Cloud Crystals

The sharpest ever measurement of ice crystals in clouds will help to improve climate change predictions. Scientists have created an instrument designed to help determine the shapes and sizes of tiny ice crystals typical of those found in high-altitude clouds, down to the micron level, comparable to the tiniest cells in the human body.

Among the hundreds of factors climate scientists must take into account in modeling weather, the nature of clouds is one of the most important and least understood. The best researchers could do in the past to measure cloud ice crystals was to try to record images of them, but for crystals below 25 microns, the images were too blurred to allow accurate determination of the crystal’s shape.

Researchers need to know the shape and sizes of these ice crystals because their sizes and shapes influence how much incoming sunlight gets absorbed in the atmosphere and how much gets reflected right back out into space. This, in turn, can have a huge impact on the magnitude of possible greenhouse warming.

Now scientists from the University of Hertfordshire and the University of Manchester in the United Kingdom and Colorado State University in the United States have developed an optical scattering instrument that can evaluate the size of the crystals in a different way. Using this instrument, the researchers have been able to determine sizes and shapes of the kind of ice crystals in clouds all the way down to the tiniest micron levels. (See the accompanying picture of cloud crystals; the crystals now being measured are much smaller than this.)

The research team actually has built two versions of the instrument: one designed to operate on ground-based cloud simulation chambers or to operate in the fuselage of research aircraft; the other, an aerodynamic version that fits under the wing of the aircraft and measures the cloud particles directly as the aircraft flies through the cloud (see the accompanying picture). Neither instrument attempts to make a full image of the ice crystal, since this would suffer the same resolution limits of existing instruments. Instead they record the detailed pattern of scattered light from each individual crystal and then interpret these patterns using either theoretical models or by comparison with recorded patterns from known crystal shapes. From this data a crystal census of varying sizes and shapes can be made.

“The new instruments should help map out a more complete understanding of complex crystal shapes found in atmospheric clouds, especially cirrus clouds which on any day can cover more than 20 percent of the Earth’s surface,” says one of the researchers, Hertfordshire scientist Paul Kaye. “Our findings show that this optical scattering instrument could help climate modelers reduce one of the greatest areas of uncertainty in interpreting current weather trends and in making more accurate climate predictions.”

In addition, recent reports have examined the effect that pollution and the clouds caused by pollution have on reducing solar radiation reaching the ground, a development that may counterbalance global warming to some extent, and this new technology could help scientists better monitor and understand this situation.

So far the new measurements, reported recently in the journal Optics Letters, have been made only in the lab, but will soon be made in actual clouds.

(Journalists can obtain the text from www.aip.org/physnews/select )