Why does hail become very large sometimes?

Summer thunderstorms can cause a lot of damage. Lightning can set houses on fire, flash floods can cause a lot of water damage, and they are not the only ones. Thunderstorms can produce a wide variety of weather phenomena that are all destructive. Hail can be one of them.

While many storms produce hail of some size, a small amount of thunderstorms produce very large hail stones. Why is this the case? How comes that some storms produce very large hail stones while most of the storms do not have hail or have small stones? Those are questions that we will answer in this article.

A large hail stone. License: public domain, nssl0001, National Severe Storms Laboratory (NSSL) Collection.

First, we will look at a basic concept from physics – being that an object moves in the direction of net force. When we understand this, we can see how water droplets in a cloud remain there rather than all of them falling to the ground. Subsequently, we extend this view to the freezing level of a storm cloud – or more precisely, the area in between 0 degrees Celsius and -13 degrees Celsius, which is most productive for (large) hail. Blending all this information together, we’ll be able to answer that question mentioned earlier:

Why can hail stones be very large every now and then?

Back to physics: net force working on an object

Suppose that we have an object, such as a tennis ball. As we can see in any tennis game, it’s very much possible to move the ball. However, we’ll have to put effort into moving it:

  • If we want to serve, we’ll have to throw the ball into the air, before we can hit it.
  • If we want to let the game continue, the two (or four) players must hit the ball back-and-forth in order to pass it along.

In both cases, we’re applying a force to the ball – muscle based force, all the time. In the first scenario, we first use our muscles to throw the ball into the air, and subsequently use them to hit the ball with our tennis racket. The latter also happens when the game has started, and players hit the ball in that back-and-forth fashion we know tennis to be.

Did you know that another force is also working on the ball? In fact, we’re all too familiar with it: it’s gravity. Gravity means that two objects – the ball, and the Earth – attract each other, while the largest object exercises most force on the other object. The result: the ball will naturally fall towards Earth vertically.

If we don’t want that to happen, we’ll have to make sure that a net force works on the ball in the other direction. This muscular force we apply is therefore slightly stronger than gravity, which means that our ball will rise. Air resistance will slow it down and eventually equal forces and then let gravity be stronger again. The same goes for hitting the ball, which adds a horizontal force, ensuring that the ball is passed to the other side of the field.

Altogether, from this example, we know that any object moves into the direction where a net force is available. If it doesn’t move, it means that all forces working on the object are in equilibrium. Now, let’s take a look at what happens in clouds, so that we can apply our knowledge to see why hail stones can get very big sometimes.

Water droplets and freezing: hail formation

From our article about the updraft and downdraft of a thunderstorm, we know that convective cloud formation (and eventually, thunderstorm formation) can be set in motion because of heat from the sun, orography (i.e. mountainous terrain) or a dynamic trigger such as a weather front. What happens generally speaking is that air is forced to rise through these causes.

A beginning updraft. Photographer: Abdallah Maqboul, Pexels License.

Sometimes, and this always happens when a thunderstorm strikes, air has risen to the Level of Free Convection (LFC) – here, the upward force is no longer necessary for air to rise, because the cooling and rising air parcel will remain warmer than its environment for some time. Only when it reaches the Equilibrium Level, or EL, it will stop rising – and the cloud will grow until then. The top of any thunder cloud reflects where we can find this EL.

Now, as we’ve discussed in that other article as well, both the atmosphere and the rising air parcel will cool down, the first a bit faster than the second. This means that they will eventually reach 0 degrees Celsius (or 32 degrees Fahrenheit) – i.e., when water in liquid form will freeze into ice.

This also happens in clouds! Once water droplets reach the 0-degree-level, as we call it in the metric system, they start being supercooled. That is, while the temperature will remain below 0, it’ll remain a liquid. This happens because no particles are available onto which ice can form. However, as no cloud is sterile, some so-called nuclei are available for freezing to happen.

