SINGAPORE - Residents in the west of Singapore scurried for cover on Tuesday as a rare hailstorm uprooted trees and disrupted traffic.
The phenomenon - which was last reported here in 2008 - may or may not be linked to haze, said the National Environment Agency (NEA) and weather experts.
But it is unlikely to have been triggered by cloud seeding in Indonesia to extinguish fires in Riau, they said, as the seeding took place hundreds of kilometres away, and hail is a local phenomenon.
"Hail is produced by sizeable cumulonimbus clouds located in very high altitude where temperature is very low," said atmosphere scientist Erwin Mulyana of Indonesia's Agency for the Assessment and Application of Technology.
"Above Riau, the cumulonimbus clouds are thin and small, and are not at a high altitude."
Both the NEA and external experts did not rule out the possibility that fine haze particles in the air had helped to produce hail.
Weather researcher Matthias Roth, a National University of Singapore associate professor of geography, said haze particles could have acted as tiny nuclei for water to condense and freeze around.
"But I cannot say for sure - cloud microphysics is a very complicated scientific field," he said.
Tuesday's storm was the first since the haze episode began last week, and was accompanied by heavy rain and powerful winds.
The NEA said there may be more afternoon thunderstorms in the next few days. It warned that while the rain may have washed coarse haze particles from the sky, it will take longer to wash out finer particles - like those smaller than 2.5 microns in size.
The NEA and experts said the rain was not toxic to people, though it may have contained haze particles. Water agency PUB has stepped up its water quality monitoring since last week.
Today, the 24-hour Pollutant Standards Index is expected to be moderate (51 to 100) until 6pm. But the health advisory is for unhealthy conditions, based on yesterday's 24-hour PM2.5 readings.
Additional reporting by Wahyudi Soeriaatmadja in Indonesia