India, May 13 -- 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.

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, cov...