Hazy conditions back, but stay in moderate range

Grace Chua Straits Times 18 Sep 12;

THE haze has returned.

At 3pm yesterday, the three-hour Pollutant Standards Index (PSI) average hit 50, and stayed in the "moderate" range in the evening.

The higher pollution levels came after about a week's reprieve, after the PSI hit a one-year high of 79 on Sept7.

Air quality is considered unhealthy when the index crosses 100.

The hourly reading, given on the National Environment Agency (NEA) website between 7am and 7pm daily, is the average of the previous three hours' air pollution levels.

At 4pm yesterday, the 24-hour PM2.5 level - a measure of very fine particles in the air and another indication of air pollution - ranged from 28 micrograms per cubic m in the central region to 45 in the west.

A 40 to 65 range carries a health advisory by NEA for people to limit prolonged or heavy exertion.

These very fine particles are more able to penetrate the lungs and bloodstream than larger ones, rendering them more hazardous to health.

The annual haze is caused by farmers and logging companies in Indonesia who burn forests to clear land for cultivation.

The NEA said on its website that a current spell of dry weather is aggravating hot spots in southern Sumatra.

This week, both southern Sumatra and Kalimantan have been burning, according to National Aeronautics and Space Administration satellite data.

Dry spells are common in the current south-west monsoon season, which typically lasts from June to September or early October.

The prevailing wind is from the south-east or south-west. This carries smoke from the fires in Indonesia to Singapore.

Conditions are likely to persist this week.

"For the coming week, hot spot activities are expected in southern Sumatra due to dry weather conditions and the prevailing low-level winds are expected to blow from south-east or south-west," the NEA said.

"Singapore may experience hazy conditions on some days when the winds blow from the south-west."

The impact of the smoke haze on Singapore is dependent on factors such as the proximity and extent of the fires, the strength and direction of the prevailing winds, and the incidence and amount of rain, it added.

During this monsoon, periods of dry weather are common, interspersed with occasional thundery showers in the afternoon and "Sumatra" squalls in the pre-dawn and early morning.