Clouds aren’t always what appear to be: Two clouds that look the same on the outside might indicate a dry day or a rainstorm, all depending on where they are. Cloud scientists have begun peering into what’s happening inside at a microscopic level, and they’ve found that the particles in clouds aren’t nearly as homogenous as they thought.
Scott Spuler, a research engineer at the National Center for Atmospheric Research, developed the holography technology in the study, “an instrument that could see and make a picture of roughly a thousand particles.” The three-dimensional imaging technique would allow the researchers to examine a few cubic centimeters of cloud, rather than cubic meters.
“We were never able to make measurements about clouds on these scales before,” says Spuler.
To create the holographic images, Spuler and the research team developed a tool that they could attach to the wing of an airplane. This way, it would collect data each time the airplane passed through a cloud.
The device needed to withstand the temperature change from the ground, at about 30 degrees C, to the clouds, at about -60 degrees C. It also had to be small, and as aerodynamic as possible.
“You don’t want to have a big block out there because as you’re flying pretty fast, you would make a pretty disturbed picture of what the cloud’s like,” says Spuler.
The team, led by scientists at Michigan Technological University, selected small clouds to sample so that they could understand how cloud particles grow to form rain clouds. Too big, and the clouds might already be producing rain.
The researchers also wanted the clouds to be out on their own, rather than components of larger weather systems.
“We wanted to look for clouds that were isolated so we could identify them and fly back to them,” explains Jeff Stith, a senior scientist at NCAR.
The images don’t actually reveal the particles themselves; they show how beams of ultraviolet light bounce off of the particles in the sample area. The resulting picture looks a bit like a puddle during a rainstorm, and take a long time to analyze: about a month of math to process an hour’s worth of holographic images. The results are published in Science.
Once all the math was complete, the researchers had a more complete understanding of how clouds are composed, with larger particles toward the middle and smaller particles toward the edges. As more water vapor condenses and the particles collide with each other, the cloud’s particles grow bigger.
“The scientists like myself and people that model this are trying to determine how cloud particles end up growing from tiny little cloud particles up to millimeter size raindrops,” says Stith. “They’re trying to trace the history of the growth of a particle.”
The findings contribute to climate science and atmospheric research, but also to day-to-day weather prediction. They explain why a meteorologist might predict a rainstorm for one cloud and a dry day for another.
“For example if you go over the tropics...you could have a rather small cloud that would rain like crazy,” says Stith. “Here over Colorado, a cloud that looks very similar wouldn’t rain at all.”
