Clouds’ snow zones

Look up photos of “atmospheric halos” and you’ll find spectacular photos of bright colorful rings etched in wispy clouds by a hazy sun or moon. The circular halos are caused by light refracting through small ice crystals in the air, much smaller than snowflakes. The shape of the ice crystals determines what kind of halo is produces. Hexagonal ice crystals will create common 22-degree halos. Rare Parry arcs are formed by irregularly-shaped crystals. The ultra-rare Scheiner’s halo may come from crystals with a trigonal symmetry. The various ice crystal shapes can also affect how much of the sun’s energy the ice crystals can scatter, a key component in atmospheric models projecting future climate change.

University of Utah chemists Arpa Hudait and Valeria Molinero modeled water molecule behavior under cloud-like conditions to see what factors affect ice crystal shape. Ice forms from super-cooled water droplets but grows by freezing the water vapor in the air, with the shape dependent on the temperature and amount of vapor in the air. In cirrus clouds, hexagonal ice is favored, but in the colder stratosphere, disordered ice crystals and trigonal symmetry are more likely. Researchers can use these results to predict what the shape of ice in different layers of the atmosphere. Graduate student Hudait is available to comment on the methods and implications of this study, published in the Journal of the American Chemical Society.

Arpa Hudait | 801-864-0753|