Months of January and February are considered as  winter period .  Extented effects of the  'North  East Monsoon activity' in the south  and the 'Western Disturbances' in the north of the country are the features of this period.
                       
                        The extra- tropical low pressure systems passing through the northern  part of the country from west to east, mainly during November to April ,is termed as 'Western Disturbances. Winter rains and severe cold  waves  are the result of the movement  of these systems.

                       In Tamil Nadu January is the coldest month when the daily minimum temperature for the state as a whole is  21C varing from about 16C in the north to about 24C in the south.   The lowest temperature ever recorded  at an individual station in the plain is 10.2C. at  Tirupattur on 15th December 1974 which is 5.9C lower than the normal of the coldest month.  In the ghat areas Uthagamandalam registered the  lowest minimum  temperature of -2.1C on 7th January 1976 which is 7.3C lower than the normal of the coldest month.