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How snowflake type influences snow measurements

A close-up image of a snowflake.

Snowflakes are formed when updrafts carry water droplets through a cloud base to heights within the cloud where the temperature is below freezing. There the droplets form ice crystals by condensing onto tiny dust particles or other ice nuclei. The crystals grow as water vapor accumulates on them and as they fall and collide with supercooled water droplets (droplets that remain liquid at temperatures below freezing until they encounter a surface on which to form ice). Snowflakes may resemble plates (such as the dendrite pictured here) or columns depending on the cloud-level temperature and the strength of updrafts. (Image ©UCAR.)

A new study by NCAR scientists looks at the accuracy of snow gauges, instruments used for collecting and measuring snow. Making accurate measurements of snowfall is critical for weather forecasting, climate studies, and hydrology. But a combination of snow gauge geometry and weather conditions can lead to systematic measurement errors.  

For example, the higher the wind speed during a snowfall event, the lower the collection efficiency of the snow gauge, since snow tends to blow over the gauge instead of falling inside. To prevent this, a shield is often installed around the gauge. But since the gauge and the shield are obstacles to natural airflow patterns, smaller and lighter snowflakes may interact with the gauge and the shield differently than larger ones.

NCAR’s Julie Thériault and colleagues looked specifically at how wind affects the different types snowflakes collected by a snow gauge. During a field experiment in February 2010 at NCAR’s Marshall Field Site, Thériault and colleagues collected snowflakes inside and outside a gauge, both with and without a shield around it, and photographed them every 20 minutes in order to analyze crystal type and size. They also ran a theoretical study in which they used a computer model to simulate wind flow around the gauge and show the trajectories of different crystals falling through the atmosphere.

Preliminary results from the study, presented at the 2011 annual meeting of the American Meteorological Society, show that wind speed indeed influences the different crystal types collected in a snow gauge. The gauge’s collection efficiency was reduced more dramatically for plate-like crystals (which include dendrites, the classically shaped flakes), which tended to flow around the gauge, as opposed to irregular crystals (including graupel, columnar crystals, and irregular ice), which fall faster.

“When we started this project, we thought it was size of a snowflake that matters, but we discovered that it’s not only the size of the flake but the speed at which it falls, which is a result of density, shape, and other factors,” Thériault says.

The team’s next step is to try to identify the threshold at which collection efficiency drops in relation to the speed of snow crystals.