Because of that origin, the air is cool and comparatively dry — Page 387, Lesson 357

I want to walk you through the monsoon systems of Asia, starting with the Northeast Monsoon. This wind blows from the winter Siberian High, so it originates over a cold, continental landmass. Because of that origin, the air is cool and comparatively dry. As it moves southward, it brings clear weather over Bangladesh, Burma, and Thailand. Now, the same Northeast Monsoon also affects southeastern India, Sri Lanka, and the east coast of West Malaysia. But here, something important happens: the air has a long over-sea track before it reaches those areas. As it travels across warm ocean water, it picks up moisture. When that moist air then encounters coastal mountain ranges, it is forced upward, producing heap-type clouds — that means cumulus and cumulonimbus — along with thunderstorms and heavy precipitation. So the same wind can give clear skies in one region and heavy rain in another, depending entirely on its path over water. Let's move to the Northwest Monsoon. This is really an extension of the Northeast Monsoon. As the Northeast Monsoon crosses the Equator moving southbound, it backs — that means it turns counterclockwise, or shifts direction anticlockwise — and becomes a northwest wind. This northwest flow brings cumulus, cumulonimbus, and thunderstorms to northern Australia and New Guinea. So you can see the same basic air mass, but its behaviour changes as it crosses the Equator. Now the Southwest Monsoon. This is produced by the Southeast Trade Wind crossing the Equator. When the Southeast Trade Wind crosses into the Northern Hemisphere, it veers — that means it turns clockwise — to become a southwest wind. It then flows toward the summer Baluchistan Low, a thermal low-pressure area over the Indian subcontinent during summer. Because this wind has a very long sea track, it is extremely moist. It produces a great deal of heavy cumulus and cumulonimbus cloud with large-scale thunderstorms. The Southwest Monsoon affects all of India, Sri Lanka, Burma, and the exposed coasts of West Malaysia. From a flying perspective, the Southwest Monsoon has a more serious effect on aviation than the Northeast Monsoon. It brings heavy thunderstorms, a low cloud base, and severe turbulence. So if you're operating in those regions during the Southwest Monsoon season, you need to be prepared for significantly degraded flying conditions. The Southwest Monsoon also affects the West African coast, notably Guinea, Ghana, and northern Nigeria. Let me summarise the global rainfall picture. The world's rainfall is produced principally by weather from the Intertropical Convergence Zone, or ITCZ, and the associated monsoons, as well as from the two cool temperate zones. In contrast, the two subtropical high-pressure belts and the two polar high-pressure belts will usually be dry. However, these general patterns can be altered significantly by local topographical features — mountains, valleys, coastlines — that force air to rise or block moisture. The extremes of rainfall around the world are shown on the chart at Figure 20.29. So to tie it all together: the Northeast Monsoon is cool and dry from the Siberian High, but picks up moisture over the sea to affect southeast India and Malaysia with thunderstorms. The Northwest Monsoon is the same air backing after crossing the Equator, bringing storms to Australia. The Southwest Monsoon is the moisture-laden trade wind veering into the Baluchistan Low, producing the heaviest rain and the most challenging flying conditions across India and West Africa.