Although not as strong as the urban heat island effect, measurements have shown that cities can be up to two degrees cooler than their rural surroundings on some mornings. Until now, the cause of this cooling has not been clear, and explanations have ranged from the shadowing effect of tall buildings, to differences in land cover altering the surface energy balance, or cooling due to higher aerosol concentrations in the air.

One hypothesis is that the cool island effect is linked to the differing dynamics in the atmospheric boundary layer in cities and rural areas. This turbulent layer of air, closest to the Earth's surface, generally cools and is shallower at night, reaching a typical thickness of just 100m in rural areas.

But over urban areas the atmospheric boundary layer is often much thicker at night (around 400m) due to the release of heat stored during the day by buildings and surfaces. The theory suggests that when the sun comes up in the morning, it takes longer to heat the deeper boundary layer over the city, meaning that the countryside warms up more quickly. Measurements suggest the effect tends to peak around four hours after sunrise and can last until the early afternoon.

To test whether the atmospheric boundary layer differences were contributing to the urban cool island effect, Natalie Theeuwes from Wageningen University in The Netherlands and her colleagues coupled a land-surface model with an atmospheric boundary layer model and input summertime measurements from the Swiss city of Basel.

Sure enough, the team’s model showed that the urban cool island effect could be generated by differences between the early morning atmospheric boundary layer depth over the city and surrounding countryside. And certain conditions made the chances of an urban cool island developing more likely. "The weather conditions that promote a strong urban cool island are basically the same as the ones that promote a strong urban heat island – low wind speed and few clouds," said Theeuwes.

The magnitude of the urban cool island and its duration depend strongly on the shape, height and layout of buildings, and the amount of vegetation. Without a doubt the atmospheric boundary layer has a significant impact on urban climate, and Theeuwes and her colleagues stress the importance of including these boundary layer effects in urban climate studies.

What’s more, the researchers think the results have implications for urban planning, health and air quality. "The strong link between the urban cool island magnitude and the urban morphology indicates that the urban cool island can be employed as an efficient tool in urban planning and health," they write in Environmental Research Letters (ERL).

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