Correspondence 551.578.46 : 311.214

SNOWFALL MEASUREMENTS IN NORTHERN CANADA

By RICHMOND W. LONGLEY

Mr. C. I. Jackson's article (Quart. 1. R. Met. SOC., 86, p. 273, 1960) emphasizes once again the problem of precipitation measurements in the Arctic. I wonder whether the source of error he discusses is not minor compared with other sources of error.

The frequency of falling snow crystals in the polar regions is amazing to anyone who goes there, no matter how much he has studied the recorded weather reports. The total deposit from these cannot be negligible. Moisture also comes from direct sublimation on the snow. There seems no method by which one can measure these accumulations.

But moisture from such sources is not the only precipitation that cannot be measured. Even when a ' measurable ' amount falls, it is seldom a ruler is used to determine that amount. There are two reasons. Daily totals of recorded measured snow are 0.2 in. or less on over 50 per cent of the occasions in the far north. The frequency of six-hourly totals of these small amounts must be much greater. Even when such small amounts lie uniformly over the surface, the value is usually estimated. Large amounts are usually accompanied by winds of 15 mi hr-l or more. These will carry the falling snow into drifts, to make it necessary that the observer use his judgment in recording the total snowfall. When the wind reaches 20-25 mi hr-l, snow from the ground is carried aloft. Under these circumstances there is no method by which an observer can state with confidence the amount that fell from the clouds.

Another error in the determination of the water content is in estimating the density of the fresh snow. The snow particles at Resolute, N.W.T., and probably at many other Arctic stations, were almost never the star-like aggregations of ice crystals familiar to residents of temperate latitudes. On most occasions they were snow pellets of 0.2 to 0.5 mm diameter. I have suspected that many of these were formed in the turbulent eddies above areas of open water from which some of the moisture came. With the light snow deposits that occur in the far north, it is im- possible to obtain a sample of this snow in order to determine the density.

There would appear to be another method of determining the moisture from the snow. In these areas there is no melting during the winter, and very little evaporation. The snow cover of 30 April then equals the total winter precipitation to date. The water content should be determined using the mean snow depth on the ground and the mean density. Using the total accumulated snow fall, one might then determine a ' mean density of fresh snow.' Moisture from sublimation and traces would be included when this value was used with the recorded snowfall.

The difficulty with such a method concerns itself with measuring the snow on the ground. A study I made at Resolute during the winter of 1957-1958 (to be reported in the Journal of Glaciology) showed that the snow depth was a function of topography and the direction of the wind during the last major storm. Except in ravines and other protected places, it did not continue to accumulate as the winter progressed. A representative ' depth of snow on the ground ' over these barren lands then becomes impossible.

As a result of my two years at Resolute, and my consideration of this problem, I have con- cluded that we cannot know the moisture that comes to these north lands. The present method gives an underestimate, I am sure. At least the data from year to year are comparable.

Department of Mathematics, University of Alberta, Edmonton, Alberta, Canada. 20 June 1960.

REPLY

By C. I. JACKSON

I am grateful to Mr. Longley for his remarks on the problem of snow measurement and look forward to the appearance of his own paper on the subject.

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