A fierce hail storm that fell on Tuesday afternoon in Catalonia, Spain, caused the death of a baby of 20 months to caused by a head injury caused by the impact of an ice ball.
Hailstones from the storm -some of which were up to 10 cm in diameter- destroyed windows, roofs and power lines.
The phenomenon that occurred in the province of Girona -which lasted for a little more than minutes- it also left dozens of wounded.
This art This article by David Hambling of BBC Future, published in March this year, investigates why hail storms will become a more frequently seen phenomenon.
The night of 21 July 2021, hailstones the size of golf balls fell suddenly from the sky in Leicestershire, UK, smashing windows and hitting cars.
Although the This hail storm, caused by strong updrafts of clouds high in the atmosphere, was unusual in its severity, being mild compared to another that hit Calgary, Canada, in June 92.
And it is that climate change is altering the pattern of hail storms. In Texas, Colorado and Alabama, in the United States, the record for the largest hailstones was broken in the last three years, with ice balls reaching 10 cm. In 2020 in Tripoli, the capital of Libya, hail fell up to 18 cm.
But why might global warming be causing an increase in the amount of ice falling from the sky? And is there a limit to the size a stone can reach?
Weight, size and speed
Hail forms when water droplets are carried upwards in a thunderstorm. Updrafts carry them to parts of the atmosphere where the air is cold enough to freeze the droplets.
Moisture in the air accumulates on the outside of ice droplets as they move through the air, causing hail to grow in layers like an onion.
How fast a hailstone grows depends on the amount of moisture in the air. It will continue to grow until the updraft is no longer strong enough to hold it in the air.
An updraft of 103 km/h supports hail the size of a golf ball while a stream a 20% faster can create hail the size of baseballs, according to the US National Oceanic and Atmospheric Administration. (although, as we’ll see in a moment, the size of a hailstone is not always directly related to its weight).
More moist air and more powerful updrafts will bring larger hail. Larger hailstones will often fall closer to the updraft, while smaller hailstones will fall farther away, blown there by crosswinds.
Trigger
Destructive storms that produce hail of more than 25 mm of diameter require a specific set of conditions, says Julian Brimelow, a physical scientist at Environment and Climate Change Canada, a Canadian government department that has studied how climate change affects hail formation.
They require sufficient humidity, powerful updrafts and a “trigger factor”, usually a weather front.
This is why severe hail storms are usually limited to particular regions, such as the Great Plains in the US and the Gold Coast of Australia.
These regions generally have cold, dry air in the upper atmosphere above the surface air Warm and wet. This unstable situation causes strong updrafts and the formation of thunderstorms.
These places are particularly prone to a type of thunderstorm known as supercells (or supercells), which can generate very large hail due to the powerful rotating updrafts they create.
But as climate change alters the temperature of Earth’s atmosphere, it also changes the amount of moisture in the air. Warmer air can hold more water vapour, while higher temperatures also mean more water evaporates from the Earth’s surface.
This is thought to lead to more intense rainfall and more extreme storms in some parts of the world.
“As the planet continues to warm, the areas conducive to hailstorms are likely to change,” says Brimelow. “An area now where sufficient moisture is a limiting factor may become wetter and consequently the frequency of hail storms may increase.”
Greater intensity
A combination of observations of changes that are already taking place and climate models has led researchers to conclude that storms hail will be more frequent in Australia and Europe, but there will be a decrease in East Asia and North America.
But they also found that hail storms will generally become more intense.
And while hail storms may become less frequent in North America, it is likely that the gr hail may be larger, according to a separate study by Brimelow and colleagues that looked at how hail conditions in North America might change in a warmer world.
A One of the reasons is that the height at which hailstones start to melt as they fall will increase, so small hailstones will melt and turn into rain before they hit the ground, but larger stones will pass through too quickly. warm enough to melt.
“In fact, we have already seen evidence of this, with hail data in France indicating a change in hail size distribution,” says Brimelow.
Hail pads are blocks of soft material that are placed outside during storms and deform when impacted by hail, allowing a record of the size and quantity of hail to be obtained. the area.
“Fewer days with small hail have been observed with warming, but there have been more days with larger hail.”
Density
But pinpointing exactly which areas will see the most hail damage is difficult, says Brimelow.
The temperature and humidity level in the air in which hail forms can influence its density.
In very cold air, water freezes as soon as it hits hail , but this can create a lot of air and mix 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 it doesn’t all freeze instantly – the air bubbles have time to escape.
This causes a clear ice to form which tends to be denser. Small hailstones are only half as dense as pure ice, as they have a lot of air mixed in with them and tend to move rapidly through the atmosphere before falling back.
Larger hailstones are They often consist of a complex mixture of ice sheets that form as they move in the air column.
Look at a cross section of the The ice can reveal a lot about how it formed, while the lobe-like structures on the outside of the hail also provide clues as to how it might have been spinning during the storm.
Size
Hail density also affects its size. The heavier it is, the more likely it is to topple over due to an updraft. And it will also fall faster, because the bigger a hail is, the less drag it experiences per unit of weight.
The heaviest hailstone ever recorded fell in the Gopalganj district of Bangladesh in 1986, with a weight of 1, 02 kg. The storm killed 30 people and injured others 40, according to reports at the time, but later reports suggest that until 92 people could have lost their lives due to this incident.
But how big can a hailstone get?
The maximum possible
Matthew Kumjian, meteorologist at the State University of Pennsylvania, in the USA, estimates that the largest hail could measure 27 cm wide (“the size of a bowling ball”), depending on models of simulation.
Nothing that big has been recorded yet, however, and he says he’s working with some colleagues to refine the estimate. Yes ok 27 cm is at the high end of the estimates, a hailstone of those proportions would be very irregular in shape.
But he explains that the ingredients needed to create such a large hail (strong updrafts, lots of supercooled liquid water, and lots of time to travel in cold air) exist in the today.
“The strong storms of ‘supercell s’ that produce the largest hailstones in the world already have many of these ingredients together, so the strongest of these storms today is probably capable of producing a supergiant stone”.
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