Last Tuesday, a severe ice storm in Catalonia, northern Spain, caused the death of a 20-month-old girl, who succumbed to injuries when a large piece of ice fell on her head.
The diameter of some snowflakes, in that storm, reached 10 centimeters.
The storm lasted in the Girona region, for a little more than 10 minutes, but left dozens of people injured. So why are snowstorms becoming more frequent than ever?
BBC Future’s David Hambling wrote an article published in March this year explaining why:
On the evening of July 21, 2021, snowballs the size of golf balls suddenly fell from the sky over Leicestershire, central England, smashing windows and cars.
And the gardens, which moments before the hail were packed with people enjoying the sun before sunset, were badly damaged by the heavy snowfall.
While the blizzard caused by the rising air current, which leads to the formation of high clouds in the atmosphere, was unusual in its intensity, it was moderate compared to the storm that hit Calgary, Canada, in June 2020.
The tennis ball-sized hail damaged at least 70,000 homes and vehicles, destroyed crops and caused $940 million in damage to the area.
The hailstorm, which lasted for 20 minutes, was one of the worst weather conditions to take its toll on the country.
Climate change is beginning to alter the pattern of hailstorms. In the US states of Texas, Colorado and Alabama, hailstones broke the record in the last three years and reached 16 cm in diameter. In 2020, a hail storm hit Tripoli, the capital of Libya, with beads about 18 cm in diameter.
But why might global warming increase the amount of ice that falls from the sky? Are there any limitations on the size of hailstones?
Weight, volume and speed
Snow forms when water droplets rise during thunderstorms. The updraft carries them to parts of the atmosphere where the air is cold enough to freeze the droplets. Moisture in the air collects on the outside of the ice droplets as they move through the air, causing the hailstones to grow into layered balls like onions.
The speed of the growth of snowflakes depends on the amount of moisture in the air. It continues to grow until the rising air current weakens, and it is no longer able to keep it high in the atmosphere.
An air current rising at 103 km/h can carry snowballs the size of a golf ball, while a 27% faster current can carry chunks the size of a baseball, according to the US National Oceanic and Atmospheric Administration (although we’ll soon will see that the size of these pieces is not always directly related to its weight).
Air with higher humidity and stronger updrafts bring in larger pieces. Large chunks usually fall close to the updraft while smaller hailstones fall away from it, often blown there by crosswinds.
“Destructive storms that produce chunks of ice larger than 25 mm in diameter require a specific set of conditions,” said Julian Primelo, a physical sciences specialist at the Canadian government’s Department of Environment and Climate Change, who has studied how climate change affects snow formation.
It requires sufficient moisture, strong updrafts, and a “stimulating factor”, usually an air front.
This is why dangerous ice storms are usually limited to certain areas such as the Great Plains in the United States and the Gold Coast in Australia.
The air in these areas is usually cool, dry in the upper atmosphere above the warm, moist air at the surface. This unstable situation leads to strong updrafts and thunderstorms.
These locations are particularly vulnerable to a type of thunderstorm known as a supersled, which can produce very large ice shards due to the intense rotating updrafts they produce.
But as climate change changes the temperature of the Earth’s atmosphere, so does the amount of moisture in the air.
Warmer air can hold more water vapor, while higher temperatures also mean more water evaporates from the Earth’s surface.
It is expected to bring heavy rain and more severe storms in some parts of the world.
“As the planet continues to warm, the areas most likely to experience blizzards are likely to change, and an area where the lack of sufficient moisture is currently a limiting factor may become wetter later,” says Primelo. may become more frequent.”
A combination of monitoring climate changes already occurring and modeling the expected changes led the researchers to conclude that ice storms will become more frequent in Australia and Europe, but that there will be a decrease in East Asia and North America will be.
But they also found that ice storms would generally become more intense.
Although these storms may occur less frequently in North America, the snowpack is likely to increase in size as it falls, according to a separate study by Primello and colleagues focused on how conditions in North America will change in a warmer world.
One reason for this is that the height at which the chunks begin to melt will increase as they fall, so the smaller chunks will melt and turn into rain before reaching the ground, but the larger chunks will move through the warm region so quickly that melting will not occur. not have much effect on them.
“We’ve already seen evidence of this, with ice sheet data in France indicating a shift in its size distribution,” says Primillo.
Icicles are clumps of smooth ore left in storms and deformed when they hit the ice, giving an idea of the size and number of snowflakes in the area.
This could mean that annual damage from ice could also increase. But it’s hard to say which areas will see an increase, according to Primelo.
The temperature and humidity of the air in which hailstones form can affect their density.
In very cold air, the water freezes as soon as it hits the hail, but this can result in a lot of air and ice mixing. If water freezes more slowly, perhaps because the air is warmer or because the amount of moisture in the air is high, meaning it doesn’t freeze completely right away, the air bubbles have time to escape.
This results in clear ice that tends to be denser. Small hailstones are half the density of clear ice, and there is a lot of air in them because they tend to move quickly through the atmosphere before falling again.
The largest hailstones often consist of a complex mixture of layers of ice that form as they move through the air column.
Looking at a cross-section of the ice can reveal a lot about how it formed, while the ice-like structures on the outside of a hailstone also give clues about how it circulates in a storm.
The density of a hailstone also affects how much it grows, the heavier it is, the more likely it is to fall from the rising current. It will also fall faster because the larger the hailstone, the less resistance it will have per unit weight.
The heaviest hailstone ever fell was recorded in Gopalganj district of Bangladesh in 1986, and it weighed 1.02 kg.
The hail storm killed 40 people and injured 400 others, according to media reports at the time, but later reports indicated the death of about 92 people.
But how big can hailstones grow?
Pennsylvania State University meteorologist Matthew Kumjian estimates that the largest hailstone could be 27 centimeters in diameter, larger than a football, based on simulated weather data.
However, nothing this large has yet been recorded, and he says he is working with some colleagues to improve the accuracy of the estimates. While 27 cm is the upper end of the estimates, pellets in these proportions would be highly irregular in shape.
But he says the ingredients needed to make such a large hailstone — strong updrafts, lots of supercooled liquid water, and a long time of movement in the cold air — are now in place.
“Powerful thunderstorms that produce the world’s largest hail contain many of these components together, so perhaps the most powerful of these storms could produce giant hail today.”