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NIR enters winter wonderland

With winter coming to the northern hemisphere and snow likely to fall over the next few months, US scientists have unveiled a new portable NIR spectroscopy instrument for measuring the size of snow grains in snow drifts.

Developed by a team led by Daniel Berisford at Nasa's Jet Propulsion Laboratory, the instrument consists of a long aluminium tube containing a halogen light and a reflectance probe attached to a fibre optic cable that leads up to an NIR spectrometer. Both the tube and the NIR spectrometer can be carried in one or two backpacks, making the whole system perfectly portable.

The idea is to make a small hole into the snow pack and then insert the tube into the hole; a snow brush at the head of the tube helps to sweep snow out of the way. Light is then shone into the surrounding snow through slits in the side of the tube, with this light either produced by the halogen light or sent down the fibre optic cable from an external light source.

The surrounding snow reflects this light back to the tube, where it is captured by the reflectance probe and transmitted up the fibre optic cable to the NIR spectrometer, which can determine grain size from the spectra of the reflected light. In contrast, other techniques for determining grain size employ much bulkier equipment and require large pits to be dug into the snow, which is time-consuming and significantly disturbs the surrounding snow.

In a paper in Cold Regions Science and Technology, Berisford reports testing the instrument at two different sites in the Colorado mountains and finding that it produced similar measurements of grain size to other, less user-friendly techniques. These revealed that, as suspected, snow grain size generally increases with depth, but also that snow grain size can differ greatly between even closely-located sites, especially when the surface of the snow is not perfectly flat.

As well as producing useful information about the distribution of different size snow grains, this instrument could also help interpret microwave-based satellite observations of snow cover over large areas.

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