How’s the heat in Phoenix?

British, Swiss and US scientists have devised a new model that explains how the undesirable effects of urban heat islands vary across seasons.

Unlike existing models, which require lots of information and are computationally demanding, this “coarse-grained” approach provides general insights into how changes in rainfall, solar radiation and vegetation conditions affect the intensity and timing of surface urban heat islands at a city-wide scale.

“With just two equations our model can describe all these complex interactions,” says Gabriele Manoli from University College London, lead author of a paper in the journal Proceedings of the National Academy of Sciences.

And it provides new evidence, Manoli says, that seasonal variations in the intensity of urban-rural surface temperature differences – which until now have been observed but not clearly explained – are controlled by time lags between solar radiation, temperatures and rainfall.

If solar radiation occurs in conjunction with water availability, summer conditions cause strong surface urban heat island intensities due to high rates of evaporative cooling in surrounding rural areas. This is typically what we see in cities like Paris or London, which have relatively wet summers.

But in cities where rainfall is scarce during summer, such as Phoenix or Madrid, the opposite can occur. With less rainfall and vegetation to spur cooling, rural areas heat up and the city experiences an “oasis effect” in which, though it may still be blisteringly hot, it’s nonetheless one or two degrees cooler than the surrounding countryside.

These seasonal patterns of warming and cooling have significant implications for heat mitigation strategies.

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