Gamma Rays, and Lump Graupels

 By Bill Oudegeest

Maybe we take snow for granted since we get so much of it, but it’s such serious business for those who study the subject, that they have an annual Western Snow Conference to discuss it. There they discuss such erudite snow topics as: “Northern Latitude Snow Pillow Installations”, “Snow Accumulation and Ablation Under Fire Altered Lodgepole Pine Forest Canopies” and “Snow Chemistry and Physics”. Attending it you may learn interesting facts like adding bacteria to water will make snow-making easier because you can use higher temperatures. Mostly they’re concerned with the dependability of snow and measuring it. Since the west is so dry and sometimes drought strikes, what falls in the mountains can be the whole year’s supply of water. 75% of the irrigation in the west comes from snow melt. The snow has to fall at the right time in the right form too. If it falls and melts to soon, the reservoirs won’t hold it and it’s wasted, as far as storage for drinking and irrigation goes. If snow is washed away by subsequent rains, the same thing can happen. How dry the ground is before snow falls is also a factor. To see what there is, hydrologists use the Mt. Rose Sampler, invented right at Lake Tahoe. It is a hollow pipe pushed down into the snow to measure the depth. The extracted core can also be tested to discover water content. To cut down on the labor intensiveness of snow surveying, new methods have been invented like gamma ray surveys, the amount of reflectivity (albedo), and snow pillows. Using planes and maybe eventually satellites, the National Weather Service measures the amount of gamma radiation given off by the earth. The more water in snow, the less radiation that can be measured. Above 10-12” though, you can’t do it since there’s no more radiation to measure. I’ll leave it to you to decide whether they use that method a lot around us. Snow pillows are 6-10 feet square pillows of rubber filled with antifreeze. Laid out in the forest, falling snow compresses the pillows and sensors measure the amount of compression and then transmit the data. Our local Central Sierra Snow Lab has tested two different kinds of pillows. Though they seem like a snow information junkie’s fix for information, they do have problems. Ice can bridge them, transducers are delicate, side snow will support the snow on the pillow, they are delicate to maintain, and they are filled with ethylene glycol which needs replenishing. All these cause accuracy problems and so snow pillow results need to be corroborated by on site inspections. Nevertheless since they are always looking for ways to remotely measure snow fall, they are, says Randall Osterhuber of our Central Sierra Snow Lab, the “best thing going” now.

Snow forms when water vapor forms an ice crystal (which is hexagonal) around a nucleus which is some small particle floating in the atmosphere. There are thousands of particles in each cubic centimeter. Those particles might be clay, dust, silica, volcanic ash, or even meteor fragments. Last winter’s snow fall might indeed have something to do with a far off volcanic eruptions in the Indonesia. Besides the temperature change effects, the extra dust in the atmosphere might help increase snow forming. After about 48 hours the snow flakes begin to lose their crystalline form and by the time snow is two months old, it is firmly compacted. If it lasts two years it reaches a stage between snow and firn. After three years it is compacted more and is called névé. After it has been around four years, it becomes glacial. Eventually glacial ice melts, wanders around as water for awhile, evaporates, and maybe becomes snow crystals again. Snow is a whole ecosystem, which is something that eludes us as we ski across it, but there is life in the snow. Algae in the snow feed on nutrients that fall with it. The algae excrete organic material which is eaten by bacteria. Snow worms eat the algae. The red and yellow tinges you sometimes see in snow are the algae.

For those of you of the more technological persuasion and who are hooked up to the world through your computer, I understand that the National Snow and Ice Data Center contains lots of useful cold information. Use your browser's search feature to look them up. (The old listing I had doesn't work any more...). They say they exist “to help broaden understanding of snow and ice, or their properties, characteristics and contexts, and of their significance for human activities...” but you get the idea.

This information has mostly come from an article in Atlantic Monthly January, 95 “In Praise of Snow” supplemented with a trip to our local snow lab and some other information.