Researchers also said heat waves have been prominent in recently on record. The research was published in the Institute of Physics Publishing's journal Environmental Research Letters. The research was undertaken by a group of researchers at the Indian Institute of Technology Gandhinagar (ITTGN), Northeastern University, University of California, Los Angeles and the University of Washington.

The researchers obtained daily observations for rain, air temperature and wind speed from data produced by the National Climatic Data Center (NCDC). Heat waves were defined as periods where the daily maximum temperature was hotter than 99 percent of days between 1973 to 2012, for a consecutive period of six days.

The results showed that there were statistically significant increases in the number of heat waves in urban areas during the last four decades. Out of the five years with the largest number of heat waves, four were the most recent years on record (2009, 2010, 2011 and 2012).

Results also showed a general decline in cold waves and around 60 percent of urban areas experienced a significant decline in extreme windy days.

Around 17 percent of urban areas experienced a significant increase in daily precipitation extremes and around 10 percent witnessed an increase in annual maximum precipitation.

Professor Vimal Mishra from IIT Gandhinagar said, “Our results show significant increases in heat waves and the number of hot days and warm nights and at the same time declines in cold waves and extreme windy days in many urban areas over the last 40 years.”

“Over half of the world's populations now live in urban areas; hence, it is particularly important to understand how the climate and climate extremes, in particular, are changing in these areas.
And, urban areas make up a relatively small part of the global land area; however, they are the centre of wealth, so damage to urban infrastructure could result in potentially large economic losses,” he added.

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