In the last issue we finished up our overview of thunderstorms and what must happen, or rather, what ingredients need to be in place for strong storms to develop.
In this issue we are going to dig deeper into the topic of severe summer weather by looking at some of the severe weather that is associated with thunderstorms.
I tend to revisit these topics every few years, and looking back, it seems like it has been a couple of years since I’ve gone deep on this issues topic. This one is probably the most feared and costly summer severe weather event, and that would be hail.
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If you have spent any significant amount of time living on the Prairies, then there is a good chance you have probably experienced a hailstorm.
While hail can occur pretty much anywhere across North America, there are two main regions where the chances of experiencing a hailstorm are significantly higher. The first region is the central United States, and the second region is the Canadian Prairies, and particularly Alberta.
For those of you who routinely read my column, then you know I have a fair number of weather pet peeves. Well, guess what, I have another one and you guessed it, 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. You need to have a thunderstorm for hail to occur. The why will be explained if you keep reading.
One of the first questions I get asked about hail is: “Can it be too warm for hail?” The answer is yes. If the upper atmosphere is really warm, then the freezing level in the atmosphere is very high up. If a thunderstorm does develop, and if hail forms in the storm, chances are that 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 that it is not too high off the ground.
This is one of the reasons why Alberta and the higher elevation of the American mid-west experience more than their share of hail. The higher elevation often means that the freezing layer is lower to the ground, resulting in a greater chance that hail will survive and not melt as it falls to the ground.
Most thunderstorms will produce hail, the question is whether or not 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 keep 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 Manitoba.
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. Now, 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.
Picture a popcorn machine, or better yet, an old fashioned bingo machine. The balls, or hail stones, are continually moving up and down due to the strong updraft. This is why a cross section of a large hailstone often has rings.
The hard part to try to understand is just how strong of an updraft you need to keep a hailstone in the air. I did some poking around and looking at some of the equations needed to calculate this and I must admit, it was a little beyond me. So, I started asking questions to one of the AI chat programs.
At first it did not help much, but with the right series of questions I was eventually able to come up with these numbers. A 250-gram object, which is a little smaller than the Canadian record of 292 grams, would take an updraft of about nine meters per second. Convert that into kilometers per hour and you get an updraft of around 32 km/hr. Updrafts in thunderstorms are often in the 10 to 20 m/s range and can, at times, approach 40 m/s – which is over 100 kph!
This means the speeds needed are usually present in most thunderstorms to keep even the heaviest hailstones in the air. If updrafts speeds are not usually a problem, then what limits the size of a hail stone? It is the size or area of the updraft and its duration that sets the limit as these two things in turn determine how long the hail stone can remain in the updraft.
When it comes to hail, size really does matter! Pea-sized hail will do little if any damage to structures or plants, while golf-ball sized hailstones can literally destroy everything in their path.
When it comes to measuring hailstone size things become a little strange. That is, you don’t usually hear that the hail will be around 50 millimetres in diameter. Instead, you hear that the hail was the size of a golf ball or an egg.
Of all the things we measure in regard to weather, hail has by far the most descriptive measurements. Here are some of the more common descriptive terms used for hail and the approximate size that hail stone would be.
Pea – 5 mm
Marble – 10 mm
Grape – 15 mm
Ping-pong ball – 40 mm
Golf ball – 45 mm
Egg – 50 mm
Pool Ball – 60 mm
Tennis ball – 65 mm
Baseball – 70 mm
Grapefruit – 100 mm
Softball – 115 mm
So far during this summer’s severe weather season we have not seen much in the way of severe hail. Let’s hope that continues.
In the next issue, depending on the weather, I will either do an early monthly weather review or we’ll continue our severe summer weather series with a look at what is often the most damaging event, straight-line winds.
