SINGAPORE – The air quality in the central region of the island slipped back into the unhealthy range overnight, after dipping temporarily to the moderate range at 11pm on Oct 3.
The 24-hour Pollutant Standards Index (PSI) reading, which stood at 100 at 11pm on Oct 3, began inching upwards two hours later and reaching 103 at 3am on Oct 4.
At 7am on Oct 4, the PSI reading in the central region was 102.
The PSI readings for the other parts of Singapore remained in the moderate range, ranging between 78 and 90 at 7am.
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.
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 29 and 43, across all regions at 7am.
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.