White is a cool colour. In California flat-roofed buildings are now required to have a white roof, to reflect sunlight and help minimize global warming. Other places are thinking of following suit, but are these white roofs really helping to keep the planet cool?

Last year white roofs sparked controversy when research published in the Journal of Climate suggested that the extra reflected light from widespread use of white roofs may decrease cloud cover, thereby increasing rather than decreasing the urban heat-island effect. But not all white roofs cause this problem. Now an experimental study carried out in India has shown that, in some locations at least, white roofs do reduce absorbed radiation.

Francisco Salamanca, from Lawrence Berkeley National Laboratory, US, and colleagues compared upper-atmosphere radiation above freshly painted white roofs and unpainted roofs in two regions of north-east India: G B Pant University in Pantnagar and the Aryabhatta Research Institute of Observational Sciences in Nainital, both in the central Himalayan region.

Satellite images taken over five cloudless days show a mean increase of 50 Watts per square metre of outgoing shortwave radiation at the top of the atmosphere for every 0.1 increase in albedo (reflectivity) of the underlying roof. They also indicated that the cooling effectiveness depended on how polluted the atmosphere was, with a higher aerosol load reducing atmospheric transmissivity. Nonetheless, even in highly polluted air, a significant fraction of radiation was reflected back to space. The findings are published in Environmental Research Letters.

In addition to reflecting radiation, white roofs offer other benefits. In particular they keep buildings cooler, reducing the need for air conditioning. This reduces electrical power usage, and hence pollutant emissions from power plants.

Salamanca and colleagues' measurements matched model predictions for the amount of radiation transmitted back to space to within 10%, helping to boost confidence in the model results. Previous estimates have suggested that widespread application of reflective urban surfaces could reduce warming by the equivalent of two years' worth of carbon-dioxide emissions. However, this calculation does not take into account the impact that white roofs may have on clouds.

"Recent models have shown that increasing surface reflectivity can potentially affect atmospheric turbulence and cloud formation," said Marc Fischer of Lawrence Berkeley National Laboratory. "These processes can affect regional reflection from clouds, and in some cases reduce precipitation, so the overall environmental benefit of reflective roofs needs to be evaluated on a regional basis."

There are many locations, including north-east India, where white roofs appear to be an obvious choice. "Low-latitude sites with clear sky conditions will produce the most radiative benefit," said Fischer. Future studies will help to resolve the issue of whether white roofs affect clouds, but for now, people living in sunny locations can be confident that reflective roofs are a good thing.