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There is endless fascination with things that fall from the sky. The very term meteorology comes from the Greek word meteoron, referring to stuff high in the sky. Of that stuff, hailstones are among the more engaging things that drop on us.

I remember rushing out to collect them in one of the rare hailstorms we had during my boyhood in New Jersey, and my own children did the same in Illinois. It’s exciting to keep them in the freezer like a hoard of gemstones. If you crack them open, they have onion-like layers inside.

That’s the fun bit. But if hailstones as large as refrigerators came down, we would be in real trouble – especially since the bigger they are, the faster they fall. Doubling the size of a hailstone increases its damage-producing energy by a factor of roughly 16. Which is why it’s important for us to know how global warming might influence this kind of weather.

As it is, hail cannot get that big on our planet (), but it can still do plenty of damage. The great hailstorm of 1788 is . A dropped baseball-sized hail and caused damage to homes and cars equivalent to more than $1 billion dollars today. Those caught without shelter risk life and limb: hail killed 25 people and injured 200 during a violent storm in China’s Henan province in 2002.

How hail is made

Global warming changes everything about our climate, and that includes hail. You might think that in a warmer world it would be harder to make great balls of ice, but that is not the case. One clue is that severe hailstorms mostly occur in the heat of summer.

To understand the impact of rising temperatures, we have to understand how hail is made. First you need water, and warmer air can hold more water. Second, you need to condense the water vapour and freeze it; this can happen even in summer because air temperature falls with height (on this factor, global warming inhibits hail, because the freezing level is higher).

Third, you need thunderstorms with strong enough updrafts to keep hail suspended while it grows; it takes an 8-metre-per-second updraft to grow hail the size of a golf ball while 10 metres per second will give you ice the size of a cricket ball. Global warming favours larger hail because the warmer, wetter air results in greater condensation of water vapour in the lower atmosphere, which releases extra energy to strengthen thunderstorm updrafts.

Integrating all these factors is a challenge, but a group of Canadian researchers has just published a study with this aim ().

Heavy weather

Using high-resolution models of anthropogenic climate change over North America and models capturing the detailed physics of hail formation, it forecasts changes in hailstorm properties over the coming century.

The key finding is an increase in the fraction of severe storms involving large hailstones. Some southerly regions may see a reduction in hail damage due to the major hail belts shifting north and, overall, the number of hail days will be reduced.

But taken together, these results predict a 40 per cent increase in damage potential over North America. This is in line with an emerging theme of human-caused climate disruption: the average amount of rainfall doesn’t increase too much, but it becomes increasingly concentrated in a few severe events (leaving other areas in drought).

This is unlikely to be the last word on hail, nor is it the worst or most certain consequence of climate disruption – for that, my money is on more frequent killer heatwaves.

Nonetheless, together with other studies of its type, this paints in more worrying details on the blurry canvas depicting the climate our descendants will have to live with unless we can curb emissions quickly.