Monsoons are large-scale, seasonally changing wind circulations that form due to the temperature differences between land and ocean, like a giant sea breeze.

Monsoons dominate the climate in the tropical regions of Asia, including Singapore. They are known locally as the Northeast Monsoon and the Southwest Monsoon, and are responsible for much of the variation in rainfall over the regions throughout the year.

The Northeast Monsoon develops during the northern hemisphere winter between December and March, when the land over much of continental Asia is colder than the ocean. This causes sinking cold air over land to blow towards the ocean, replacing the void left by the rising warm air over the ocean. As the cold air flows southward towards the low pressure systems in the southern hemisphere, it warms up and moistens, leading to the formation of dense rain clouds in the equatorial region.

The start of the Northeast Monsoon usually heralds the wettest months (wet phase), while its tail end brings the driest months (dry phase), as the ITCZ begins to migrate south.



The Southwest Monsoon develops during the southern hemisphere winter between June and September, when cold subsiding air flows from the high pressure systems in the southern hemisphere to the warm northern hemisphere.

Monsoon winds are influenced by the Coriolis effect such that surface winds are deflected to the right in the northern hemisphere to become northeasterly winds during the Northeast Monsoon. During the Southwest Monsoon, they are deflected to the left to become southwesterly winds.

In between the two monsoon seasons is an inter-monsoon period which is characterised by light and variable winds and warm temperatures. Singapore experiences two inter-monsoon periods in April-May and October-November.