By Ray Petelin

PITTSBURGH (KDKA) — Now that it is officially spring, we are getting into the time of year that features thunderstorms.

Some of them are nasty, with strong winds and big hail. The two pretty much go “hand-in-hand”.

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To make hail, you need to get a lot of moisture up high into the atmosphere.

We all know warm air rises, so that is a start, but you need more lift to get hail in the atmosphere — a lot more lift.

That is where strong, upper atmospheric winds come in.

As winds blow at high speeds high in the sky, it creates low pressure at surface, or ground level.

That low pressure creates an upward wind, or updraft, that lifts moisture up into the sky.

To show how this works, here is a bowl of paper towel balls, which will represent moisture with a plastic tube which represents a column of the atmosphere.

when I use my air compressor to blow a high-speed stream of air across the top, you can see the updraft that occurs, lifting the paper towel balls up.

That is just step one.

The second step is that updraft needs to be strong enough to push that moisture high enough where it hits freezing temperatures.

This causes that moisture to freeze.

The updrafts cause that ice to bounce up and down, creating additional layers of ice.

The updrafts hold this ice in the air thanks to what is called the Bernoulli Principle, named after the scientist who came up with it.

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It is pretty simple, too. It states that when a fluid that is moving, like air, comes into contact with a constriction the velocity increases, causing a decrease in pressure.

This is one of of those things that is easier to show.

With a hair dryer turned on, when you place the ping pong ball into the stream of air, it seems to float in place.

If you did not know the Bernoulli Principle, you might think the air from the hair dryer cancels the force of gravity, allowing the ball to seemingly float.

That, though, is not the case.

As the stream of air from the hair dryer hits the bottom of the ping-pong ball, the speed of the air increases to get around the sides of the ball.

As that velocity goes up, the pressure above the ball goes down, causing low pressure above the ball.

This means the ball is actually being lifted.

To prove this, you can tilt the hair dryer, steering the ball.

You can do this, until the force of gravity is too strong, causing the ball to fall to the ground.

The same is true with hail.

It will bobble around in the sky, growing bigger and bigger until the hail is too big and heavy, or until the updraft stops.

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