Clouds , Nature reminds us, “ are one of the biggest sources of uncertainty in efforts to measure and predict global warming. They have two opposite effects: increasing warming by absorbing heat radiated from the planet's surface (which is why cloudy nights are warmer), while offsetting this by reflecting sunlight back into space from cloud tops...modeling studies typically try to distinguish between cloudy and cloud-free regions of the atmosphere. “
But the distinction between fair sky and cloudy has just gotten un-clearer. A paper by Ilian Koren et al in Geophysical Research Letters. reports that digitized images reveal an invisible nimbus of damp but not dripping aerosols can extend kilometers beyond the limits of what the eye can see- it takes the equivalent of a Photoshop contrast boost to reveal the tenuous halo that extends much further than previously imagined.
"People may have seen these extended haloes anecdotally," Lorraine Remer of the NASA Goddard Space Flight Center told Nature. "But thanks to a new generation of instruments, the satellite observations have got much better, and we can look on larger scales, with more sensitivity and at finer resolution."
Satellite images of clouds over the Atlantic Ocean show that the sky's reflectance falls very gradually with increasing distance from cloud edge for at least 20-30 kilometres away, Koren's team says. The twilight zone study also examined years of images from a global network of ground-based lightmeters called AERONET, usually used to monitor solar brightness . estimate that for typical global cloud coverage, the halo could encompass as much as two-thirds of the sky usually classed as cloud-free.
Some climate models 'grow' such halos automatically if they do a good job of capturing humidity variations . But simpler models might neglect the effect., leading to suspicions that the overall cooling effect of aerosols may have been underestimated. But NASA admits that it is too early to say how significant an impact it might have on climate predictions.
"Right now there is a discrepancy between what global models predict for aerosol effects and what satellites measure," says. Lorraine Remer of the NASA Goddard Space Flight Center "This might be part of the reason for that."