On a hot summer day it might be the last thing you expect, but it happens almost every year: hail. Tuesday is also predicted to hit. The KNMI even warns locally for hailstones up to two centimeters in size.
In Luyksgestel, Someren and Asten, one specific hailstorm is etched on the retina. In June 2016, stones the size of tennis balls wreaked havoc. Houses, cars and greenhouses were destroyed. The KNMI also warns for large hailstones for this Tuesday. But how do those ice balls actually arise, when it is more than 25 degrees on the ground?
“Hail requires cold,” says weatherman Johnny Willemsen of Weerplaza. That sounds contradictory with the current temperatures. “But high in the air it is also very cold in the summer,” he explains. Ice can therefore always form, but if there is a thunderstorm in the air, the chance is much higher. “From 3700 meters it already freezes. Storm clouds are about 13 kilometers high.”
High in the air
The origin of hail is a fairly technical story. In a rain cloud, air flows in circles: cold air flows down and warm air rises. Hailstones form at the top of the cloud, where it is coldest. From there, air currents take them down again.
Arrived lower in the cloud, it is a lot warmer and so the hailstones melt a bit. Meanwhile, warm air takes them up again. “Back in the freezing cold, they clump together and freeze together,” says Willemsen. “So the hailstones get a lot bigger. Then they keep moving back and forth in the cloud until they become too heavy and come down.
Larger stones in the summer
“In winter, hailstones don’t get much bigger than a centimeter,” says Willemsen. “In the summer there is a lot of heat coming from the earth. So there is a lot of warm air to push the hailstones back up. Because the air current up is stronger, the hail stays in the cloud longer. So they have more time to grow before they fall.”
For example, hailstones become much larger in the summer, up to ten centimeters. Once they fall to the ground, they melt because of the heat. But now they are already so big that they can still do a lot of damage.
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