Ting Sun from Tsinghua University in Beijing, China, and colleagues analysed observations from two flux towers, one in Beijing and one situated in farmland about 50 km outside of the city, to investigate why urban areas magnify heatwaves. Focusing on the summer season – June, July and August – during 2009 and 2010, the researchers collected measurements of radiation, turbulent flux, temperature and humidity.

As expected, the results showed that during heatwaves temperatures were much higher in both central Beijing and the rural location, compared with non-heatwave periods. More interesting, however, is that the increases in urban temperatures are much higher than the increases in rural temperatures, particularly at night. The reason for this disparity can be largely explained by city layout (streets of tall buildings acting as heat-trapping canyons), building materials (tarmac and concrete absorbing heat without evaporating), and human activities (air-conditioning and refrigeration releasing yet more heat).

"During night-time, the excessive heat will be released and thus causes a greater urban heat-island effect," said Sun.

Comparing the measurements from the two towers showed that the urban site received more incoming radiation than the rural site, and that more sensible heat flux was observed at the urban site under heatwaves. In particular, the team noticed that the latent heat flux was higher at the rural site, due to the availability of water (used to irrigate crops). Meanwhile, heat storage was greater at the urban location, causing larger heat releases into the atmosphere at night during heatwaves. The findings are published in Environmental Research Letters (ERL).

As climate warms, the problem of heatwaves is likely to worsen, with cities with humid climates being most vulnerable to the enhanced heatwave effect. Even small increases in temperature can be significant. Previous epidemiological studies have found a 4.5% increase in mortality risk for every 1 °C increase in heatwave intensity, and a 0.38% increase for every one-day increase in heatwave duration in the US.

But there are ways of reducing the impact of heatwaves on urban areas. "Installing green roofs and planting trees, which increase the water availability in cities, are strongly needed to alter the surface-energy partitioning by increasing evapotranspiration," said Sun. Painting roofs white, to reflect more solar radiation, has been shown to be effective too.

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