Eric Bieber, age 11, of Mundelein, Illinois, for his question:

 

Why are some hailstones bigger than others?

A record sized hailstone may measure 1.7 inches around the waist and weigh 1 1/2 pounds. When a shower of such icy ammunition hits the earth it can do enormous damage to crops and even buildings. Animals and people may be injured or even killed. Fortunately, such giant hailstones are rare. Most of them are much smaller. However, even these can ruin crops when great numbers pelt down with great force.
The details of hailstone creation are somewhat mysterious, though meteorologists have a general idea how they are formed. However, they are fairly certain why they come in different sizes. It seems that the larger pellets need the strongest updrafts, massive concentrations of supercooled moisture and a very long fall from perhaps ten miles.
You might expect a fallen hailstone to be a solid pellet of hard ice. But if you slice it in half, you see that it is built in onion skin layers. Usually the inside has several layers of hard, clear ice alternating with layers of half frozen mush. This structure suggests that it grew by adding new jackets in different air masses.
Hailstones form in the cumulonimbus cloud we call a thunderhead. This turbulent storm provides contrasting pockets of warm, cool and supercooled air, plus moisture and wild winds whirling in all directions. Its base is held aloft on warm currents of air rising from the heated ground. Its supercooled top may reach up six to ten miles. There the air is below freezing.
Most meteorologists suspect that hailstones most likely start to form at or near the top of a thunderhead. Perhaps they form as small ice crystals, suitable to act as nuclei to gather more frozen or partly frozen moisture. As they fall, they pass through contrasting layers of air, adding jackets of hard ice and soggy mush. Lower in the cloud, they encounter updrafts. But if these rising currents are not too strong, they break through and crash down to the ground as small or medium sized hailstones.
However, sometimes the updrafts are strong enough to keep medium hailstones from falling. They even may whisk them aloft for another whirl through the cloud, perhaps all the way to the top. More icy jackets are added and these hailstones become giants. At last they gain enough weight to break through the rising updraft and come plummeting down.
The size of a hailstone depends upon the amount of frozen moisture built around it. This most likely depends on how long it stayed aloft in the cloud. If it starts falling from ten miles and a very strong updraft holds it aloft, it is likely to stay in the cloud a long time. There it gathers more and still more frozen moisture. When hailstones falling from great heights are held aloft a long time, they are likely to be whoppers.