When a small hail stone has formed, it will move through the cloud because of the updraft pushing it upward. Recalling what we’ve learnt about the tennis game, the upward force of the updraft is stronger than gravity, so it remains in the cloud. While moving around, the hailstone can collide with other supercooled water droplets, which immediately freeze onto the hail stone. When this happens, a relatively transparent layer of ice forms. However, sometimes, the hailstone gets into an area of the cloud with primarily water vapor. Then, it’ll be a very opaque layer of ice that’s available. What fun, that you can distinguish cloud behavior from the structure of its hail!

Very turbulent updrafts ensure that hail remains in the cloud

Now that we know how hail forms, we still don’t know why some hail stones become very big, while others don’t.

However, funnily enough, we’re now really close to the answer – and I think that it’s likely that some people have already figured things out by now.

If you didn’t, don’t worry – because we’ll combine the facts that we’ve seen up to now into a coherent answer as to why large hail can occur.

Recall the storm updraft, which ensures that a storm is fed with hot and humid air and serves as the main energy source of the storm. In some cases, where so-called instability is very large and severe thunderclouds can grow, the upward motion is very strong. When this happens, hail stones that have formed will remain in the cloud for a very long amount of time – simply because the net upward force is stronger than gravity, which is also dependent on the mass of the object.

A supercell storm, with a clear incus cloud. Those clouds have very strong updrafts and can therefore produce large hail. Photographer: Greg Lundeen.

However, as we know by now, because the hail stone collides with supercooled water droplets, other water droplets and water vapor, it keeps growing and growing. When it does, its mass increases, and indeed – the strength of the gravity force becomes stronger. Eventually, gravity is stronger than the updraft – either because the storm dies off (by means of a blocked updraft) or because the thriving updraft is no longer strong enough to keep the hail afloat. Only then, hail falls down.

Now, because strong updrafts can keep hail in the sky for much longer, the hailstones can become very big. Because those strong updrafts are relatively rare, we see small hail much more often. However, when those situations do occur, hail can become very large.

A recap: why hail can become very large

In this article, we have answered the question why hail stones can become very large sometimes, but not always. We looked at this matter from three different angles. First, we looked at a game of tennis – and the forces that work on a tennis ball during the game. By doing so, we have understood how a net force working on an object will ensure that the object moves in that particular direction. It’s all a very nuanced balance!

Then, we looked at hail formation in general. Once convective clouds grow above the 0-degrees-Celcius level (or 32 degrees Fahrenheit), water will be supercooled up until approximately -13 degrees, when most water has turned into ice. In the area between those two levels, hailstones will form and have the natural tendency to fall towards Earth because of gravity. Only the updraft will keep them in the sky, because altogether, it produces a net upward force that makes sure the hail stays in the cloud.

When this happens, hailstones collide into water droplets and water vapor, allowing them to grow. Because they could eventually grow too large for the updraft to keep them in the sky, either because the updraft dies or the hail becomes too large, they will fall towards Earth. Because some updrafts are really strong, large hail can be produced – while many aren’t so strong, and we see small hailstones.

I hope you have learnt something from reading this article. If you did, I’d love to know – so please feel free to leave a comment in the comments section below! Please do the same if you have any questions, remarks or other comments. I’ll happily answer your question if possible. Thank you for reading MisterWeather and enjoy the weather today!


 Hail. (2002, January 20). Wikipedia, the free encyclopedia. Retrieved September 8, 2020, from https://en.wikipedia.org/wiki/Hail#Layer_nature_of_the_hailstones

Supercooling. (2003, July 31). Wikipedia, the free encyclopedia. Retrieved September 8, 2020, from https://en.wikipedia.org/wiki/Supercooling

How does a thunderstorm form? (2020, August 19). Mr. Weather. https://mister-weather.com/2020/08/19/how-does-a-thunderstorm-form/

What are the updraft and downdraft of a thunderstorm? (2020, September 7). Mr. Weather. https://mister-weather.com/2020/09/07/what-are-the-updraft-and-downdraft-of-a-thunderstorm/

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