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Digital holography takes to the skies

The technique allows researchers to image the small-scale structure of clouds.

Viewed from afar, clouds can look like floating, wispy puffs of cotton candy. What do they look like close up on the scale of a few cubic centimeters? Cloud physicists have long wanted to know how water droplets and other aerosols are spatially distributed in clouds and how turbulent mixing of moist air inside clouds with the dry air surrounding them affects those distributions. Raymond Shaw of Michigan Technological University, Jacob Fugal of the Max Planck Institute for Chemistry, and their colleagues have turned to digital holography for definitive answers. Their instrument, called the Holographic Detector for Clouds or HOLODEC (the black-tipped instrument mounted below the airplane wing in the photograph), takes holographic snapshots of clouds, 15 cm3 at a time and at a rapid-fire clip of 200 per minute. On two separate flights, the researchers used their instrument to image water droplets in cumulus clouds. Back on the ground, they digitally reconstructed some of those holograms, and for each reconstructed volume, they determined the sizes and positions of the thousand or so droplets inside. The group found that whereas the average size of the droplets was constant, the droplet number density was highly inhomogeneous, counter to the assumption made in many cloud models. Armed with the new technique, cloud physicists should be able to image other types of clouds and help meteorologists and climate scientists build models with more realistic representations of clouds. (M. J. Beals et al., Science 350, 87, 2015; photo courtesy of the National Center for Atmospheric Research.)

Digital holography takes to the skies - figure 1

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