Urban heatwaves have become more frequent over the last 40 years, scientists reported on Friday.

A weather database of cities around the world reveals "significant" increases in periods of extremely hot days and falls in the number of cold days, they found.

Previous research found that, in the four decades covered in the study, man-made global warming stepped up a gear.

But, in urban heatwaves, additional factors can play a role, the authors cautioned.

These include local climate variability, the design and spread of a city, and land cover beyond it.

"Over half of the world's population now live in urban areas," lead author Vimal Mishra, a professor at the Indian Institute of Technology (IIT) Gandhinagar, said.

"It is particularly important to understand how the climate and climate extremes, in particular, are changing in these areas."

The data trawl covered around 650 cities, defined as areas with a population greater than 250,000, with weather stations that reported to a US-run meteorological report called Global Summary of the Day (GSOD).

The researchers were left with 217 cities for which there were complete records for 1973-2012.

Heatwaves were defined as periods lasting six days consecutively or more -- their daily maximum had to be greater than 99 percent of the temperatures recorded at that time of the year, as measured over the whole 40 years.

From 1973-2012, the number of heatwaves per urban area rose by 0.3 of an event, a "statistically significant" increase, the researchers said.

Of the five years with the most heatwaves, four occurred in the most recent years -- 2009, 2010, 2011 and 2012.

Over the 40 years, more than half of the weather stations reported an increase in the number of individual extreme hot days.

And almost two-thirds showed significant increases in the number of individual extreme hot nights.

The five years with the largest number of cold waves were towards the start of the study period -- 1973, 1974, 1976, 1981 and 1983.

The new study appears in a British journal, Environmental Research Letters.

From 1900 to 2003, the number of urban areas with more than one million population increased from 17 to 388, it said.

A 2008 estimate found that more than half of the world's population were urban dwellers. This proportion is expected to rise to 60 percent by 2030 and 70 percent by 2050.

In its landmark Fifth Assessment Report, the UN's Intergovernmental Panel on Climate Change (IPCC) last year said it was "likely" that heatwaves had already become more frequent in Europe, Asia and Australia, although it did not filter out urban from non-urban heatwaves.

The IPCC said it was "very likely" heatwaves would become more frequent and last longer in the future, with big implications for health, businesses and urban design.

Cities that are vulnerable to heatwaves often suffer from a phenomenon called urban heat island.

Part of the cause is heat which is stored during the daytime in concrete buildings and tarmacked roads, and released at night.

As a result, a heatwave easily turns the city into a 24-hour hotspot, for it fails to cool down adequately after sunset.