When the mercury rises in the UAE during the summer, the last things residents expect to see falling from the sky are hailstones – but they have appeared this week.
Social media users in Al Ain have posted videos of hailstones that appear to be sized somewhere between a pea and a golf ball.
The sudden flurry created an unlikely winter wonderland at the height of another sizzling summer in the UAE desert.
It came amid a spell of cloudy weather and rain that primarily affected Abu Dhabi and Al Ain.
Why are we seeing hail in the summer and why are updrafts important?
Crucial to the formation of hail are updrafts of air, which keep ice particles suspended in the atmosphere and allow them to grow into hailstones before they fall to Earth.
Surprisingly, the conditions that are ideal for hailstone formation are more likely to be seen in summer in the UAE.
“With warmer temperatures, the updrafts are stronger and sustained in time,” said Dr Diana Francis, an assistant professor and head of the Environmental and Geophysical Science (Engeos) Lab at Khalifa University in Abu Dhabi.
“That’s why the UAE witnesses hailstones when convective clouds occur during summer, when the temperatures are very high.
“The warm air naturally rises up, which fuels and maintains strong updrafts within the convective clouds.”
David Schultz, a professor of synoptic meteorology at the University of Manchester in the UK, said a number of factors, including a weather front or a sea breeze, could create an updraft capable of keeping ice particles in the atmosphere to form hail.
“What you need to start that updraft is usually a convergence of air coming from various locations. There are a number of ways that could happen,” he said.
The terrain could also cause an updraft, he said, with mountains able to cause the air movement required.
How do hailstones form and grow?
According to US space agency Nasa, the starting point for hailstones can be aggregated ice particles, known as graupel, or frozen raindrops.
The water exists in frozen form because temperatures 5km or 6km up in the atmosphere are below freezing, said Prof Schultz.
“Any cloud producing rain or snow at the surface of the middle or high latitudes … will start out as ice or snow,” he said.
“The conditions that form hail – you will have very strong updrafts into these clouds.”
“You can imagine that [hailstones] several centimetres across require even stronger [updrafts] in the atmosphere to keep [them] suspended in the air,” Prof Schultz said.
Dr Francis said that large hailstones were the result of several down-and-up movements of the hail that had initially formed within a cloud.
“They get tossed back up to the top of the storm by another updraft,” Dr Francis said. “Each trip above and below freezing adds another layer of ice until the hail becomes heavy enough to fall down to the surface.”
Nasa notes that each hailstone can experience both dry growth, when their surface remains dry, or wet growth, when a water-ice mesh forms. The combination creates a layered internal structure.
Why don't hailstones melt as they fall to Earth?
Eventually the hailstones become so large and heavy that the updrafts cannot keep them suspended any longer and they fall to Earth.
“Because they're falling quite rapidly, several metres per second, they can fall relatively quickly before they have time to melt,” Prof Schultz said.
He said hailstones could fall even in equatorial regions, as long as there was a strong enough updraft and they fell too fast to melt on the way down.
According to Nasa, hailstones are typically about 1cm in diameter, but some as large as 15cm in diameter have been recorded.
