Cooler nights in city flummox researchers

With rapid urbanisation and population boom, the minimum temperature usually tends to increase.

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According to a study, the night time (minimum) temperature in Mumbai has dropped over the century, as contrary to other metros like Delhi, Kolkata and Chennai.

The study, Long-term temperature trends at the four largest cities in India during the 20th century, was commissioned by the University Grants Commission. But what has flummoxed researchers and climatologists is the fact that with rapid urbanisation and growing population, the minimum temperature tends to increase.

According to Amit Dhorde, a lecturer at the department of geography, University of Pune, and Anargha Dhorde, a lecturer at the Nowrosjee Wadia College in Pune, while other metros indicated a clear rise in the minimum temperature, in Mumbai, it was the reverse.

“The decrease was observed during all seasons, but it was significant during winter and the monsoon,” said Amit Dhorde. The 100-year data for the study were collected from the Indian Meteorological Department (IMD).

Dhorde said although the data has been taken till 2000, the trend would remain the same in the past nine years too.

As per the data, the annual minimum temperature in Mumbai has dropped by 0.4 degree Celsius. During the summer months, the drop is almost by 0.28 degrees Celsius, while in the monsoon months, it is 0.3 degrees Celsius. In the post monsoon months, the drop in the minimum temperature is 0.32 degree Celsius. The maximum drop was noticed during winter (0.55 degree Celsius).

Interestingly, in Delhi, Kolkata and Chennai, the minimum temperature increased by 0.99, 0.72 and 0.54 degree Celsius respectively.

The study was published in the International Journal of Climate Change, The Journal Of Indian Geophysical Research Union, and presented at the International Scientific Congress on Climate Change held at Copenhagen in March this year.

“Given the rapid pace of urbanisation, the night time (minimum) temperature in Mumbai should have increased. However, it is a paradox for which one needs to do a detailed analysis of the cloud and frequency of winds coming from a particular direction. We also need to study the sea surface temperature as Mumbai is located on the coastal belt,” said Dhorde.

He said the next phase of the study would involve an attempt to understand this variation.
Director of the Indian Institute of Tropical Meteorology (IITM) BN Goswami said there is a need to do a detailed analysis of the local factors such as cloud cover affecting radiation, and soil moisture. “Factors such as increase in the sea surface temperature also to needs to be examined,” he said.

Deputy director general (weather forecasting) of India Meteorological Department (IMD) AB Majumdar said “One needs to do a detailed analysis of the factors like cloud cover, land and sea breeze.”
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