SINGAPORE: Strong winds and heavy rain swept across Singapore on Tuesday night (Sep 17), toppling trees in various places.
The sudden storm was likely brought about by a Sumatra squall, experts told CNA.
What is this weather phenomenon and how often does it happen?
What is a Sumatra squall?
A squall is simply a sudden burst of strong winds that are accompanied by heavy rain. It is brief but intense, and it can die down as quickly as it started.
A Sumatra squall has characteristics that are specific to this region – it is a line of thunderstorms that develops over Indonesia’s Sumatra island or the Strait of Malacca, then moves eastwards to affect Singapore and Peninsula Malaysia.
In a typical case, a thunderstorm line or squall line can bring about one to two hours of thundery showers.
Wind gusts can reach up to 80kmh.
Because of their linear structure, squalls affect a much larger area along their path compared with conventional storms. But their narrow width means the heavy rain and winds pass quickly.
“Squall lines are mesoscale weather systems and can be hundreds of kilometres long but are typically only a few tens of kilometres wide and tend to pass quickly,” said Professor Matthias Roth from the National University of Singapore’s (NUS) geography department.
“Their formation is related to the descent of air cooled by radiation on the high ground of northern Sumatra.”
When do squalls occur?
A Sumatra squall typically occurs during the southwest monsoon and inter-monsoon periods – between April and November – when winds are normally from the southwest.
“It’s strongly associated with southwest wind,” said Dr Wang Jingyu, an assistant professor of physical geography at the National Institute of Education (NIE).
“The southwesterly winds are forced to ascend over the Bukit Barisan mountain range, generating linear-shaped convection systems that propagate across the Malacca Strait toward the Malaysian Peninsula.”
Dr Wang added that on average, squalls occur five to 10 times per month during these peak seasons.
According to a 2020 report by the Meteorological Service Singapore, the country experiences 45 squalls per year on average.
Squalls often occur in the pre-dawn hours or early morning.
