When you see hail fall from the sky, those chunks of sky ice can vary in size.
In order to understand what causes the difference in hail size, you have to first understand how hail is formed.
Hail is precipitation that is formed when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere and freeze.
The National Weather Service says once a hailstone starts to form, it continues growing as it collides with super-cooled water droplets that freeze on contact.
If the water freezes instantaneously when colliding with the hailstone, cloudy ice will form as air bubbles will be trapped in the newly formed ice. However, if the water freezes slowly, the air bubbles can escape and the new ice will be clear.
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The longer the hailstone stays suspended in the top of the cloud colliding with water droplets, the larger the hail will become.
The stronger the updraft, the longer the hail stays in the cloud and the more water droplets freeze onto it. The hail will then fall when it becomes too heavy to remain in the cloud or when the updraft weakens.
So once hail begins to fall to the ground, it can range in size from that of a pea to a baseball to a grapefruit.
Hail size is often estimated by comparing it to a known object. Most hailstorms are made up of a mix of different sizes, and only the very largest hailstones pose a serious risk to people caught in the open.
According to the NWS, when reporting hail, estimates comparing the hail to a known object with definite size are good, but measurements using a ruler, calipers, or a tape measure are best:
- Pea = 1/4 inch diameter
- Mothball = 1/2 inch diameter
- Penny = 3/4 inch diameter
- Nickel = 7/8 inch
- Quarter = 1 inch — hail quarter size or larger is considered severe
- Ping-Pong Ball = 1 1/2 inch
- Golf Ball = 1 3/4 inches
- Tennis Ball = 2 1/2 inches
- Baseball = 2 3/4 inches
- Tea cup = 3 inches
- Softball = 4 inches
- Grapefruit = 4 1/2 inches