The strength of what meteorologists call the updraft in a thunderstorm is what determines the size.
A storm forms at a place in the atmosphere where the air is moving up very rapidly.
When the air cools in the cold upper atmosphere, its water vapor condenses into a storm cloud.
Eventually precipitation forms in the cloud, first as a snowflake, then as a raindrop as it falls.
If this raindrop is caught in the updraft once more, it moves up past the freezing level again and becomes a little ball of ice.
It then takes on added ice from droplets in its environment. Then, when it is heavy enough, it falls again, perhaps to be caught up once again in the turbulent motion of the air.
With each trip up and down, the hailstone adds material.
A fresh hailstone can actually be sliced to reveal layers like a tree ring, showing how many trips it has made.
An updraft of around 100 miles an hour is needed to support hailstones of 5 inches in diameter or more. Areas of great air turbulence thus give birth to the largest hailstones.
The hail capital of the United States is Wyoming, especially the southeastern part, where a convergence of dry air currents from the mountains to the south and cold air currents from southeastern Wyoming conspires to produce remarkable hail.
It is possible for hail to reach the size of a grapefruit. This grapefruit is not necessarily perfectly round, but may have strange protuberances.
One notable hailstone that fell at Coffeyville, Kansas, in 1979 weighed 1.67 pounds and was about 7 1/2 inches in diameter.
Hail forms only in thunderstorms, which arise in part from air currents generated by the heat of the summer sun.