Published on: June 1, 2022

ONSET OF MONSOON

ONSET OF MONSOON

NEWS : The southwest monsoon has arrived in Kerala three days before its normal onset date of June 1

ONSET OF MONSOON

NEW DELHI: SW MONSOON
: PTI GRAPHICS(PTI4_15_2020_001020)
  • As per the India Meteorological Department (IMD) records, this is only the fourth time since 2010 when the monsoon has arrived well ahead of its normal date
  • The onset of the monsoon over Kerala marks the beginning of the four-month — June-September — southwest monsoon season over India, which brings more than 70 per cent of the country’s annual rainfall
  • According to the IMD, the onset of the monsoon marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, and the 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 PREVAILING CONDITIONS

RAINFALL

  • The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record 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
  • The 14 enlisted stations are: Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru

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

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) in the box confined by 5-10ºN latitude and 70-75ºE latitude

OTHER DETAILS

  • In general, the Andaman and Nicobar Islands start receiving monsoon rainfall between May 15 and May 20 every year, and it usually starts raining along the Kerala coast in the last week of May. However, the onset is not officially declared until the prescribed conditions (above) are met
  • 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
  • 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