From UPSC perspective, the following things are important :
Prelims level : Various terms related to Monsoon
Mains level : Indian Monsoon
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.
- 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.
- 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.