More than a silver lining

Light reflected by clouds can brighten air kilometres away.

There's more to clouds than meets the eye. Credit: Punchstock

A study looking at why clouds make the air near them glow more brightly suggests climate models may need to be revised.

Atmospheric scientists already account for the brighter air close to clouds, thanks to a 2007 study1 by Ilan Koren and his colleagues at the Weizmann Institute of Science in Rehovot, Israel. The team showed that cloud droplets, attached to dust and smoke particles, float in a halo kilometres wide around clouds, bouncing sunlight back out of the atmosphere. Seen from a satellite, that means air close to clouds looks brighter.

But Tamás Várnai and Alexander Marshak of NASA's Goddard Space Flight Center in Greenbelt, Maryland, have found that a related factor has been left out of the current generation of algorithms used to interpret satellite images2.

Using observations made by a pair of NASA satellites, the researchers identified evidence of an effect called three-dimensional radiative interaction, in which sunlight is reflected horizontally by clouds and scattered from nearby air molecules back out of the atmosphere.

Clouding the issue

The effect was a theoretical possibility, but nobody knew whether it was strong enough to be worth accounting for in satellite image-processing algorithms.

"We showed that actually this effect cannot be neglected and makes up a significant fraction of the enhanced radiation" near clouds, Marshak says.

Aerosols — small particles in the air — are thought to affect local temperatures, and are one of the least understood parts of climate models. They can come from natural processes such as volcanoes or forest fires, but also from human pollution.

They can have a cooling effect by "scattering sunlight back out to space", explains Koren. But they also "absorb some heat and can therefore warm air locally and change circulation".

To detect aerosols, atmospheric scientists measure the brightness of the unclouded pixels in their satellite image and subtract the amount of light reflected from the surface of Earth, attributing much of the remainder to aerosols.

Koren's findings prompted researchers to treat the region around clouds as extensions of the clouds, with declining brightness over a distance of many kilometres. Still, much of the brightness was attributed to the presence of aerosols, with the rest attributed to increased humidity and some instrumentation errors.

Blue-sky approach

But Várnai and Marshak found that air molecules actually have a much larger contribution. They found that the light reflected from the cloud's halo had a bluer tint than should have been the case if sunlight was being reflected equally. The bluish tint implies that only small atmospheric molecules, and not the larger aerosols, were responsible for the light reflected back out of the atmosphere.

"They showed that the short light waves have significantly more three-dimensional effects than the longer waves," adds Koren.

Leaving such three-dimensional radiative interactions out of the equation might mean that scientists are overestimating the amount of aerosols in the sky, throwing off their predictive models.

Várnai and Marshak "are addressing an issue that we have been stuck with for many years", says Olivier Boucher, a climate scientist at the UK Met Office. The findings "will have a direct impact on studies of aerosol's indirect effects" in climate models, he adds. The assumption that three-dimensional radiative interaction was negligible "has been widely made by everyone and has not been questioned enough".


  1. 1

    Koren, I., Remer, L. A., Kaufman, Y. J., Rudich, Y. & Martins, J. V. Geophys. Res. Lett. 34, L08805 (2007).

  2. 2

    Várnai, T. & Marshak, A. Geophy. Res. Lett. 36, L06807 (2009).

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Tamás Várnai

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Laursen, L. More than a silver lining. Nature (2009).

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