KUALA LUMPUR, Oct 21 — Malaysia, particularly the Klang Valley, has been experiencing bouts of heavy rain and thunderstorms in recent days, but the haze continues to linger. Why?
According to Malaysian Meteorological Department (MetMalaysia) director-general Mohd Hisham Mohd Anip, rainfall can provide only temporary relief as long as smoke continues to arrive from Indonesia.
“The haze currently affecting the country is caused by transboundary haze, whereby smoke from fire-affected areas in neighbouring countries is carried into Malaysian airspace.
“At the same time, the country is experiencing the monsoon transition phase, characterised by weak winds blowing from various directions.
“These conditions are conducive to the formation of rain clouds and thunderstorms in the afternoon and early evening.
“Rainfall during the monsoon transition can only temporarily reduce haze conditions. Therefore, haze may persist despite rainfall if smoke continues to be carried in by winds from other areas,” he told Malay Mail.
Universiti Malaya meteorologist Professor Datuk Azizan Abu Samah said the prevailing wind direction was a key reason the haze could persist even after rainfall.
As long as winds continued to blow from the south, south-west or west, smoke from Indonesia would continue to be carried into Malaysia, he explained.
“So even if the rain cleared due to wet deposition, new smoke will be blown back from Sumatra,” he said.
According to deforestation monitoring site Nusantara Atlas, fires in Indonesia burned more than 1.9 million hectares between January and the end of September this year, an area nearly 12 times the size of London.
Forest and peatland fires in Indonesia, particularly in Sumatra and Kalimantan, have been blamed for much of the haze affecting Malaysia, with smoke carried across borders by prevailing winds.
The haze has disrupted daily life in Malaysia in recent weeks, with worsening air quality prompting health concerns and school closures.
Prime Minister Datuk Seri Anwar Ibrahim has said Malaysia remains prepared to extend assistance to Indonesia in tackling the forest and land fires responsible for the transboundary haze.
Mohd Hisham explained that heavy, prolonged rainfall over a wide area could remove more pollutant particles from the atmosphere than brief, localised drizzle, although its effectiveness also depended on the size and properties of the particles and atmospheric conditions.
“A greater volume of rainwater and a longer duration of rainfall can enhance the removal of pollutant particles. However, the effectiveness of rainfall is also influenced by the size and properties of pollutant particles, as well as atmospheric conditions.
“Heavy rain does not necessarily guarantee that haze will clear completely if smoke continues to be carried into Malaysia from fire-affected areas in neighbouring countries.
“In the context of transboundary haze, changes in wind direction that reduce the transport of smoke into Malaysia, together with sufficient and sustained rainfall in affected areas, are crucial to improving air quality,” he said.
Explaining why the haze could return after rain, Mohd Hisham said smoke from forest and land fires typically contained fine suspended particles such as PM2.5 and PM10, including black carbon and organic carbon produced by burning organic matter.
PM2.5 particles are particularly fine and can remain suspended in the air for long periods, allowing them to travel considerable distances with the wind.
Certain gases released during combustion can also react in the atmosphere to form secondary pollutant particles, he said.
“Rain can remove some of these particles, but not all particles are necessarily removed at the same rate. When fresh smoke continues to enter Malaysia due to winds from fire-affected areas, pollutant concentrations can remain high or rise again,” Mohd Hisham said.
Azizan said the scale of the forest and peatland fires in Sumatra meant there was little technology could do to resolve the haze quickly.
“The scale of the environmental disaster is too big for a technological fix,” he said, adding that rainfall in Sumatra during October and November could help extinguish the fires.
Mohd Hisham also warned that hot and dry conditions could increase the risk of forest and land fires, particularly in areas with dry vegetation and peat soil.
“Ongoing fires will release more smoke and pollutant particles into the atmosphere, making it more difficult to reduce haze.
“The presence of a very strong Super El Niño phenomenon will further worsen weather conditions in the region and increase the risk of transboundary haze, such as the situation currently being experienced,” he said.
But a change in the weather could finally offer Malaysians some relief.
Mohd Hisham said the haze was expected to gradually subside next week following an anticipated change in wind direction.
During the Northeast Monsoon, stronger north-easterly winds and more widespread rainfall were expected to help improve air quality, he said.
Azizan said Malaysia would have to wait for the Northeast Monsoon to bring a more lasting improvement, with easterly and north-easterly winds expected to carry smoke away from the country.
He said the north-easterly wind belt was already present over the South China Sea near Ho Chi Minh City, Vietnam, and was expected to reach Peninsular Malaysia in early November.
“We just need to stay safe and let nature take its course,” he said, estimating that the country would have to endure the conditions for another two to three weeks.