When I first moved to Colorado, one of the things that totally amazed me was the size of the hailstones that occasionally pounded the area. I also noticed that there are businesses which advertise repairing hail damage to vehicles. It dawned on me that, if businesses existed to address such damage, it must be a reasonably frequent occurrence. It is! Large marble sized hail is quite common, golf ball sized hail happens frequently, and baseball/cricket ball sized hail occurs a few times a year. There have even been reports of grapefruit sized hail.

     In the early evening of 21 July 2021, hailstones the size of golf balls pelted suddenly from the sky, smashing windows and battering cars. Gardens that were a few moments earlier filled with people soaking up the evening sun, were left badly damaged by the downpour of ice. AND THAT WAS IN ENGLAND. Even more unbelievable, in 2020, Tripoli, yes Tripoli, Libya, was struck by hailstones that measured nearly 18cm (7.1 inches) across. 7.1 inches. The heaviest hailstone ever recorded fell in Gopalganj district of Bangladesh in 1986, weighing 1.02kg (2.25lbs). The hailstorm killed 40 people and injured 400 others, according to reports at the time, but later reports suggest as many as 92 people may have lost their lives.

     Time to buy that crash-helmet! A hailstone measuring 4.83in (12cm) at its widest point was collected after a storm in Bethune, Colorado, US, in 2019 (Credit: National Weather Service, Goodland Forecast Office).

     Hail forms as droplets of water are carried upward into a thunderstorm. Up-draughts carry them into parts of the atmosphere where the air is cold enough to freeze the droplets. Moisture from the air accumulates on the outside of the drops of ice as it moves through the air, causing the hailstone to grow in onion-like layers.How fast a hailstone grows depends on the amount of moisture in the air. It will continue to grow until the up-draught is no longer strong enough to keep it aloft. A 103km/h (64mph) updraft supports hail the size of a golf ball, while one 27% faster can create hailstones the size of baseballs, according to the US National Oceanic and Atmospheric Administration, although as we will see in a moment, the size of a hailstone doesn’t always directly relate to its weight.      

     The temperature and the level of moisture in the air a hailstone forms in can influence how dense it is. In very cold air, water freezes as soon as it collides with the hailstone, but this can lead to a lot of air and being mixed with the ice. If the water freezes more slowly, perhaps because the air is warmer or the amount of moisture in the air is high, meaning not all of it freezes instantly, the air bubbles have time to escape. This leads to clear ice that tends to be denser. Small hailstones are only half as dense as pure ice, as they have a lot of air mixed in as they tend to move rapidly up through the atmosphere before falling again.

     As climate change alters the temperature of the Earth’s atmosphere, so too is the amount of moisture in the air. Warmer air can hold more water vapour while higher temperatures also mean more water is evaporated from the Earth’s surface. This is predicted to lead to heavier rainfall,more extreme storms, and bigger hail.

     Time to buy that crash-helmet! A hailstone measuring 4.83in (12cm) at its widest point was collected after a storm in Bethune, Colorado, US, in 2019 (Credit: National Weather Service, Goodland Forecast Office).

     Not everyone allows the sky to bombard them with hailstones without firing back, deluded though that reaction might be. An arsenal of cannon and rockets have been deployed over the years in an attempt to prevent damaging hailstorms. In the 1890s, vineyards across Europe deployed hail cannon, a device pointed into the sky to break up storm clouds by projecting the force of an explosion via a large horn. These cannon were intended to prevent grapes from being damaged by hail. In the 1900s, the Italian government carried out extensive research and concluded that hail cannon had no effect, a finding repeated by other researchers. Surprise, Surprise! How do they know what to aim at?

     Some European vineyards still use hail cannon!

     In the Republic of Georgia, the national Anti-Hail Service uses weather radar to track storm clouds and deploys silver-iodide bearing rockets against them to stave off hail. The government has installed 85 of the anti-hail missile systems and the service claims over 90% effectiveness, but the results have yet to be verified. Again, surprise, surprise!

     Finally, the speed at which a hailstone hits is not the same as its falling speed. For one thing, there may be a horizontal component – side winds can increase the impact speed of a hailstone compared to if it had hit in freefall. The most damaging hail events are downbursts, driven by powerful downdraughts – where air rapidly descends from storms and spread outwards when they hit the ground, producing very high wind speeds. Downbursts are typically only a few kilometres or miles across and last a matter of minutes, but can feature vertical wind speeds of 70-80m/s (156-179mph) with correspondingly destructive hail. Large hailstones travelling at these sorts of speeds have the power to punch through roof tiles, smash car windows and tear off cladding on buildings. They can devastate crops, injure people and animals. They pose a particular threat to aircraft. Now there’s a comforting thought!

     All of these factors put together mean that scaled-up hailstones can cause significantly more damage than expected, but there is hope. A 2021 study led by Leila Tolderlund at the University of Colorado highlighted the potential for green roofing as hail protection. This consists of a waterproof membrane with a thick layer of soil planted with vegetation. Green roofs provide insulation, reduce heat in summer and absorb CO2, but they also turn out to be excellent hail armour. The study found that in a simulated severe hailstorm, all the non-protected roof surfaces were damaged, while those with green roofing remained unharmed. It gives “Roof Gardens” a whole new level of importance. Maybe flat roofs are not such a stupid architectural idea afterall. 

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