DES MOINES, Iowa — Despite hot temperatures at the ground level, below-freezing temperatures can still be found thousands of feet in the air. This is where hail first develops. Dirt and dust collide with supercooled water droplets and freeze into a small piece of ice.

The strength of the updraft determines the size of the hailstone.

The wind we feel day to day, whether traveling east to west, north to south, or vice versa all moves in the horizontal plane. An updraft is wind that travels in the vertical plane. When you jump, you are also moving in the vertical plane.

The updraft can vary in strength, but the stronger it is, the bigger the hail can become. After a hail stone forms, it can grow by colliding with other rain drops within the thunderstorm.

We can reenact this with a hair dryer and a ping pong ball.
When the hair dryer is plugged in, turned on, and pointed so that the air is moving up (updraft), a ping pong ball (hail) placed above the hair dryer where the air is coming out will float up and down in the air when you let go.
But, imagine if we put a golf ball in the place of the ping pong ball.
The golf ball would be too heavy and fall to the ground.

This is what we see in thunderstorms, too. If an updraft is strong enough to support the weight of the growing hail, it will continue to grow, but as soon as the hail becomes too heavy, it falls to the ground.

Measuring Hail

When measuring hail, it’s important to use a point of reference. Measuring the diameter of the hail stone with a tape measure or ruler will tell exactly how large the hail is, but other items like coins or sports balls can serve as a good point of reference.

Note: Taking a picture of a hail stone in your hand or next to marbles is not a helpful description of how large a hail stone is, because hands and marbles can be different sizes.

Below you’ll see the different reference points and the diameter of these objects. For example, a quarter measures one inch even. That is also the threshold for severe thunderstorm warnings. Any storm with hail stones at or larger than 1.00″ in diameter is considered severe. A ping pong ball is 1.50″ wide while the slightly larger golf ball measures 1.75″. Baseball size hail is 2.75″ and a softball is 4.00″ wide.

The largest hail stone on record

On July 23, 2010 an 8.00″ hail stone fell from the sky in Vivian, South Dakota. The previous record for the largest hail stone was held by Aurora, Nebraska when a 7.00″ wide hail stone fell on June 22, 2003 That measures about the diameter of a basketball.

For more on the different types of extreme weather we see here in Iowa, click here.