The word 'monsoon' comes from the Arabic word mausim, which means season. It is applied to the seasonal reversals of wind direction along the shores of the [M004 / Indian Ocean], especially in the Arabian Sea but also in the Bay of Bengal. These winds blow from the southwest during one half of the year, and from the northeast during the other.
There are actually [I008 / many other areas of the world] where a monsoon [G 03 / climate] is present, but in none of them is the monsoon so dramatic or powerful as in India. Only the Indian monsoon can truly be called a wonder of the world.
The definition of a monsoon climate calls for the wind to reverse its direction between summer and winter. The wind must blow inland from the cooler oceans toward the warm continent during the summer season, and from the cold continent toward the warmer oceans during the winter. The definition also calls for the summer to be [P 001 & P021 / very wet], with [P 004 & P007 / high rainfall], and the winter to be very dry.
There are three basic mechanisms which work together to produce the phenomenon of the monsoon.
The most important factor is the [L2 201 / differential heating] of the land and the ocean. These differences result in a [G31 / pressure gradient] which draws winds to flow from areas of high pressure to areas of low pressure.
All of the heating that the earth receives comes from the sun. However, there are variations in the amount of heating that an area receives. These variations are caused by changes in the relative positions of the earth and the sun, due to the annual cycle of the earth's orbit around the sun. They are also a result of which kind of surface is being heated. The two basic types of surface are water and earth.
Water and earth react very differently to solar heating. Water requires a lot of energy to heat it and also loses heat energy slowly. Soil or rock, on the other hand, can be heated very quickly by the sun, but also suffers from rapid heat loss.
During the summer, the sun, at its strongest, heats both the sea and the land. The land becomes hotter, and in turn heats up the air above it. This air expands and rises, so to equalize the pressure, air from above the sea rushes in. This air picks up moisture from the sea as it moves toward land, and then lets it fall as rain once it is over the land.
The next basic mechanism of the monsoon is related to the [L2 202 / moisture] that has just been deposited on the land. The rain cools down the land, and makes it more difficult for the sun to heat the land. The land therefore becomes more 'ocean-like' in the area where rain has fallen, and the pressure gradient continues to push the cooler sea air deeper into the hotter interior of the continent. This is why the monsoon advances as it does.
The [N 001 / advance of the monsoon] is remarkably consistent. It first appears over the southernmost Indian state of [M001 / Kerala] around the first of June. It advances at approximately 10 miles per hour, and by June 10 has reached Bombay. At the same time, another branch of the monsoon is sweeping in from the Bay of Bengal, on the eastern coast of India. [P 002 / Calcutta's monsoon] arrives usually three days before Bombay's. Soon after this the two branches of the monsoon merge and become indistinguishable.
The variance of the onset date of the monsoon is, on average, seven days. One can generally predict with confidence that the monsoon will reach India within a week of the first of June.
The final basic mechanism of the monsoon is the [L2 203 / Coriolis effect]. This is the deflection of the wind resulting from the rotation of the earth. In the Northern Hemisphere, moving bodies are deflected to the right. The cool ocean air flowing toward India from the Arabian Sea is therefore pushed to the right, helping the south westerly flow to penetrate further into the continent.
During the winter, when the sun is shining at a flatter angle to the earth, the land does not receive as much heat, and consequently cools down. The air above the land also cools and becomes denser, and creates a zone of high pressure. The winds now flow in the opposite direction, from the northeast, toward the sea. This is known as the northeast monsoon. Because it is coming from over land, it has little moisture and so produces little [G 18 / precipitation]. This northeast monsoon tends to follow the [N 002 / withdrawal of the southwest monsoon], which is much more gradual than its onset.
There are, of course, many other factors involved in the making of the Indian monsoon. There are [G 12 / jet streams], high altitude air currents that shape the flows of air around the globe. The [L2 204 / N005 / Himalayas] to the north of India are also of significant importance. The amount of [M 002 / annual precipitation in South America] has been shown to bear a relationship to the onset date of the Indian monsoon, although the reasons for this remain unclear. The amount of forest cover on the Tibetan Plateau is also regarded as having a role in the speed of progress of the monsoon.
Scientists are still struggling to agree on the relative importance of many of these factors, and to fully understand this huge and extremely complex organism that is the monsoon.
