A monsoon is a seasonal change in the usual wind direction. Its rainfall depends on the wind's moisture supply and the position of the rain belt.
Monsoons develop as land and oceans heat differently through the year. This shifts regional pressure patterns and the tropical rain belt. A monsoon changes the usual, or prevailing, wind direction over a season. A sea breeze changes coastal winds within a day.
In Northern Hemisphere winter, continental Asia cools and high pressure develops. Air moves towards warmer, lower-pressure regions farther south. Earth's rotation deflects moving air to its right in the Northern Hemisphere, helping produce northeasterly winds. Crossing warm seas adds moisture, which can bring heavy rain to equatorial Southeast Asia.
In Northern Hemisphere summer, the Asian landmass warms and lower pressure develops farther north. Higher pressure occurs in the winter Southern Hemisphere. Southeast trade winds may cross the equator northwards. Earth's rotation deflects them left in the Southern Hemisphere and right in the Northern Hemisphere; after crossing north, they can become southwesterly.
This deflection is called the Coriolis effect. Left and right refer to the air's direction of travel, not a fixed side of a map. The effect is weakest near the equator.
Singapore's Northeast Monsoon generally lasts from December to early March. The Southwest Monsoon generally lasts from June to September, although local surface winds often come from the south or southeast. The regional season name does not describe every local wind.
The Northeast Monsoon has a wetter early phase and a relatively drier later phase as the rain belt shifts. Between monsoons, winds are often light and variable, but strong afternoon thunderstorms still occur. To explain rainfall, follow the air's route over water and identify what makes it rise.
Step by step
Seasonal heating changes
Land and sea warm differently, shifting regional pressure patterns.
Winds change
Air moves from higher towards lower pressure, deflected right north of the equator and left south of it.
Rainfall changes
Air gains moisture over warm seas. Uplift and the rain belt affect where that moisture falls as rain.
Worked example: Why December and February can differ
December and February are both within Singapore's Northeast Monsoon, but Changi's normals show much more rain in December. Monsoon surges and the nearby rain belt favour the early wet phase. The rain belt moves farther south later, helping make the later phase drier.
Watch out for this
Coriolis always bends wind to the east.
It deflects motion to the right in the Northern Hemisphere and left in the Southern Hemisphere. The compass result depends on the initial movement.
Check your understanding
Singapore has southerly wind during the Southwest Monsoon. Which conclusion is best?
- The observation must be wrong because the season is named Southwest.
- There can be no rainfall unless winds blow from the southwest.
- A local wind can differ from the regional season name.