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By Daniel Collins

The snowstorm that covered much of the South Island with a white veil on Wednesday cut power to over a thousand homes and closed many schools and businesses. But in the midst of the disruption was a rare opportunity to do some important science: to measure the amount and density of the snow that fell at low elevations.

To get as much of this data as possible we asked for your help, and the results are now in. 42 people, at last count, sent in measurements, from Dunedin to Rangiora; a few more data are to be added.

CitizenSnowData6June2012Here is a Google Map of the measurements we’ve received. Ross Woods has helpfully compiled the largest measured depth at each location submitted.

Sampling is obviously biased towards Christchurch, because that’s where our social and scientist networks are largest. It’s not that snow didn’t fall elsewhere, just that no one there made the measurements for us. (We know it fell widely from aircraft and satellite photos shown in the media.)

The measurements are colour-coded — orange low, yellow intermediate, and blue high. The lowest measurements were near Leeston, Lyttelton and a few places around Christchurch. The highest were on the northern flank of the Port Hills (Christchurch south) and further north in Canterbury. In Christchurch, measurements made of asphalt or the like were shallower than over grass.

The Press explains why just north of Canterbury received the deepest snow.

A few people also made some handy density measurements. 21% in Spreydon (that is, percent relative to that of liquid water), 26% in Sydenham, 15% in Burnside in the morning and than 26% in the evening, and several others. Yes, snow density increased during the day. These numbers are invaluable in estimating snow loading on buildings.

The range of values we see across Christchurch, during the day, and even just a few centimeters apart, tells us that snow depth is quite variable. It is therefore important that we don’t just take one measurement but many. Replication. As we accrue more numbers, we get a richer and more accurate understanding of snowstorms.

And so a big thanks to all those citizen and professional scientists who gathered the data and sent them in. It all helps to push the envelop of hydrological knowledge a little bit further. But winter has just begun, so when it snows, we welcome more of your measurements.