Monsoons have been in the news lately, largely because of heavy rains that have occurred in southern Asia. Monsoons are seasonal weather phenomena that occur in many places around the world, and they have a very simple cause. From the Glossary of Meteorology:
monsoon: A name for seasonal winds.It was first applied to the winds over the Arabian Sea, which blow for six months from northeast and for six months from southwest, but it has been extended to similar winds in other parts of the world. Even in Europe the prevailing west to northwest winds of summer have been called the "European monsoon." The primary cause is the much greater annual variation of temperature over large land areas compared with neighboring ocean surfaces, causing an excess of pressure over the continents in winter and a deficit in summer, but other factors such as the relief features of the land have a considerable effect. The monsoons are strongest on the southern and eastern sides of Asia, the largest landmass, but monsoons also occur on the coasts of tropical regions wherever the planetary circulation is not strong enough to inhibit them. They have been described in Spain, northern Australia, Africa except the Mediterranean, Texas, and the western coasts of the United States and Chile. In India, the term is popularly applied chiefly to the southwest monsoon, and by extension, to the rains which it brings.
Before we get into translating this definition into English, let's take a short vacation to Los Angeles, California as a thought experiment. On a typical summer day in LA, temperatures warm up through the morning and the early afternoon. But often in the afternoons, a refreshing sea breeze will arrive to bring in cooler air and relief.
The sea breeze in coastal areas is caused by the simple fact that, all else being equal, a land mass will heat up faster than a body of water. The air over land becomes hot and rises, creating a small area of low pressure. Air rushes in from the surroundings to replace the rising hot air, causing the winds to blow onshore and bringing us the sea breeze.
At night, the situation can reverse. All else being equal, the land mass will cool off more quickly than a body of water. The air over the land cools and sinks, creating a small area of locally high pressure. Air flows outward from the high, turning the winds around to offshore and creating the land breeze - the opposite of the sea breeze.
Usually the sea breeze-land breeze circulation oscillates once per day. That is, we get the sea breeze during the afternoon, and the land breeze at night. Other factors may have a modulating effect on the circulation, such as nearby thunderstorms, frontal boundaries, terrain, or larger scale highs and lows. But the main force that drives the sea breeze-land breeze circulation is the differential heating between land and ocean, operating on the short timescale of a day.
Now imagine a larger area, such as India, and consider what happens over a longer period of time, such as several months. As the seasons change from spring to summer, the land surface of India will gradually grow warmer and warmer. The surrounding ocean also gets warmer, but far less quickly than the land surface. Over time, a large area of low pressure develops over the land, causing the winds to become onshore over a larger area. In winter, the situation reverses. The land cools more quickly than the surrounding ocean, causing the development of relatively high pressure over land, which results in a large area of offshore winds. The seasonal change in the winds is the monsoon. In essence, the monsoon circulation is just a larger and more drawn out version of the typical daily sea breeze circulation.
So, in strict terms, the monsoon is really just the seasonal reversal of the prevailing winds as a result of the differential heating between land and ocean. Winds are onshore in the summer, and offshore in the winter. The reversal of the winds will bring changes to the weather. The onshore winds usually are rich with moisture from the tropical oceans, and as a result the summer phase of the monsoon is typically a very wet time. India receives much of its yearly rainfall during the onshore phase of the monsoon. In winter, offshore winds stem the flow of moisture and bring an end to the rains.
The monsoon in India is the most textbook example, but as noted in the definition above, monsoons do occur in other regions of the world as well. In North America, a monsoon brings summer rains to the Southwest. The northern coast of South America is subject to a monsoon, as is the northern part of Australia. Africa, and to a lesser extent Europe, also have monsoons as a major feature of their weather and climate.