THE MONSOON AND THE PEOPLE:
The Indian monsoon and the [L2 205 / P025 / Ganges River] have an incalculable importance to the people of India. For thousands of years people's ways of life have been fixed to the coming and going of the monsoon, and it is deeply rooted in their culture, [P 023 / religion] and economy.
Because India is a [G 24 / tropical], developing country, it relies very heavily on [P 018 / agriculture] to feed itself. It is estimated that 70% of India's population is involved in agricultural production. Agriculture is [P 003 / critically sensitive to the weather]. Too much or too little rain can have a devastating effect on crop production, and can be a matter of life and death for millions of people.
In [M004 / Southeast Asia], 65% of food-grain growing is done during the summer monsoon. Seeds need [P 017 / plenty of moisture] during their critical first week in the ground. Farmers, therefore, [P 019 / plant the seeds] after the first rain of the monsoon in the expectation that more rain will follow. The consistency of the monsoon allows them to make that decision with some degree of confidence.
The [P 022 / arrival of the monsoon is celebrated] all over India. Before the rains come, the country becomes painfully hot. Tempers become short; there are fights, murders and suicides; and [P 008 / everyone prays for rain]. After its arrival people throw parties outside, give blessings to their gods and generally give thanks for the rain.
In Goa, on India's west coast, the arrival of the monsoon generally coincides with the Feast of St John. It is said that St John jumped twice in his mothers womb. In Goa, young men wearing laurel wreaths go around jumping twice into people's wells. Not only is this a piece of religious devotion, it also celebrates the monsoon. Without the rains, the wells would be empty, and consequently there would be no food and no people.
For centuries, the winds of the monsoon were used by seamen for navigation. Before the advent of steamships in the nineteenth century, winds were the driving force of trade, and Indian, Arab and Persian ships conducted commerce between India, the [M003 / Persian Gulf] and Zanzibar using the winds. An Arab pilot with a sound knowledge of the winds guided the Portuguese explorer Vasco da Gama to India in 1498 - the first European power to arrive in India.
A feature of the monsoon that also helped traders is the mysterious phenomenon of the mudbanks along the coast of Kerala. About five miles off the Keralan coast there is a large amount of soil lying on the seabed, that has been washed from the coast in rivers. When the monsoon arrives it stirs up the sea. The soil is lifted off the seabed and becomes suspended in the water. This has the effect of calming the water to an extraordinary extent. The sea above the mudbanks becomes as flat and calm as a millpond, although the sea around it can be very violent. This provides a wonderful natural harbor for ships to transfer cargo and take on fresh water, and to avoid dangerous seas.
Another interesting feature of the mudbanks is that shrimps seem to be attracted to the mud, and consequently catching them becomes particularly easy. [P 016 / Fishermen] can take thousands of dollars worth in a single catch. The phenomenon is known in the area as Chakara, and for Keralan fishermen has all the aspects of a gold-rush as they compete to harvest the bounty of the monsoon.
However, not all of the aspects of the monsoon are so pleasant for the people of India. The waters of the monsoon can be life-threatening as well as life-giving. The main agent for this is [P 006 / flooding]. As 80% of India's rain falls during the monsoon, this is obviously the season when flooding occurs. It is also a problem that seems to be getting worse in terms of human life as the population encroaches more and more on [G 08 / floodplains] with increasing urbanization. Between the years of 1953 and 1985, the average loss of life due to flooding each year was 1,448 people.
The main cause of flooding is the [G 06 / cyclones] which occur during the monsoon period. These can have winds of 75 miles per hour, and can bring with them extremely heavy rainfall. They also bring an associated 'storm-surge,' a rising of the sea level. It is estimated that 300,000 people died in Bangladesh because of the storm-surge caused by a cyclone in 1970 in the [M001 / Bay of Bengal].
Cyclones and flooding ruin crops, damage buildings, kill livestock and people, [P 009 & P010 / disrupt transport], close factories and cause [P 011 / major health hazards]. Unfortunately, it is very expensive to build adequate fortifications to limit the damage done by flooding, although governments are starting to put money into elevating buildings, zoning floodplains, building shelters and, most importantly, more accurate weather forecasting.
The winds associated with the monsoon can also be very dangerous for airplanes, as they can be gusty, unpredictable and very strong. The large cells of [G 05 / cumulonimbus] [P 014 / clouds] the monsoon brings can also generate dangerous amounts of turbulence, and airplane crashes are not uncommon in India during the monsoon.
