Holograms Show That Puffy Clouds Have Sharp Edges

A laser-based imaging technique let scientists see what happens to water droplets at the borders of cumulus clouds

Cumulus clouds don't literally have silver linings, but their edges are sharper than we thought. Bennewitz/iStock

A child’s drawing of the sky usually shows puffy, balloon-like clouds standing in stark contrast to a blue background. Now, a holographic device strapped to an airplane wing has shown that simple sketch may actually be a pretty good likeness of real cumulus clouds.

The high-tech system imaged the tiny droplets at the edges of clouds and found that there is a sharp transition between cloud and sky. The results may help scientists build better climate and weather models that more accurately represent the effects of clouds.

All clouds are collections of tiny water and ice crystals that are small enough that they don’t fall too quickly. Depending on how those particles scatter the sun’s light, clouds can look bright white or dreary grey. And when the droplets collide and coalesce, water and ice fall out as precipitation.

“How clouds scatter light and how fast they can form precipitation depend very much on the number and size of the particles contained inside,” notes study co-author Raymond Shaw, an atmospheric scientist at Michigan Technological University. But until now, scientists haven’t been able to see what is happening to those particles.

Shaw and his colleagues made use of a device called the Holographic Detector for Clouds, or HOLODEC, that gets strapped to the edge of a plane’s wing. As the airplane flies through a cloud, a laser beam makes a three-dimensional record of the particles inside.

“We can measure hundreds, even thousands, of cloud droplets, including their sizes and how they are clustered in space, all within a volume about the size of a highlighter marker,” Shaw says.

The researchers flew the HOLODEC through cumulus clouds over Colorado and Wyoming. They focused on isolated puffs so that they could fly back through several times to see droplets at different stages of a cloud’s life cycle, notes co-author Jeffrey Stith of the National Center for Atmospheric Research.

The technology was able to “provide a remarkable view of clouds at the microscopic level, much like you might see if you were able to bring a microscope into a cloud to view individual droplets as they encounter dry patches of air that mix into the cloud,” he says.

Holograms Show That Puffy Clouds Have Sharp Edges
The Holographic Detector for Clouds is seen here strapped to the underside of a plane wing. NCAR

Current weather and climate models assume that all the particles inside a cloud behave the same, but the new study, appearing this week in Science, found that a subset of the droplets completely evaporates away, creating distinct edges.

“We all know that clouds can appear to have sharp edges on the large scale, but seeing that sharpness when zoomed in 10,000 times closer is fascinating,” Shaw says. Such knowledge could lead to improved weather and climate forecasts.

Creating 3-D images from the HOLODEC data requires sophisticated computer programs that make use of high-end graphics cards similar to the ones used by video gamers, Shaw adds.

“I suppose we should thank all those generations of teenage kids for doing their part to contribute to this deeper understanding of clouds,” he says.

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