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What's At The Edge Of A Cloud?

Using an instrument they've named the HOLODEC, for Holographic Detector for Clouds, scientists can now see in fine detail the way air and water-droplets mix at a cloud's wispiest edge.

Scientists have just made a breakthrough in understanding how clouds interact with the surrounding air by studying some of the most boring clouds you can imagine in unprecedented detail.

"If you ask a child to draw a cloud they would draw a white puffy cloud floating in the air all by itself — and that's the kind of cloud we were looking at," says Raymond Shaw, an atmospheric scientist at Michigan Technological University.

A life studying clouds is not what you would have predicted for Shaw, who remembers struggling to get the weather badge when he was a Cub Scout.

"We had to memorize the types of clouds and I really hated it," he recalls.

But later he got interested in snowflakes — which, of course, come from clouds — and he ended up studying cloud secrets. What he wants to understand, at a very fine level of detail, is how the cloud interacts with the surrounding air.

His team has been flying through clouds in an airplane equipped with a new instrument that they call the Holographic Detector for Clouds, or HOLODEC.

"The instrument creates a small, three-dimensional picture of everything inside of a volume," says Shaw.

The volume isn't a big amount of space — it's like a cigar-sized section of the cloud — but it's enough to be revealing.

"We can not only see how many droplets there are, and how big they are, we can also see how they are distributed in space," explains Shaw.

They focused on the clouds' edges — where dry air is mixing in. You might think all the droplets at the edge of a cloud would evaporate a little bit and that they would all shrink. But that's not what happens.

Shaw says dry air seems to creep in and evaporate some droplets completely, while others survive in wispy cloud filaments surrounded by clear air.

"The droplets that are remaining are just as big as the droplets that are in the center — kind of the protected core of the cloud, where no evaporation has taken place," he says. "That was a surprise to us."

They report on their work in this week's issue of the journal Science. Shaw says, getting this kind of intimate look at clouds is just neat.

"But the amazing thing is that it really does have consequences," he adds. "Those little details in the cloud eventually form a link in a chain that leads to a weather forecast, or an understanding of how climate will change."

The findings amazed Marcia Baker, a retired cloud physicist from the University of Washington in Seattle. Back in the 1980s, she and a colleague named John Latham first proposed that this is exactly how dry air would mix with a cloud.

"This new instrument seems like a real tour de force," says Baker. "To me, it's quite astonishing that they were able to make measurements down to this level — and it is of course very gratifying."

Other kinds of clouds — such as colder clouds with ice crystals — might behave differently, Baker says. Shaw says he's really interested in understanding those clouds, too.

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