What is a monsoon?
The word "monsoon" is a term from early Arabs - "mausin", or "the season of winds". It refers to the winds that seasonally shift in the Indian Ocean and the surrounding region, including the Arabian Sea. These winds blow constantly from the northeast during the northern winter and from the opposite direction during the northern summer. The period between the monsoons is called inter monsoon period. The monsoons play a major role for the formation of the climate in India, Bangladesh, Southeast Asia, and, to a lesser extent, northern Australia and Central Africa.
What causes monsoons?
Monsoon is caused by the different temperature of the land and the sea, which is due to the sun’s heating. Monsoons bring large amounts of rain in the summer and dry and sunny weather in the winter. All monsoons share three basic physical mechanisms: different heating between the land and the ocean; the ability of water to sore and release energy when changing from a liquid to vapour and back; Coriolis forces due to the rotation of the Earth. These three mechanisms together produce the monsoon’s characteristic reversals of high winds and the amount of rain.
The Indian Monsoon
The Indian subcontinent is the classic place where the monsoon climate can be found. It is known that the temperature of the water takes more time to change than does the temperature of the land. This is the reason why at the beginning of winter the subcontinent cools down rapidly while the Indian Ocean is still warm. The air over the ocean is heated by the warm water and as a result it rises. This movement causes cool air to draw from northern India and the Himalayas to the Indian Ocean. These winter monsoon winds are the reason for the cool, dry, and sunny weather in India during the winter.
As summer comes, the pattern reverses. The land in southern Asia warms faster than the water of the Indian Ocean and the air over the land is heated to a higher temperature. Hot air over the land rises and cool, humid air from the ocean moves onshore to replace it. These southwest winds blow in moisture that condenses and brings heavy and long-lasting rainfall. This rainy period begins in June and continues to September.
The Himalayas form a barrier that does not allow the warm and humid air to reach northern Asia. This is the reason why the southern slopes of the Himalayas receive heavy rainfall while the northern part receives smaller amounts of rain. This effect also occurs along the southwestern coast of India. So, some parts of the country receive great amounts of rain during the summer monsoon season.
The Malaysian-Australian Monsoon
The monsoonal system combining southeast Asia and northern Australia is a little bit different in comparison to the others. The reason for this is the strange and rather symmetrical distribution of landmasses on both sides of the Equator. That is why the Australian northwest monsoon is unique. The considerable amount of water lying between Australia and Asia has a moderating effect on temperatures and makes the summer monsoon weaker. The large number of islands provides a great variety of topographic effects.
It is typical for the monsoons to extend up to 25 degrees to the North. Further north, the summer monsoon is weaker and cannot overcome the effect of the subtropical traveling anticyclones. Therefore, in June and in late August and September monsoonal rains occur, separated in July by mild drought. These monsoonal streams are pretty shallow. They bring rain only when submit to substantial cooling.
The summer monsoon in Thailand and Vietnam is well developed because of the wider expanses of overheated land. Indonesian monsoonal winds are weaker because of the low latitude and the large area of water. Simple monsoonal patterns can be observed in Australia because of its small size and shape.
The West African Monsoon
The southwest winter monsoon flows as a moist layer of surface air. The West African monsoon succeeds the southwesterly wind and the surface harmattan. The whole year is more or less dry at about 20 degrees north. Further south the drought becomes less lasting and complete. The southwest monsoon may occur at any time on the south coast. The results, however, are quite unusual. From December to February, when the midday Sun is at its lowest, the weather is dry and cloudy. In the high-sun season (April-June) there is a lot of rain because the frequent southwest wind builds up clouds.