Measuring Ice Crystals

Climate change studies currently are hampered by incomplete knowledge of the radiative behaviour of ice crystals in clouds. The tendency of the crystals to flutter about a horizontal alignment affects their optical properties, and scientists from the Institute of Atmospheric Optics in Tomsk, Russia, and from Hebrew University of Jerusalem have developed a technique to measure their parameters.

Illumination from the sun reflects specularly from horizontally oriented ice crystals, forming a pillar of light in the valley. Courtesy of A.S. Kolevatov, Institute of Atmospheric Optics in Tomsk, Russia.

The approach uses low-cost equipment: A standard xenon lamp projector acts as the light source beaming up toward the horizontal ice platelets. A CCD camera some distance away images the light scattered at various angles and times. By scanning the projected beam across the sky, the researchers can separate the bright stationary spot of specular reflection from the halo of diffuse light that moves with the beam. When light reflects in a specular direction from the tiny mirrorlike crystals, the mathematics is simplified, and measurements of the angular radius of the specular spot for red light of wavelength 0.7 μm and for blue light of 0.4 μm allow the calculation of both size and flutter.

(Optics Express, 26 May 2008, pp. 7625-7633)