The annual southwest monsoon in South Asia

The southwest monsoon is one of the oldest, most anticipated and economically important weather phenomena to affect the Indian subcontinent each year.

The onset of the southwest monsoon begins towards the end of May and moves from the southern tip of India and Sri Lanka, to the far northwest, Pakistan, Bangladesh, Bhutan and Nepal during the summer months through September.

It is one of the most tracked and studied weather patterns across any region of the world, as the rains are vital to the overall wellbeing of the people and the economy.

The word monsoon is derived from the Arabic “mausim,” meaning “season”, and really refers to the reversal of wind direction. The India Meteorological Department defines it as “the seasonal reversal of the direction of winds along the shores of the Indian Ocean, especially in the Arabian Sea, which blow from the southwest for half the year and from the northeast for the other half”.

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The monsoon delivers about 70 percent of India’s annual rainfall and is critical for crops such as rice, cane, corn, cotton and soybeans, as nearly half of the country’s farmland lacks irrigation. And with two-thirds of the 1.3 billion population relying on agriculture for their livelihood, India is heavily impacted by the annual monsoon rainfall.

Each year, the India Meteorological Department (IMD) issues a predicted forecast for the monsoon, or ‘the onset’, with specific dates for the arrival of the rains across the subcontinent.

Just this week, the IMD released an updated forecast, stating that the 2017 monsoon rains are expected to be “average” – an improvement over their initial April 18 forecast. India defines average, or normal rainfall, as between 96 percent and 104 percent of the 50-year average.

Each year before the annual southwest monsoon, temperatures soar across parts of India, Pakistan, Nepal, and Bangladesh. This year, the thermometer has regularly reached more than 40C across many cities and villages across India, with a highest temperature recorded so far of 47.2C at Banda in East Uttar Pradesh. The arrival of the monsoon rains brings not only much needed water for crops, but welcome relief from the searing heat.

Pre-monsoon showers and some thunderstorms have already arrived in certain dry areas across the south of India, bringing a welcome taste of what is, hopefully, soon to come.