A rare hailstorm fell over Tseung Kwan O yesterday afternoon, with online videos and photos shared by residents showing the area being pounded by one-centimeter balls of ice.
The phenomenon was caused by high temperature and unsettled weather, the observatory said.
Amid thunderstorms, a swimmer in his 60s was fatally struck by lightning while trying to seek help from a passing boat near Green Egg Island in Clear Water Bay.
The observatory hoisted the thunderstorm warning at 11.56am yesterday and at 1pm said rainfall would be heavier over parts of the city due to the influence of showers and thunderstorms triggered by high temperatures.
Over the past 20 years, the observatory has recorded 11 reports of hail.
Online users posted photos of the hailstorm, with some saying they initially thought naughty children were merely throwing ice out windows. Some said: "No need to pay an extra HK$2 for an iced drink." But another said the hailstones were "larger than the bubbles in bubble tea."
Some Tseung Kwan O residents, however, were worried if the hailstones would break windows.
The observatory said more than 3,000 lightning strikes were recorded over Hong Kong from noon to 3pm, with around 95 percent of them recorded in the New Territories.
"Rain was particularly heavy in Tseung Kwan O in Sai Kung district, where the highest hourly rainfall of 91 millimeters was recorded," the observatory said.
It warned it may cause serious flooding but there were no reports of such.
The heavy rain and thunderstorm caught many off guard, including the man who was struck by lightning. Police said they received multiple reports from kayakers at around 2pm that a man was struck by lightning while swimming near Green Egg Island.
The man and his wife were canoeing and they took shelter on the island, but he was struck by lightning when he swam to seek help from a passing boat.
A police marine patrol vessel responded to the scene and lifted the unconscious man from the water. He was then airlifted to Pamela Youde Nethersole Eastern Hospital in Chai Wan by a Government Flying Service helicopter.
He was rushed to the emergency ward on a stretcher, but was declared dead shortly after.
The last time Hong Kong recorded hail was in July 2016. This was the third time hail hit the city in September - the other times were in 1983 and 2002.
Hail is more likely to be seen in March, April and July. The highest record was in March 1983, when Hong Kong recorded four hailstorms in a month.
The former director of the observatory, Shum Chi-ming, shared an online video of hailstones hitting a playground in Po Lam. "What a pity I didn't witness it in person," he said.