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How fast did the snow melt?

Welcome to Snow Lab!

Results so far

We had a modest fall of snow on the night of 13-14 January 2013, and a bit more in the afternoon of 14 January. Four schools in Cambridgeshire sent in measurements of the snow, and very good they were too! I also made some measurements in my garden, even though I don't quite live in Cambridgeshire (I am over the border in Essex). I used these five sets of measurements to make our very first snow map. This map shows the mass of snow per unit area, measured in kilograms per square metre. This number is like measuring the amount of rainfall in millimetres - it measures the same amount of water. Our measurements showed this amount ranged between 2.4 kilograms per square metre and 5.0 kilograms per square metre.

Snow map

More snow!

More snow fell in the area from Friday 18 to Sunday 20 January. It stayed cold for a few days after that, so the amount of snow lying on the ground did not change much. More schools sent in measurements (thank you) so we were able to make a new map. The total amount of snow was now quite a lot more than in the previous week - from 5 to 14 kilograms per square metre. (The large crosses on this map show where the snow measurements were made.)

new snow map

New map

More schools sent in measurements in the next few days after the snowfall on 18-20 January. Since the total amount of snow on the ground didn't change very much over this period, we could make a more detailed map than before. Here is our new map.

snowmap 3

The map shows that over most of the region we had about 10 kg of snow per square metre, an equivalent amount of water to 10 mm of rain. It was a bit less around Cambridge itself, perhaps because Cambridge is warmer than its surroundings. There seems to have been a lot of snow around Kings Lynn!

How fast did the snow melt?

We made a graph to show how the amount of snow changed from day to day, by combining all the measurements from the different schools. Here it is:


The graph is labelled 'SWE' which means 'snow water equivalent'. This is the rainfall, in millimetres, that would give the same amount of water as melting the snow, and it is the same number as the amount of snow in kilograms per square metre. What the graph shows is that the first snowfall on 14-15 January gave around 2.5-5 mm SWE. The second snowfall of 18-20 January increased this to about 10 mm. In the next few days, the SWE decreased to about 7 mm, so only about a quarter of it had melted by 26 January.

How fluffy was the snow?

'Fluffy' isn't exactly a scientific word! Instead, we talk about the density of snow, which is the mass of snow in a volume of one cubic metre. Pure ice has a density of about 917 kilograms per cubic metre, but the density of snow is always smaller than this because of the air mixed with the ice in snow. Very 'fluffy' snow has a low density. As snow gets older, its density gets bigger as the crystals of ice stick more closely to each other and squeeze out the air between them. We used the Snow Lab measurements to calculate the way the average density of the snow changed from day to day. The results are shown in this graph:


Apart from that surprising value of nearly 200 kilograms per cubic metre measured on 15 January (maybe it was a mistake!), we can definitely see the way that the density of snow increases over time. At the time of the biggest snowfall, on 20 January, it was about 100 kilograms per cubic metre. A few days later it was about 150 kilograms per cubic metre.