How Hail Forms in a Thunderstorm

Everyone watches thunderstorms and they are fascinating to watch. But along with thunderstorms there can be hail. Hailstorms can be destructive, cause injuries and in rare cases even cause death. The larger or taller a thunderstorm is, the more chance there will be large hail. But not all thunderstorms cause hail. Here is how hail forms in those pretty billowing thunderheads.

Thunderstorm clouds can be beautiful to watch, but along with thunderstorms there can also be hail. Hailstorms can cause a great deal of damage, injuries and in rare cases even death.

Hail can range from pea size to hail the size of grapefruits. The larger the hail, the larger the thunderstorm is that forms the hail. This is how hail forms inside of those beautiful thunderheads.

Thunderheads, Cumulonimbus Clouds and Hail

A thunderhead starts out as a cumulus cloud, those billowy pretty clouds. If conditions are right, this cumulus cloud will start to billow and expand upwards; these are called towering cumulus and usually become a thunderstorm.  If the towering cumulus cloud continues to grow until the top reaches the upper level winds, it then becomes a cumulonimbus cloud and is usually a strong or severe thunderstorm. This is when you see the anvil shape at the top of the thunderhead. The anvil shape is caused by strong upper level winds blowing the top of the thunderhead or towering cumulus.

A cumulonimbus cloud can rise as high as 50,000 feet or more above the ground. So high that commercial jets have to fly around them. The higher these cumulonimbus clouds or thunderheads are, the more severe a thunderstorm will be and the larger the hail is likely to be.
Towering Cumulus Cloud            Photo by Tim/Flickr 

How Hail Forms in a Thunderstorm

You can watch the towering cumulus thunderhead billow and grow upwards because of the updrafts within the thunderhead. It is these same updrafts that form hail and cause the hail to become larger and larger.

A hailstone starts out as a droplet of water. The updrafts carry the water droplets to the top of the thunderhead where the temperature is much colder than at the base of the cloud and the water droplets freeze.

These frozen water droplets then fall back down toward the base of the thunderstorm and the updraft will once again blow this frozen water droplet upwards to the top of the cloud. As this hailstone moves up and down in the towering cumulus or cumulonimbus cloud, it picks up more supercooled water that freezes onto the hailstone making it grow in size.

This process will continue over and over again until the hailstone becomes too heavy for the updraft to carry it upwards and the hailstone will fall to the ground. The larger the cumulonimbus cloud, the stronger the updraft. And the stronger the updraft, the more times a hailstone will be carried up and down inside of the thunderstorm freezing more layers on the hailstone, making the hailstone bigger.

Updrafts and the Largest Hailstone

Updrafts in a severe thunderstorm can reach speeds as high as 103 mph (166 km/h). An updraft this strong would cause the hail to go up and down many times in the thunderstorm resulting in hail the size of softballs. Marble size hail would have an updraft of 35 mph (56 km/h) and golf ball size hail would have an updraft of 64 mph (103 km/h).

If you get a large hailstone and can cut it open, you will see rings of ice, since each time the hail is carried upwards by the updrafts, a new layer of ice is formed on the hail. Each ring indicates an up and down trip in the cumulonimbus cloud. Just don’t run out in the middle of a hailstorm to get a piece of hail. You could be hit by hail or lightning.

The largest hailstone on record in the United States occurred on July 23, 2010 in Vivian, South Dakota. It weighed 1.9375 pounds (0.8788 kg) with a diameter of 8 inches (20.32 cm) and a circumference of 18.5 inches (46.99 cm).
Largest hailstone from Vivian, SD

Where Hail Occurs   

Hail can happen anywhere, but is most prevalent in certain areas. China, India and Australia are known to have severe hailstorms. In the United States where Colorado, Nebraska and Wyoming meet is known as “Hail Alley” having the most hailstorms with Cheyenne, Wyoming averaging the most at 10 hailstorms per year.

Not all thunderstorms cause hail; this is because of the freezing level in the thunderstorm and the ground elevation. Hail can melt quickly as it falls, the lower in the thunderstorm the freezing level is, the more chance it will hail. This is why the eastern slope of the Rocky Mountains and High Plains get more hail than Florida.

Worst Hailstorms in the United States

Hailstorms cause a lot of damage to crops, livestock, gardens, businesses, homes and cars. Hail can also cause injury and in rare cases even death. On May 13, 1939, a farmer in Lubbock, Texas was on his tractor in the field when a hailstone struck and killed him.

On July 30, 1979 in Fort Collins, Colorado, hail the size of baseballs fell as a mother was holding her three month old baby when the baby was hit in the head by one of these large hailstones and killed. Many others were injured by this same hailstorm.

The last person to be killed in the United States by hail was on March 28, 2000 in Lake Worth, Texas, when a 19 year old man was hit in the head by softball size hail. He was running to move his car under cover.

When you see a list of the worst hail storms in the US, it is usually measured in terms of damage in dollars. Hail can also cause flash flooding when the hail drifts and blocks storm drains causing deaths. The costliest hailstorms in recent years (in 2010 dollars) include:

April 10, 2001: A hailstorm in St. Louis, Missouri cost $2.4 billion.

May 5, 1995: Fort Worth, Texas, known as the Mayfest hailstorm caused $2 billion in damage and injured at least 100 people. The storm hit when thousands of people were in downtown Fort Worth for the annual Mayfest celebration.

July 11, 1990: Denver, Colorado, known as the 7-11 Storm, caused $1.6 billion in damage and injured 50 people who were stranded on rides at an amusement park when the power went out. While stuck on the rides, they were hit with golf ball to baseball size hail.

Forecasting Hail

You can look at a thunderstorm and at times forecast if there will be hail. The base of the thunderstorm will appear to have white bumps or white ridges, at sunset these ridges can be orange. At times there might be a greenish or turquoise color in the clouds.

Looking at a thunderstorm, you will often see a rain shaft, those dark lines moving from the base of the thunderstorm to the ground. If these rain shaft lines are white in color, it is a good chance those are hail shafts and it is hailing where you are looking.

Copyright © 2014 Sam Montana

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