The dense vegetation in the Western Ghats determines the amount of rainfall that Tamil Nadu gets during the summer monsoon.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/western-ghats-forest-cover-vital-tamil-nadu%E2%80%99s-south-west-monsoon-rainfall
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/western-ghats
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forests
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monsoons
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/indian-institute-technology-iit
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/tamil-nadu