‘Halo effect’ causes formation of unusually bright patches of sky

March 31st, 2009 - 1:22 pm ICT by ANI  

London, March 31 (ANI): A new research has shown that unusually bright patches of sky, observed up to several kilometers away from clouds, are a result of the ‘halo effect’, which is light reflected off the cloud and bouncing off the particles in the air.

This seemingly innocuous finding could have a surprisingly big knock-on effect because it means there may be fewer cooling particles in the sky than previously thought, and that could change the way climate change is modeled.

According to a report in New Scientist, to discover why the air near clouds appears so aglow, Tamas Varnai and Alexander Marshak at the NASA Goddard Space Flight Center in Greenbelt, Maryland, used MODIS satellite observations from a piece of sky above the Atlantic just southeast of the UK.

To work out the amount of particles suspended in the air, Marshak’s team looked at the number of clear sky pixels picked up by MODIS and subtracted the reflection that was estimated to come from the planet’s surface and air molecules.

“This leaves us with the remaining reflection bouncing off aerosol particles, and so we can estimate their density,” explained Marshak.

Using this idea, it makes sense to assume that where the sky appears brighter, light must be being reflected off more or bigger aerosol particles.

In their current analysis, Marshak and Varnai found that the bright sky effect was stronger on the sunlit sides of clouds or when the clouds were denser.

Because more light reflects off a denser or sunlit cloud, this suggests that the clear sky brightness near clouds is caused by extra light reflecting off the clouds sideways and then scattering again between the particles in the clear sky area before reaching the satellite.

“It’s essentially extra energy bouncing off the clouds that enhances the glow of the clear sky,” he said.

This effect - called 3D radiative interaction - had been previously identified as a factor cranking up the sky’s brightness, but the new data elevates it to the most important factor.

This, in turn, means that many estimates of aerosol density may be plain wrong, because most clear sky analyses are close enough to clouds to be affected by the effect, according to Marshak.

“Overestimating aerosol density means that climate models will be wrong if they assume a certain amount of aerosol is needed, when in fact it is less,” said Varnai. (ANI)

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