SINGAPORE - The hazy conditions seen in Singapore on Saturday are expected to persist on Sunday (Oct 4), although air quality could improve slightly.

In an update on Saturday evening, the National Environment Agency (NEA) said that the 24-hour Pollutant Standard Index (PSI) for the next 24 hours is expected to be between the high end of the unhealthy range and the low end of the very unhealthy range.

There may be a slight improvement to the hazy situation tomorrow, with readings dropping to the mid-level of the unhealthy range if wind directions shift, said NEA.

The PSI ticked upward on Saturday as haze from the surrounding region was blown in by the prevailing southerly wind. Widespread moderate to dense haze was still seen in central and southern Sumatra, the NEA said.

At 5am, the 24-hour PSI was 131-164, which falls in the unhealthy range. The three-hour PSI, which is not tied to the health advisory, was 64. The 24-hour PSI earlier at noon on Saturday was 128-161.

On Friday, air quality had turned out better than forecast. The 24-hour PSI fell to 100-130 at 6pm on Friday, and dipped into double digits briefly. At 11pm, it re-entered the unhealthy range.

The NEA, in an update on Friday evening, said that the 24-hour PSI over the next 24 hours was expected to be between the middle and high sections of the unhealthy range, and could enter the low end of the very unhealthy range if denser haze is blown in to Singapore.

Visibility is likely to get worse, NEA said, especially on Saturday morning when humidity is high. This is because moisture has a swelling effect on the haze particles.

This year's haze crisis could become one of the worst on record, the US National Aeronautics and Space Administration (Nasa) said, due to a prolonged dry season.

"Conditions in Singapore and south-eastern Sumatra are tracking close to 1997," said Dr Robert Field, a Columbia University scientist based at Nasa's Goddard Institute for Space Studies, in an Agence France-Presse report yesterday.

"If the forecasts for a longer dry season hold, this suggests 2015 will rank among the most severe events on record."

Event organisers have been forced to review their plans due to the haze. The Pedestrian Night at Orchard Road on Saturday has been cancelled. Saturday's Fina Swimming World Cup finals in Singapore has been cancelled as well.

For updates, visit the NEA website and the haze microsite, and follow NEA's Facebook page or Twitter account, or download the myEnv app.