Weathermen attribute the cool temperature at nights to radiation cooling which occurs because of zero clouding in the day.
"Sun's radiations that reach the earth's surface are trapped by clouds. Since there is little or no clouding during the day, radiation escapes into the atmosphere, paving the way for colder nights," said Regional Meteorological Centre former deputy director general Y E A Raj.
The minimum temperature fell to about 15 degrees Celsius in parts of northern Tamil Nadu like Tirupathur and Tiruttani on Tuesday.Nungambakkam recorded 20.6 degrees on Tuesday and 20.5 degrees Celsius on Monday.
Raj said, "As Chennai lies near the coast, there is moisture from the sea which helps trap some of the radiation. If you move away even 15-20km from the coast and towards the interior districts, the minimum temperature begins to fall lower." At the Meenambakkam centre, which lies away from the coast in the southern suburb of the city , the minimum temperature recorded was 18.4 degrees Celsius on Tuesday .
Raj said that the drop in temperature was not unusual during the months of January and February .
The maximum temperature in the city has gone up as far as 33 degrees Celsius, while the minimum temperature has fallen to nearly 20 degrees Celsius over the last week, officials said.