Urban heat makes Salt Lake City 3.8°C hotter than predicted Zoonar GmbH/Alamy

People living in cities are disproportionately affected by extreme weather, becoming exposed to higher than predicted temperatures during heat waves.

Leiqiu Hu at the University of Alabama in Huntsville and her colleagues analysed temperature and census data from 16 metropolitan areas in the US. They found that during heat waves people living in urban areas experienced temperatures an average of 1.9°C higher than what was forecast. The difference was highest in Salt Lake City, where urban temperatures were 3.8°C higher than predicted.

The discrepancy is significant considering that a heat wave is already associated with temperatures on average 3.6°C hotter than normal, says Hu.


Different areas within a city have temperature discrepancies throughout the day, which need to be taken into account to make accurate predictions about a population’s heat exposure. “If we consider the population movement and population distribution within a city, actually we have to add another 1.9°C to really represent the city’s exposure temperature to extreme heat,” she says.

Much of this was due to the urban heat island effect, in which metropolitan areas are warmer than rural areas as a result of human activity. Materials commonly used in urban areas for roads and roofing, for example, absorb more solar radiation than natural land surfaces.

This also has a warming effect in cold weather. The researchers found that during cold waves, urban areas were on average 0.6°C warmer than predicted.

The researchers estimated the temperature variability by combining weather and census data, including patterns of daily commuting within urban areas. They found that exposure to heat waves is more likely to be underestimated in spread-out cities such as Los Angeles, where the daily commute between residential areas and urban centres was associated with a large discrepancy in exposure temperature.

In future, city planners and policy-makers will need to factor in climate adaptation strategies to mitigate against exposure to extreme heat in urban areas, says Hu.

Journal reference: Science Advances, DOI: 10.1126/sciadv.aay3452