As people around the world swelter in rising heat hitting new highs, spare a thought for those living in city centres - where research has found that temperatures can be considerably higher than in nearby green spaces or surrounding rural areas.

This phenomenon is known as the urban heat island (UHI) effect - where dense clusters of concrete buildings and infrastructure such as roads absorb, retain and radiate more heat than green areas, leading to temperatures that are often several degrees hotter.

A recent study of six cities - Cairo, London, Los Angeles, Madrid, Mumbai and New York - found that temperature variations between the hottest and coolest spot in each city ranged from 4.5 degrees Celsius (8 degrees Fahrenheit) in New York to 8°C (14.5°F) in Madrid.

An increasing number of people in major cities are enduring “worrying levels of heat”, according to researchers at Arup, the sustainable development consultancy behind the study.

“We’ve inadvertently designed many of our cities to be hot,” said Dima Zogheib, a design lead at Arup. “We’ve pushed out nature – concreted our streets, built high in steel and glass.”

Meanwhile, nonprofit Climate Central separately studied 44 US cities and found that more than half of the residents analysed lived in areas with a UHI index of 8°F (4.5°C) or higher.

This means, for example, that on a day when temperatures in a park outside a city are 90F (32.2°C), it would feel like 98F (36.7°C) or higher for downtown residents.

“In some cities, the temperature is as much as 12°F (6.6°C) warmer … on top of a temperature that is already rising because of climate change,” said Jen Brady of Climate Central.

About 200 million urban-dwellers across more than 350 cities now experience summer temperature highs of at least 35°C (95°F), and the number of cities exposed to extreme temperatures will triple by 2050, according to the C40 Cities network.

Here is what you need to know about urban heat islands:

What exactly is an urban heat island?

From buildings to roads, urban infrastructure has been designed with materials that make cities warmer.

Unlike asphalt and concrete, vegetation including grass and trees absorb less heat and release more water into the air.

Emissions from commercial and residential buildings that use air conditioning also contribute to the heat, as do vehicles.

Read the full article about urban heat islands by Thomson Reuters Foundation at Eco-Business.