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 July 02, 2019
Was Mexico's freak summer hail storm due to global warming?

 A hailstorm in Guadalajara on 30 June left some parts of the Mexican city covered in up to 2 metres of hail pellets, nearly burying cars.

No injuries were reported but two people were said to be showing signs of hypothermia. The city's usual temperature in June is 31C.

The incredible aftermath has left many wondering if global warming is to blame.

"I've never seen such scenes," the state governor, Enrique Alfaro, tweeted. "Then we ask ourselves if climate change is real."

Global warming at work?

Most studies looking at how global warming will affect hail have looked at the size of hailstones, rather than the overall mass of hail, because large hailstones do the most damage. They occasionally kill people and animals as well as doing serious damage to cars and buildings. For instance, 25 people were killed by a hailstorm in China in 2002.

These studies suggest hailstones will indeed get bigger as the world warms.

It is also possible that the mass of hail that falls in any one storm could increase due to warming. As the atmosphere warms, it holds more moisture. This means more water can fall out of the sky when conditions are right, in the form of rain, snow, hail or graupel (snow pellets).

"In a warmer future, increased amounts of moisture in the air can lead to heavier precipitation during an individual storm," says climate researcher Amulya Chevuturi at the University of Reading in the UK.

However, it's not clear that the mass of hail that fell in Guadalajara was that exceptional.

"It does appear from some of those pictures that floodwaters helped to pileup the hailstones into larger drifts," says Matthew Kumjian of Pennsylvania State University. "Some of the photos show much smaller accumulations in more open spaces."

Such pileups, or hail drifts, do occur occasionally, such as in Amarillo, Texas, in April 2012.

"We can't yet say anything about whether these storms are more or less likely owing to climate change," says Kumjian.

But he's not ruling out a link either. "We did find, as have other scientists, that these cases tend to form in environments with a large amount of 'precipitable water'," Kumjian says.