1983, hailstones reportedly as heavy as 2.2 lbs. fell in Bangladesh. It has been estimated that hailstones this large hit the ground travelling at nearly 100 mph or 160 kph! The storm in Bangladesh resulted in 90 fatalities.
So how does a hailstone stay up in the air and travel within the thunderstorm? The answer is updrafts, extremely powerful updrafts. These rising currents of air can keep these massive hailstones aloft until the updraft either dies out or the hailstone falls out of updraft and comes crashing down to earth.
While most of us have seen hailstorms, and some of us have even been caught in a really severe one, I don’t think anyone living on the Prairies has ever experienced a hailstorm lasting 85 minutes and covering the ground to a depth of 46cm. This is what happened in June of 1959 in the town of Selden, Kansas.
When it comes to Canadian hailstorms, Alberta is the hail capital, with most of our country’s hail records occurring here. If we look at the top 12 hailstorms measured by insurance claims, we can see that, with the exception of two, all were in Alberta.
I’d love to know what hail insurance costs in Calgary!
Daniel Bezte has a special interest in farm weather, which he follows from a small farm near Winnipeg, where he has his own computerized weather station. He has been a regular contributor to other farm publications including the Farmers’ Independent Weekly and the Manitoba Co-operator. Daniel has a degree in geography, specializing in climatology, from the University of Winnipeg. He welcomes questions and comments at[email protected]