3. The formation of monsoons

In winter the land is colder than the sea, therefore the density and the pressure of the air over the land will be higher. As a result, wind flows from land to sea. In summer the situation will be reversed. The land is warmer and the air over land is lower in density; wind flows from sea to land. With extensive seasonal change of wind direction, it is called a "monsoon".

A monsoon is most noticeable in the south and east of Asia. This is due to the fact that in summer the Asian inland is heated up by the Sun and its temperature rises quickly, thereby creating an extensive low pressure region, which leads the air over the Indian Ocean to flow toward the land. This air current is called a "southwest monsoon" in Asia. The southwest monsoon can bring the wet oceanic air into inland Asia and generate continuous precipitation. This will bring abundant rainfall, sometimes even floods, to regions such as Indochinese Peninsula.

In winter, a high pressure system develops over the cold Asian Continent. A large amount of cold dry air blows out from the continent, and will only absorb water vapor until it reaches the ocean surface far from the land. On the east coast of the Mainland China, south of 30° N, this prevalent northeast wind is called a "northeast monsoon". In winter the continental high pressure moves south. When the cold front edge arrives at the sea area near Taiwan via the East China Sea, along with it comes the northeast monsoon with substantially strong winds. During the northeast monsoon season, the north and northeast of Taiwan will experience cloudy and partially rainy weather.

A monsoon is most powerful in the south and east Asia, because the Asian Continent is the world's largest land-mass. Other regions in the world that also experience monsoon phenomenon are Spain, the north of Australia, Africa (except the Mediterranean region), the U.S. west coast and Chile.