Hazy conditions could ease as early as next week, with rain heralding the end of the dry season.
The Asean Specialised Meteorological Centre (ASMC), which has been monitoring the region's haze for more than two decades, told The Straits Times that the current south-west monsoon season is transitioning into the inter-monsoon season, which brings more rainfall over Singapore and the region.
"While hot-spot activities could still persist in Sumatra and Kalimantan, the haze situation is forecast to improve in the southern Asean region, with increased rainfall during the inter-monsoon period," said a spokesman for the centre.
The ASMC was established in 1993 by Asean member states and its headquarters is in Singapore.
During the south-west monsoon, winds typically blow from the south-east or south for Singapore, and rainfall is generally sporadic. But when the inter-monsoon season sets in, which the ASMC forecasts for next week, more rain is expected. Winds during this season are expected to be light and variable in direction.
The effect of the cooling of the eastern Indian Ocean - which suppresses the formation of rainclouds over the region - is also expected to ease over the rest of the year, bringing more rain.
Weather scientist Koh Tieh Yong from the Singapore University of Social Sciences said the monsoon - which has roots in the Arabic word mausim, meaning seasons - is closely tied to changing wind directions. Singapore experiences two distinct monsoon seasons - the south-west monsoon season, from June to September, and the north-east monsoon season, from November to March.
The south-west monsoon season coincides with the Northern Hemisphere summer. Because of various factors, including the heating of the Asian continent in the north, the rainband over the Equator shifts over the northern Asean region. This makes the weather drier over southern Asean, said Professor Koh.
The reverse happens during the north-east monsoon season, when the Southern Hemisphere experiences summer. This shifts the rainband south over Singapore, bringing heavier rain from November to January. "By February, the rainband has passed by us, moving southwards to Indonesia, making it the driest month in Singapore," said Prof Koh.
The change between monsoon seasons is gradual, which is why Singapore experiences two inter-monsoon seasons in a year.
Minister for the Environment and Water Resources Masagos Zulkifli on Thursday said ASMC has always given advance notice of intense or prolonged dry weather that can trigger forest fires in Asean. "It has done so diligently all these years," he said.
ASMC had raised the Level 1 alert for southern Asean countries on July 2, advising affected nations about the onset of the dry season. A Level 2 alert was raised for the same region on Aug 1, when the hot-spot count in Sumatra and Kalimantan started going up.
Level 3 alerts were raised for Kalimantan on Sept 5 and for Sumatra on Sept 9 because of increased risk of transboundary haze from the escalating hot-spot situation and deterioration in haze conditions, said a centre spokesman.
A variety of data sources are used for such forecasts and warnings.
Hot-spot data is processed from satellites operated by the United States National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration, as well as from the Himawari-8 satellite operated by the Japan Meteorological Agency.
Data from the Himawari-8 satellite is available every 10 minutes. Polar orbiting satellites such as the NOAA20 satellite allow ASMC to detect hot spots both in the day and at night. This is supplemented with ground observation reports from the various national meteorological centres and air quality data.
