Monsoons in Coimbatore

The locality where the Agricultural College and Research Institute is situated is within ten miles from the "Palghat gap", which is about forty miles from the Arabian sea. It is purely an inland area with mountains on the north, west and southern sides. The Vellingiri hills on the west, within a distance of about twelve miles, function as a rampart against the South-west monsoon. The daily rainfall data collected in the observatory, attached to the Agricultural College and Research Institute, for a period of fifty years from 1907 have been critically examined to assess the probable dates of the onset, duration of activity and the nature of pattern of rainfall during the south-west, and the north-east, monsoon periods of June to September and October to December respectively. With the details collected regarding the dates of onset of the monsoons, an onset diagram was prepared to bring out the probable periods of the onset of these two monsoons. The cyclic nature of the performance of these two monsoons has been assessed by adopting the Centre Shift Average Method for periods of one, two, three etc. up to eighteen years. In addition, the relationship between the patterns of rainfall in these two monsoons and their individual relationship with the annual pattern of rainfall were also worked out. Incidentally the nature of the break periods between the two monsoons has also been examined.