With each passing year, the city's expanding concrete jungle is adding to heat radiation. The maximum average temperature has shot up by 0.6 degree Celsius over the past three decades.

According to data from 1981-2010 analysed by the Meteorological Department, the number of hot days experienced in May is on the rise. Residents felt the intense heat in the 1960s too when the mercury level went beyond 40 degree Celsius. But, the trend prevailed only for a day or two during the month. Now, residents suffer the scorching heat for nearly a week, officials said. The situation has worsened in the past decade. During the '80s, the hot weather did not last as long as it does now

The city's average temperature for May has gone up to 37 degree Celsius. Though it is only a slight increase compared to the past three decades, it has a substantial impact on the weather, said Y.E.A. Raj, deputy director general of meteorology at the Regional Meteorological Centre.

Global warming and increased heat retention in urban areas owing to rising population and the growing number of vehicles are also cited as reasons for the rise in average temperature, Mr. Raj said.

However, the department continues to use the weather data from 1971-2000 to forecast weather patterns. The average maximum temperature derived from this period is 36 degree Celsius for Nungambakkam and 37 degree Celsius for Meenambakkam.

The department is awaiting data from National Data Centre, Pune, to incorporate it into daily weather forecasting.

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