Air quality in the central region of the island lingered between the upper end of the moderate range and the unhealthy band on the morning of Oct 4.
The 24-hour Pollutant Standards Index (PSI) reading in that area of Singapore had mostly been in the unhealthy range since 11am on Oct 3, but dipped temporarily into the moderate range at 11pm that night and then again at 9am on Oct 4.
The PSI value stood at 100 at 11pm on Oct 3, before inching upwards two hours later and reaching 103 at 3am on Oct 4.
At 9am on Oct 4, the PSI reading in the central region dropped back to 100, before dipping further to 95 at 12pm.
The values for the other parts of Singapore remained in the moderate range, ranging between 73 and 86 at 12pm.
The PSI reading is a 24-hour rolling average of six key air pollutants and is used in planning for outdoor activities the next day.
Air quality is considered unhealthy when the 24-hour PSI is in the range of 101 to 200, during which the National Environment Agency (NEA) advises the public to at least reduce outdoor activities. PSI readings of 51 to 100 are considered moderate, while values of zero to 50 are in the “good” range.
The air quality started worsening at 11am on Oct 3 when the 24-hour PSI hit 102, the first time the reading in any region here reached unhealthy levels since 10pm on Sept 30.
Meanwhile, the one-hour PM 2.5 readings, which are used as a gauge for immediate activities, stood in the normal range between 14 and 28, across all regions at 12pm.
PM2.5 readings, which indicate the current concentrations of fine particulate matter, are considered elevated if they fall between 56 and 150, and high if they are between 151 and 250.
In its daily haze advisory on Oct 3, NEA said the 24-hour PSI is forecast to be in the “moderate to low unhealthy” range.
Moderate to dense smoke plumes from southern Sumatra were observed drifting towards Singapore and the surrounding region with smoke haze also observed from fires in Kalimantan.
Dry conditions are forecast to continue over Singapore and the surrounding region on Oct 4. The risk of smoke haze affecting the Republic remains as the prevailing winds are expected to blow from the south-east or south-west.
The PSI crossed into the unhealthy range a month ago, hitting 101 in central Singapore on the morning of Sept 4. In the days that followed, readings in the central, eastern and western parts of the island occasionally crossed into the unhealthy range.
The developments in September and October mark the first time since October 2023 that the PSI in Singapore hit unhealthy levels because of haze, with the authorities earlier warning of the risk of haze between June and October as a result of warmer and drier conditions brought on by both El Nino and the Indian Ocean Dipole.
Meanwhile, fair and warm conditions are expected on several days, particularly during the first week of the month, said the Meteorological Service Singapore in its fortnightly weather advisory on Oct 1.
The total rainfall for the first half of the month is forecast to be below average across most of the island, making it drier than the previous two weeks.
However, localised thundery showers may fall over parts of Singapore in the late morning and afternoon on a few days.
Daily temperatures are likely to reach a high of between 34 deg C and 35 deg C on most days, and could exceed 35 deg C on a few days.
Some nights may be warm and humid, with temperatures above 27 deg C.
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