We thank Mr Ajit Singh Nagpal for his feedback and share his concern about the transboundary haze that affected Singapore ("Why no prior warning of haze?; last Saturday).

Forecasting the occurrence of transboundary haze is based on a combination of factors.

These include the direction of prevailing winds, the location and intensity of the fires, the density and extent of the haze, and the incidence of rain.

Forecasting wind direction in the equatorial region is a particular challenge, due to its high variability. The presence of heavy cloud cover on some days also hinders the detection of fire hot spots, plumes and haze by satellites.

During the south-west monsoon season, the winds over our surrounding region normally blow from the south-east or south-west, and the fires producing the haze in Sumatra can sometimes start or intensify quickly.

On the few days before last Friday, satellite pictures showed only a handful of fire hot spots, with no significant haze over central Sumatra.

However, the fires there intensified rapidly overnight last Thursday, and the resulting haze was blown in by strengthening unseasonal westerly winds, causing a sharp deterioration in our air quality in a matter of hours the following morning.

We assure Mr Nagpal that we are closely monitoring the haze situation in the region and endeavour to provide timely haze forecasts to the public.

Patricia Ee (Ms)

Director

Weather Services Department

Meteorological Service Singapore