Geographical and meteorological explanation From October to December each year, a very large area of South India, including Tamil Nadu, the coastal regions of Andhra Pradesh and the union territory of Puducherry, receives up to 30 percent of its annual rainfall from the northeast monsoon (or winter monsoon). The northeast monsoon is the result of the annual gradual retreat of monsoonal rains from northeastern India. Unlike during the regular monsoon, rainfall during the northeast monsoon is sporadic, but typically far exceeds the amount produced by the regular monsoon by up to 90 percent. This excessive rainfall can be exacerbated by an El Nino year, which 2015 was.[16] The coastal districts of Andhra Pradesh usually bear the brunt of heavy rains that occur during the northeast monsoon; with numerous river systems and wetlands, Puducherry and eastern Tamil Nadu are prone to flooding.[16] The city of Chennai alone experienced five major floods between 1943 and 2005, with the 1943, 1978 and 2005 floods causing particularly severe damage.[17] In addition, unplanned and often illegal urban development has led to many wetlands and natural sinks being built over; this, along with ageing civic infrastructure and poorly designed drainage systems, has resulted in an increased frequency of severe flooding.[16] The weather systems On 8 November 2015, during the annual cyclone season, a low pressure area consolidated into a depression and slowly intensified into a deep depression before crossing the coast of Tamil Nadu near Puducherry the following day. Because of land interaction and high vertical wind shear, the system weakened into a well-marked low pressure area over north Tamil Nadu on 10 November.[18] The system brought very heavy rainfall over the coastal and the north interior districts of Tamil Nadu. On 15 November, a well-marked low pressure area moved northwards along the Tamil Nadu coast, dropping huge amounts of rainfall over coastal Tamil Nadu and Andhra Pradesh with 24 hour totals peaking at 370 mm in Ponneri. Chennai International Airport recorded 266 mm of rainfall in 24 hours. On 28–29 November, another system developed and arrived over Tamil Nadu on 30 November, bringing additional rain and flooding. The system dropped 490 mm of rainfall at Tambaram in 24 hours starting 8:30 am on 1 December. Very heavy rains led to flooding across the entire stretch of coast from Chennai to Cuddalore.