The average annual rainfall of Mumbai is 2504mm. 70% of this rainfall occurs in July and August with 50% occurring in just two or three events. It has been observed that during severe flooding or inundation, it rains almost uniformly over the city. Inundation depths of 0.5m to 1.5m are common in low-lying areas.
The municipal area is highly susceptible to frequent flooding, and witnesses severe disruptions every year. Any disruption due to flooding results in economic and social disruption and loss of livelihood to the individuals and loss of business to commerce and industry.
The Santa Cruz observatory at Mumbai airport recorded 944mm during the 24 hours ending 08:30 on 27th July 2005. Santa Cruz recorded 481.2mm in four hours between 14:30 and 18:30 and hourly rainfall exceeding 190mm/h between 14:30 and 15:30.
The extremely high rainfall resulted in overflows from the already inadequate drainage system, and it was unable to drain out to the sea because of the maximum high tide level of 4.48m at that time. Over 60% of Mumbai was seriously inundated and the city came to a complete halt.