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The Monsoon: a basic guide


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The Southwest Monsoon is one of the most interesting meteorological events in the world. It is also, literally, the single biggest indicator of economic performance for the region, as most of South Asia’s farmers are reliant on the monsoon for water. 

The failure of the monsoon can be a catastrophe that has, in the past, led to terrible famines in the region. The budget of British India was described as a “gamble in rain” by the administrators of the Raj, as an estimated 70% of India’s rainfall comes from the monsoon.

The meteorology 

The monsoon usually arrives in Sri Lanka in the last week of May. Kerala, on the South-western coast of India typically gets the monsoon in early June, and it then proceeds to march up the coast, reaching Mumbai (Bombay) by early July and the northern plains by late July. It is preceded by a long period of heat and humidity with some few thunderstorms to bring relief. The meteorological basis of the monsoon is quite complex and in some ways unique to our region. 

The Indian Ocean, unlike its Pacific and Atlantic cousins, does not extend all the way to the northern icecap. The Asian landmass and the Himalayas get in the way. Thus, as the ocean warms with the summer tilt of the earth, the warmed water has no place to go as it butts up against the subcontinent, with no way of cooling.

As the sun moves north it heats up the vast plains of India, causing a low pressure system to form, which in turn causes the wind to blow from the ocean northward toward the land. The Himalayan mountain range makes the winds rise and draws moisture from the warm seas further northwards.

The system causes the moist air to pass over the much warmer landmass. Huge thunderstorms are formed in cohorts, which result in intense precipitation. As the thunderstorms come through one after the other rather than in isolation, the rain can persist for longer than usual, causing much grief for the unfortunate aviators who fly in this season. 

The intensity of the rain is often mindboggling with Mumbai experiencing a record 37 inches of rain (over one meter) in a single day, at the start of the monsoon in 2005.

In the cooler months of the year, the winds reverse, blowing from the high pressure of the subcontinent from the north-east towards the emptiness of the southern Indian Ocean. This is mostly a dry monsoon, except for parts of southern India and Sri Lanka for which it brings much needed winter rains.

Some history

The word monsoon is a mash-up of the Arabic mawsim(season) and Portuguese Monção. The Portuguese, the first Europeans to experience it, were initially astonished by the change in winds and the intensity of the rain, used as they were to the ‘prevailing westerlies’ that are the norm outside the tropics.

Initial surprise soon turned to the realisation that they could use the winds to their advantage. This was already being done by sailors in the region of course, where east-west traffic across the Bay of Bengal and the Arabian Sea followed monsoonal patterns. The Europeans with their bigger ships and better navigational techniques were able to put the monsoon to good use in the global trade system that was being developed.

Once around Cape Town in Southern Africa, the ships would use the summer monsoon to make a rapid passage to Galle and onto Batavia (modern Jakarta). In the winter months, once the winds reversed, passage back to Cape Town would be easier. Typically the Dutch ships would leave Batavia in late November and stop for provisions in Galle. 

Then, after the Christmas services, the next leg would be with the winds towards the Cape, around to the Atlantic and home. The advent of steamships of course, changed the whole equation and took the navigational importance of the monsoon away.

The monsoon today

As the provider of the most rainfall to the subcontinent the monsoon retains its historic importance for agriculture. As the impetus for trade and commerce though, its importance has diminished. However, both sailors and aviators treat the monsoon season with a lot of respect, as the accompanying weather makes their jobs more challenging. 

This is particularly so when torrential rain gathers on the runway making braking very difficult. A number of cases of aircraft ‘skidding’ off wet runways occurs every year during the monsoon.

The Indian Meteorological Department (IMD) is the primary organisation tasked with researching and tracking the monsoon. The IMD has recently switched away from a purely statistical, or historical, method – which failed to predict the failed monsoons of 2002-2006. Since 2015 they have been using a dynamic model, powered by super-computers, to better predict the onset and duration of this vital resource to the subcontinent.

The prevailing winds from the South West can be recognised by observers in Sri Lanka, from the fact that aircraft take-off and land into the wind, towards the sea. The shorter ‘winter’ months are when departures are towards the North East, and aircraft landing at Katunayake can be seen approaching the runway over the Negombo lagoon.

Sri Lanka, being an island, gets more precipitation from the ‘inter-monsoonal’ rains later in the year, but the failure of the monsoon is still a significant economic factor. This year’s thankfully, appears to be fairly normal. It has just reached Mumbai, more or less on schedule and is expected to bring sorely needed water to the parched plains of northern India in the coming weeks.


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