What this lesson teaches
I can explain how monsoons, tropical cyclones and ENSO change rainfall in the tropics.
Syllabus 9173, Topic 2.1, 1. Tropical Climates. How does synoptic scale circulation affect rainfall in the tropics?
Make a guess
In winter, northeast monsoon winds blow out of cold, dry Asia. Can they still bring heavy rain anywhere?
- On mountains far inland.
- No, because winter winds blow towards the land.
- No. Winter monsoon winds are dry wherever they go.
- Yes, where they cross a warm sea first.
Show the answer
Yes, where they cross a warm sea first.
Air straight off the land is dry, but winds crossing the Bay of Bengal pick up moisture and rain on India's southeast coast.
Seasonal heating changes pressure and winds over Asia, and rain depends on whether the wind crosses the sea and meets mountains.
In the northern summer, Asia's land heats up faster than the oceans. Low pressure forms over the continent, and the rain belt moves north. Moist air flows in from the Indian Ocean.
Air crossing the equator towards South Asia is turned to the right in the Northern Hemisphere. This creates the southwest summer monsoon. As moist air meets other winds and rises over mountains, it cools and brings heavy rain. Winds, pressure systems and upper-air circulation work together, so the monsoon is much more than a giant sea breeze.
In winter, the Asian landmass cools quickly and high pressure forms. Air flows out towards the warmer seas, often as northeast winds. Air coming straight from the land is dry. But if it crosses the sea, it picks up moisture and can bring heavy rain to exposed coasts.
The Western Ghats mountains boost summer rain on India's west coast. In the northeast monsoon, winds crossing the Bay of Bengal bring rain to India's southeast coast. So a winter monsoon is not always dry. Rain shadows, distance inland, passing depressions and year-to-year changes also affect rainfall totals.
Relief rain, also called orographic rain, explains why mountains get so much monsoon rain. When moist air meets a mountain range, it is forced to rise. Rising air expands and cools. Once it cools to its dew point, water vapour condenses into clouds, and heavy rain falls on the windward slope, the side facing the wind. After crossing the top, the air sinks. Sinking air is compressed and warms, so clouds clear. The leeward side, away from the wind, lies in a rain shadow and is much drier.
- Summer monsoon
- Land heats, low pressure forms, moist southwest winds bring rain.
- Winter monsoon
- Land cools, high pressure forms, northeast winds blow out.
- Key rule
- Winds bring rain only if they cross water and rise.
Step by step
Compare land and sea
Seasonal heating changes pressure.
Trace the wind
Include the sea crossing and Earth's spin.
Find the rain
Moisture supply, meeting winds and mountains.
Real examples to use
Mumbai and the Western Ghats (IMD)
Mumbai, on the windward coast below the Western Ghats, gets about 2,213 mm of rain a year (1991-2020). About 95% falls from June to September, when the southwest monsoon blows in from the Arabian Sea. Places inland, in the rain shadow on the Deccan Plateau, are much drier.
Mawsynram, Meghalaya, India
Guinness World Records lists Mawsynram as the place with the highest average yearly rainfall, about 11,870 mm. Summer monsoon air from the Bay of Bengal is forced up the steep Khasi Hills, so relief rain falls on their windward slopes.
Worked example: Opposite coasts of India
Summer southwest winds reach India's west coast after crossing the Arabian Sea. Winter northeast winds reach Tamil Nadu after crossing the Bay of Bengal. Both bring rain because both cross the sea.
Watch out for this
Winter monsoon winds are dry everywhere.
They start dry over land, but a sea crossing can add moisture before they reach the coast.
Check your understanding
A northeast monsoon wind crosses a warm sea before reaching a coast facing the wind. What is most likely?
- It stays dry because it started over land.
- It brings rain after picking up moisture and rising.
- Earth's spin adds water vapour to the wind.
Show the answer
It brings rain after picking up moisture and rising.
Yes. The sea crossing adds moisture.