The India Meteorological Department (IMD) announced on May 13 that the southwest monsoon had advanced to the Nicobar Islands on the day, apart from some parts of the south Bay of Bengal and parts of the Andaman Sea. What does the arrival of monsoon mean? More importantly, how does the IMD decide that monsoon has arrived at a particular place? Here are some charts and maps that try to answer these questions.

What does the arrival of the monsoon mean?

The southwest monsoon season, which runs officially from June to September, is the rainiest period for India as a whole and most of its regions. The reason why these four months are rainy is that large scale atmospheric patterns support a rainy weather. These patterns usually develop on the Kerala coast on June 1, cover the entire country by July 8, and retreat from north-western regions of India on September 30. This is why the season officially runs from June to September. However, the actual date on which these patterns develop/dissipate over particular regions is the day on which monsoon is said to have arrived over/retreated from that........

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