The Indian Meteorological Department has said the onset of monsoon over the Kerala coast will happen on June 6, six days later than the normal date of June 1. The onset marks the beginning of the four-month, June-September monsoon season over India, which brings over 70 per cent of the country’s annual rainfall.
The time of onset does not have any bearing on the quality or amount of rainfall during the monsoon season. It is just an event that happens during the progress of monsoon over the Indian subcontinent. The onset is announced when certain criteria is fulfilled anytime after May 10. If at least 60 per cent of the 14 designated meteorological stations in Kerala and Lakshadweep report at least 2.5 mm of rain for two consecutive days after May 10 — and a few other conditions relating to wind and temperature are also fulfilled — the onset of monsoon is said to have taken place.
The Andaman and Nicobar Islands generally start getting monsoon rainfall between May 15 and 20, and the Kerala coast generally begins to have monsoon rainfall sometime in the last week of May. However, if the prescribed conditions are not met, the onset is not declared.
A delayed onset has the potential to delay the arrival of monsoon to other parts of the country as well, especially in states in south India which start getting rainfall within days of it reaching the Kerala coast. However, it does not mean that the arrival of monsoon over the entire country would be delayed.
The northward progression of monsoon, after reaching the Kerala coast, depends on a lot of local factors, including the creation of low pressure areas. It is possible, therefore, that despite the onset happening late, other parts of the country start getting rains on time. After its onset over Kerala, the monsoon spreads over the entire country by July 15.
The overall amount of rainfall that India gets during the four-month monsoon season, and the regional distribution of rainfall, is not influenced by the date of the onset of monsoon.
India as a whole receives about 89 cm of rainfall in a normal monsoon season.