How a trade wind becomes a monsoon
The summer monsoon is not new air but a trade wind transformed. Drawn toward the summer continent's thermal low, a trade wind that crosses the equator comes under the opposite hemisphere's Coriolis deflection and turns: the southeast trade, crossing northward, becomes the southwest monsoon. And the journey rewrites its character: a long track over tropical sea loads the air with moisture and heat from below, so what left one hemisphere as fair-weather trade arrives as a deep, moist, unstable current — the same modification logic the air-masses lesson taught, applied across an ocean.
Monsoon weather, wet and dry
The two monsoon seasons are opposite worlds:
Summer (wet) monsoon — delivers widespread, prolonged heavy rain with embedded cumulonimbus and thunderstorms, very low cloud and poor visibility on windward coasts and high ground, severe turbulence, and waterlogged aerodromes: for aviation, weeks of instrument weather with convective violence hidden inside it.
Winter (dry) monsoon — the continent exhaling: dry, mostly fine weather in a continental outflow that often carries dust haze; flyable, but with visibility its recurring weakness.
West Africa
West Africa's year alternates between two airstreams. In the northern summer, the moist southwesterly monsoon off the Gulf of Guinea pushes inland beneath the dry Saharan air, and along and south of that boundary come cloud, rain, thunderstorms and the westward-sweeping squall lines of the previous lesson. In winter the flow reverses: the dry northeasterly Harmattan blows off the Sahara — fine weather, but with dust haze that can cut visibility severely; the Harmattan's full story belongs to the next lesson's named winds.
India
India is the monsoon's showpiece. The winter northeast monsoon brings dry, fine, often hazy weather in continental air flowing off Asia, the outflow of the winter continent's cold high. Then, around June, comes the burst of the southwest monsoon: from about June to September, deep moist oceanic air delivers torrential and prolonged rain, embedded thunderstorms, and very low cloud on the windward Ghats and the Himalayan foothills — some of the heaviest rainfall on Earth, and month after month of demanding instrument flying.
The Far East and northern Australia
The same engine runs further east:
China and Japan — the winter monsoon is a cold, dry north to northeasterly outflow of Siberian air; the summer monsoon a warm, humid south to southeasterly maritime flow with rain and thunderstorms.
Northern Australia — the calendar flips hemispheres: the southern summer, roughly December to April, brings the moist northwest monsoon (northeast trades drawn across the equator and turned) and the wet season; the southern winter restores the dry southeast trades.
Sandstorms
The dry climates contribute their own visibility hazard. A sandstorm needs strong winds over a dry, bare surface, helped by daytime instability that stirs the lifted grains through a deep layer — supplied either by a tight pressure gradient or by a thunderstorm's gust front, the wall-of-dust haboob. Sand travels mainly in the lowest few hundred to few thousand feet, while fine dust hazes to far greater heights; visibility can fall to near zero, and the flying problems are the visibility itself plus abrasion of engines, windscreens and airframes.
Cold air among the palms
Finally, the subtropics are not safe from winter. In winter, behind a deep depression, polar air can surge far into the subtropical weather systems, wherever no east-west mountain barrier stands in its way: North America above all, South America east of the Andes, and East Asia. The named examples the syllabus wants:
Blizzard — the North American polar outbreak with gales, driving snow and near-zero visibility.
Pampero — the cold, squally southerly outbreak that sweeps the Argentine and Uruguayan pampas, announcing itself with a sharp squall line and a plunge in temperature.