What this lesson teaches
I can explain what drives wind direction and speed, including pressure differences, breezes and monsoons.
Syllabus K229, Topic 2.1, Key Question 4. Why do wind direction and wind speed vary across Earth's surface?
Make a guess
In the northern summer, winds from the Southern Hemisphere cross the equator heading north. Which way do they end up blowing near Singapore?
- From the northeast.
- The winds stop at the equator.
- From the southeast, the whole way.
- From the southwest.
Show the answer
From the southwest.
Earth's spin turns the air left south of the equator and right north of it. The southeast trades become southwest winds.
A monsoon is a seasonal change in wind direction, caused by land and sea heating differently through the year.
Land and sea heat up and cool down at different rates through the year. This shifts regional pressure patterns and the tropical rain belt. A monsoon is a change in the usual, or prevailing, wind direction that lasts a whole season. That is different from a sea breeze, which changes coastal winds within a day.
In the northern winter, continental Asia cools and high pressure forms over it. Air flows south towards warmer, lower-pressure areas. Earth's spin turns moving air to its right in the Northern Hemisphere, which helps create northeast winds. Crossing warm seas adds moisture, which can bring heavy rain to Southeast Asia near the equator.
Put these in order
Put the steps that create the Southwest Monsoon in order.
- Moist southwest winds bring the season's rain where the air rises.
- Low pressure forms over Asia, while high pressure sits in the Southern Hemisphere's winter.
- The Coriolis effect turns the winds right, so they become southwest winds.
- In the northern summer, the Asian landmass heats up faster than the seas around it.
- Southeast trade winds blow north and cross the equator.
Show the answer
- In the northern summer, the Asian landmass heats up faster than the seas around it. Warm land heats the air above it, so the air rises and pressure falls.
- Low pressure forms over Asia, while high pressure sits in the Southern Hemisphere's winter. Air moves from high to low pressure, so winds start to flow north towards Asia.
- Southeast trade winds blow north and cross the equator. North of the equator, Earth's spin turns moving air to its right.
- The Coriolis effect turns the winds right, so they become southwest winds. The winds cross warm seas on their way, so they pick up moisture.
- Moist southwest winds bring the season's rain where the air rises.
In the northern summer, the Asian landmass warms and low pressure forms to the north. High pressure sits in the Southern Hemisphere, where it is winter. Southeast trade winds can then cross the equator heading north. Earth's spin turns them left in the Southern Hemisphere and right in the Northern Hemisphere. After crossing the equator, they can become southwest winds.
This turning is called the Coriolis effect. Left and right mean relative to the direction the air is moving, not a side of the map. The effect is weakest near the equator.
Singapore's Northeast Monsoon usually lasts from December to early March. The Southwest Monsoon usually lasts from June to September, though local winds often blow from the south or southeast. So the season's name does not describe every local wind.
The Northeast Monsoon has a wetter early part and a drier later part, as the rain belt moves south. Between the monsoons, winds are often light and changeable, but strong afternoon thunderstorms still happen. To explain rain, follow the air's path over water and find what makes it rise.
- Northeast Monsoon
- December to early March; wettest early on.
- Southwest Monsoon
- June to September.
- Coriolis effect
- Earth's spin turns winds right in the north, left in the south.
Step by step
Heating changes
Land and sea warm differently through the year.
Winds change
Air moves to low pressure and is turned by Earth's spin.
Rain changes
Air picks up moisture over warm seas and rains where it rises.
Worked example: Why December and February differ
Both months are in the Northeast Monsoon, but Changi gets far more rain in December. Early in the season, the rain belt is nearby and monsoon surges bring heavy rain. Later, the rain belt moves south, so February is drier.
Watch out for this
The Coriolis effect always turns wind to the east.
It turns moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Check your understanding
Singapore has a southerly wind during the Southwest Monsoon. What is the best conclusion?
- A local wind can differ from the regional season name.
- It cannot rain unless the wind is from the southwest.
- The reading must be wrong, or taken on a calm day.
Show the answer
A local wind can differ from the regional season name.
Yes. The name describes the regional pattern, not every local wind.