Monsoon

While the thermal circulation described previously is a central part of monsoons, it is not sufficient to explain the world's most pronounced monsoons, those of Asia. Since they span such a huge area and affect over a billion people, the Asian monsoons have been studied for over a century to determine their causes and reasons for their variation. Although our understanding is not complete, it is clear that the monsoons of Asia are a complicated set of circulations, which combine the sun-driven winds with large scale circulations that span the entire planet. The extremely high mountains of the Himalayas also play a role in determining monsoon behavior.

An important factor in the development of the Asian monsoons is the existence of jet streams. These are great rivers of air, that ring the Earth at levels in the atmosphere ranging from 7-12 mi (12–20 km) above the surface. Jet streams are part of the global wind circulation, brought about by the large differences in temperature between the equator and the poles. Jet stream winds blow at several different latitudes, and play a major role in determining the weather beneath them. The monsoons of southern Asia are affected by two jet streams called the subtropical and the tropical jets. The subtropical jet is a permanent feature, flowing westerly (from west to east) at an altitude of about 7 mi (12 km). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter. It occasionally splits in two. The tropical jet is a weaker easterly (east to west) flow that forms near the equator at a height of nearly 12 mi (20 km). It is found only in the summer months.