The Monsoon – a basic guide

“And then the rains came….” Which they seem to have done last night in Colombo – the Southwest monsoon making an on-time arrival.

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 administrators of the Raj described the budget of British India as a “gamble in rain”, as an estimated 70% of the Indian subcontinent’s rainfall comes from the monsoon.

Meteorology lesson – a short one

Tropical meteorology is a very different beast to the frontal system driven weather patterns of the world’s temperate regions. The weather forecast, which has traditionally followed the nightly news on TV channels all over world, popularized the frigid blue-pointed cold fronts and red-bubbled warm fronts. But down in the tropics, especially in South Asia, weather systems are very dissimilar.

As the earth’s tilt warms the mid-latitudes in the spring, warmer water moves north in both the Pacific and Atlantic Oceans, thereby distributing the warmth to the northern landmasses. The ‘inter-tropical convergence zone’ (ITCZ) an area of low pressure and rising winds, moves north and takes this warm moist air with it. The Indian Ocean, unlike its Pacific and Atlantic cousins does not extend all the way to the northern and southern icecaps as the Asian landmass gets 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 sub-continent, with no way of cooling. The rotation of the earth also causes a ‘coriolis effect’ that shifts the winds to the west; something named the ‘Hadley Circulation’ after the British scientist who first documented it.

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.following the ITCZ. The Himalayan mountain range makes the winds rise and draws moisture from the warm seas further northwards. A huge mass of warm, moist air starts flowing northward and approaches the subcontinent from both sides – across the Arabian Sea and the Bay of Bengal. The direction of the wind on west coast (from the south-west) gives the monsoon its English name, but Bangladesh and the east coast India also gets a huge amount of rainfall from the more eastern part of the same weather phenomenon.

The monsoon

The monsoon usually arrives in Sri Lanka in the last week of May. Kerala, on the Southwestern coast of India typically gets the monsoon about a week later 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 system causes the moist cool air to pass over the much warmer landmass. Being warmed from below, huge thunderstorms are formed in cohorts, which result in intense precipitation. As the thunderstorms come through one after the other rather than in isolation (as is usual in the tropics) 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 mind-boggling 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 sub-continent from the northeast towards the emptiness of the southern Indian Ocean. This is a mostly a dry monsoon, except for parts of southern India and Sri Lanka for which it brings much needed winter rains. It also produces rainfall in northern Australia, known as ‘The Wet’ in Darwin.

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 realization that they could use the winds to their advantage. Sailors in the region were already doing this for hundreds of years 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 more advanced 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 of Good Hope, around to the Atlantic and home.

The advent of steamships, 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 sub-continent 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 for pilots when torrential rain gathers on the runway making braking very difficult. A number of cases of aircraft “skidding” off wet runways occur every year during the monsoon.

The Indian Meteorological Department (IMD) is the primary organization tasked with researching and tracking the monsoon from its regional center in Kohzikode (Calicut). 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 sub-continent.

The prevailing winds from the South West can be recognized 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 Lankan, 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 has arrived on time and appears to be fairly normal. However, a late cyclone in the Bay of Bengal has led to widespread rain before the monsoon, so the possibility of flooding may be more acute this year.



2 responses to “Tired of COVID let’s talk about the weather”

  1. Peter Greenfield Avatar

    Very lucid discussion on the monsoon. Currently inter monsoon here in Penang, but the SW monsoon is on its way.

    1. Suren Ratwatte Avatar

      I don’t miss flying in the monsoon, but do love the warm rain! getting a bit cold here but has been a good couple of days. Hope you are well.

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