However, the key danger lies in the wind direction. These prevailing southwesterly winds blow directly from the island of Sumatra in Indonesia. This sets the stage for a perfect atmospheric conveyor belt for pollutants. The haze is not a natural weather event. It is a direct consequence of large-scale, illegal slash-and-burn land clearing, primarily for palm oil and pulpwood plantations in the Indonesian provinces of Riau, Jambi, and South Sumatra. During the dry season, the peatlands of Sumatra—which can burn underground for weeks—become tinderboxes.

When most people think of a dry season, they imagine clear blue skies, sun-drenched landscapes, and perfect beach weather. In Malaysia, the dry season—typically occurring during the Southwest Monsoon from May to September—does bring these benefits, but it also arrives with a persistent, hazardous, and infamous stowaway: transboundary haze.

While the eastern coast of Peninsular Malaysia (e.g., Terengganu, Pahang) and East Malaysia (Sabah and Sarawak) experience their true dry season during this period, the entire nation becomes vulnerable to a recurring air pollution crisis. This feature explores the science, source, and severe impact of this seasonal scourge. Malaysia’s weather is dominated by two monsoon winds. The Southwest Monsoon, originating from Australia’s dry winter, carries less moisture and creates stable atmospheric conditions. Wind speeds are lower, and rainfall is significantly reduced, particularly in the western states (e.g., Selangor, Perak, Kedah).