Clouding and the early onset of sea breeze

Chennai: Northern parts of the State, which experienced heat wave conditions for nearly two weeks this month, seem to have crossed the peak summer phase.Clouding and the early onset of sea breeze have brought down the mercury levels. A maximum temperature of 40 degree Celsius and above was recorded almost everyday from May 7.

The northern belt of the State is usually warmer than the south. Northern districts, particularly coastal areas, experience peak summer during the second half of May. The Nungambakkam observatory recorded the highest temperature of 45 degree Celsius on May 21, 1910. Four years ago, the observatory recorded 44.5 degree Celsius on May 22.

Two factors are responsible for higher temperatures in the northern parts. Firstly, higher latitudes mean higher radiation. Secondly, the intensity of the heat is higher as the northwesterly component is greater.This year, the cyclonic storm, Akash, that crossed the coast near Chittagong, Bangladesh around May 15, was also responsible for the heat conditions, as it triggered advection.Taking Chennai (Nungambakkam) and Vellore as examples of the northern parts of the State, a meteorologist said while Chennai recorded a maximum of 43 degree Celsius on May 15, Vellore registered 43 degree Celsius thrice — May 16, 18 and 19.It was a departure of nearly seven degree Celsius in the case of Chennai, while it was in the range of four-five degree Celsius with respect to Vellore.

The meteorologist points out that a distinction has to be made between Chennai and Vellore with regard to the maximum temperature recorded on any given day even if the value is the same. As the benefit of sea breeze is not available in the case of the latter, the maximum temperature recorded in Vellore persists for a longer duration.But in the case of the former, it lasts only until the onset of sea breeze. (Usually, sea breeze travels at a rate of 10 km per hour. By the time it reaches Vellore, about 130 km from Chennai, most of it gets dissipated.)During this time of the year, the normal time of the onset of the sea breeze is 1 p.m. in Chennai. On Friday, the onset took place at 11.45 a.m.