Statistics relating to the date of onset of the southwest monsoon over Kerala for the 100-year period 1891-1990 reveal that the mean and median dates of onset for south Kerala are 31 May and 1 June, with a standard deviation of 8.5 days. Declaring the date of monsoon onset is not a straightforward matter. However, rainfall pattern, uppper air circulation features and INSAT cloud pictures are useful guides, and these indicate that the onset date in 1990 esd 17-18 May. During 1891-1990 there have been onlyy 12 years in which the date of onset over Kerala has been on or before 18 May.
Original Source [2]
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/onset-southwest-monsoon-1990
[2] http://moeseprints.incois.gov.in/3797/1/onset%20of%20southwest.pdf
[3] http://admin.indiaenvironmentportal.org.in/category/author/r-ananthakrishnan
[4] http://admin.indiaenvironmentportal.org.in/category/author/m-k-soman
[5] http://admin.indiaenvironmentportal.org.in/category/journal/current-science
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monsoons
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/india
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/kerala
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rainfall-pattern
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/indian-meteorological-department-imd
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/weather-predictions
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/remote-sensing