I. Introduction to Meteorology

2. Monsoon and local circulation

Do you know what the word "Monsoon" means? A simple explanation is that it is the wind that comes from different directions as a result of seasonal change. Since Taiwan is located in the global monsoon region with significant seasonal wind variations, no matter what the short-term weather changes or long-term climate situations are, it is significantly influenced by monsoons!

The next question one might ask is why are there monsoons? First we must talk about "sea breeze" and "land breeze". Since the temperature changes at sea are slower than those at land, during the daytime when a region is heated by the sun, the land will quickly absorb the heat from the sun and its temperature will quickly rise. This causes the land temperature to be higher than that at sea which in turn will affect the temperature of the air; the land area will be hotter while the sea surface will be cooler.

This temperature difference will result in the flow of air. At this time, air from the cooler sea surface will flow toward the warmer land area, creating a cool "sea breeze". At night the process will be reversed. Since the land loses heat faster than the sea, the land area will be cooler than the sea surface. As a result, air will flow from the land towards the sea, and a "land breeze" will thereby be formed. So in the course of a day, there will be both land and sea breezes. When we extend the timeline from a day to a season, the different winds created by these effects are called "monsoons".

What then, is local circulation? In fact, it is also closely related to land-sea breeze. When a land or sea breeze is formed, the movement of air is at low altitude levels. However, the air above the low level air will also be affected and flow in the opposite direction. So if the lower level air flows eastward, the air above it will flow westward, resulting in a circular cycle. This is called a circulatory convection, which is also known as local circulation. Local circulation only occurs in small areas of a region where there is a high difference in daytime and nighttime temperatures, and the circulation is completed within 24 hours. For example, mountain breezes and valley breezes are examples of local circulation.