The monsoon has reached Kerala, the India Meteorological Department (IMD).

What does the “Onset of Monsoon” mean?

• The onset of the monsoon over Kerala marks the beginning of the four-month — June-September — southwest monsoon season over India.
• It brings more than 70 per cent of the country’s annual rainfall. This marks a significant day in India’s economic calendar.
• IMD announces it only after certain newly defined and measurable parameters, adopted in 2016, are met.
• Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed.

(1) Rainfall

• The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep.
• The 14 enlisted stations are: Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru.
• It records at least 2.5 mm of rain for two consecutive days at any time after May 10.
• In such a situation, the onset over Kerala is declared on the second day, provided specific wind and temperature criteria are also fulfilled.

(2) Wind field

• The depth of westerlies should be up to 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE.
• The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.

(3) Heat

• According to IMD, the INSAT-derived Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watt per sq m (wm2).
• This is measured in the box confined by 5-10ºN latitude and 70-75ºE latitude.

Is it unusual for the monsoon to hit the Kerala coast early?

• Neither early nor late onset of the monsoon is unusual.
• In 2018 and 2017, the onset over Kerala occurred on May 29 and May 30, respectively.
• In 2010, onset was realised on May 31.
• In 2020 and 2013, the monsoon was exactly on time, hitting the Kerala coast on June 1.

Does an early onset foretell a good monsoon?

• No, it does not — just as a delay does not foretell a poor monsoon.
• The onset is just an event that happens during the progress of the monsoon over the Indian subcontinent.
• A delay of a few days, or perhaps the monsoon arriving a few days early, has no bearing on the quality or amount of rainfall.

How does the monsoon spread across the country after hitting Kerala coast?

• The northward progression of the monsoon after it has hit the Kerala coast depends on a lot of local factors, including the creation of low pressure areas.
• Though this year monsoon has arrived early, it is possible that despite a late onset over Kerala, other parts of the country start getting rain on time.
• After its onset over Kerala, the monsoon spreads over the entire country by July 15.

Back2Basics:

Various terms related to Indian Monsoon

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