WX Info: How do meteorologists forecast hail size?

meteorologists forecast

Given the hailstorm that moved through parts of south Mississippi early Wednesday morning, a lot of people have been asking about how the hail gets to be that big?

The National Weather Service noted there were storm reports of hail sizes between nickels, quarters, McDonald’s sauce cups, and golf balls. Some big hail!

The forecast called for Quarter-to-Half-Dollar-sized hail, with a chance for some larger hail, but that wasn’t as likely.

This is why…

How does hail even form?

Hail forms differently that snow or sleet. Hail forms when super-cooled liquid drops freeze to something called the nuclei.

From the NWS:

Hailstones grow by collision with supercooled water drops. (Supercooled drops are liquid drops surrounded by air that is below freezing which is a common occurrence in thunderstorms.) There are two methods by which the hailstone grows, wet growth and dry growth, and which produce the “layered look” of hail.

In wet growth, the hailstone nucleus (a tiny piece of ice) is in a region where the air temperature is below freezing, but not super cold. Upon colliding with a supercooled drop the water does not immediately freeze around the nucleus.Instead liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.

With dry growth, the air temperature is well below freezing and the water droplet immediately freezes as it collides with the nucleus. The air bubbles are “frozen” in place, leaving cloudy ice.

The hailstone size is a result of how long the hailstone is in the cloud. So what dictates how long it is in the cloud? I’m so glad you asked.

Hail in the clouds

The updraft, in the image above, is the part of the storm where warm moist air is lifted up intot he cloud. It is the source of all of the moisture for the raindrops – and hail stones. And the air that is being lifted into the cloud can be moving really fast. And the fast the air moves, the more ‘stuff’ it can hold up!

On top of that, the faster it goes, the heavier the ‘stuff’ is it can hold up.

The above chart from the NWS is a look at how fast an updraft has to be going in order to hold up a certain-sized hailstone. But that is given a generic atmosphere. And the numbers were possibly averaged based off of research done by http://www.jdkoontz.com:

So, when the NWS kicks out reports of golf ball-sized hail, it is safe to assume the speed of that updraft is between 33 and 64mph.

That’s great, Nick, but how do you predict it?

Another great question! It turns out there is a cool little math trick that allows you to estimate an updraft speed.

A great example of this calculation is on Jeff Haby’s website, Haby’s Hints.

If you take the instability value, CAPE, and double it… then take the square root, it does a good job at giving you an estimation for the updraft speed.

So all we have to do is look at the CAPE value from the model data:

For this data, the “Most Unstable CAPE” was around 2000. So 200 x 2 = 4000. Then take the Square Root of 4000, and you get 63.24. But that is in “meters per second” to “miles per hour” so we need to convert it. For that, you just go to google (I’m lazy) and you get 140mph.

So the “most unstable CAPE” or the “highest possible value given the environment” is an updraft of 140mph. But that is without friction (from horizontal wind shear), air density and moisture, and all of that other stuff factored in.

As a “rule of thumb,” for me personally, I tend to cut that number in half to give a “realistic” idea of updraft speed. It isn’t scientific, there haven’t been studies to show that it works by any means, but it seems to work well for me.

In this case, half of 140mph is 70mph. And 64mph can hold up Golf Ball hail according to the NWS chart, so it works out alright.

The take home point

So the next time you hear a local meteorologist talking about hail sizes, you can now reverse engineer the instability! Say a local meteorologist calls for Quarter-sized hail.

Well, you now know that quarter-sized hail takes a 50mph updraft. So in order to figure out the “Most Unstable CAPE value, you double that number… Now its 100.

Then you convert it back to m/s… 44m/s

Now square it! You get 1936. And divide it in half. You get 968. So the MUCAPE is somewhere a round 950.

via GIPHY

SCIENCE! MATH!