We live on the south side of the street; the sidewalk is to the north of our house. We shovel the walks for the woman who lives on the north side of the street. We noticed that we could easily shovel the snow off the sidewalk on the north side; but the snow on the south side became packed and harder to shovel. Also, the next morning, there was a thin layer of very slippery ice on the bare sidewalks on the north side of the street.

Why? Before the snowstorm, when it was still sunny, the sidewalk on the south side of the street was shaded by our house, while the sidewalks on the north side were in the sun. Our hypothesis is that the temperature of the sidewalk on the north side was above freezing during the snowstorm. Hence the heat from the sidewalk melted the snow from the bottom, and the water flowed out onto the sidewalk during the night. Colder air temperatures and lack of sunlight the next morning allowed the sidewalk to cool off enough to freeze the water.

Those of you who have tried the GLOBE Surface Temperature Protocol know that shadows can cool surface temperature. It is not surprising that shadows keep sidewalks from heating up — or that shadows keep snow from melting.

The two pictures illustrate how shadows affect snowmelt. The first picture shows a striped pattern in the snow. The picture was taken in the early morning — you can see my shadow. To the left, there is a trailer with slatted sides. Did their shadows cause the stripes in the snow? The shadow of the slats makes just the right pattern. So, I waited until it was closer to noon, and took the second picture. Sure enough, you can see that the shadows are going to cover the snow strips!

Next time you have a snowstorm, watch how shadows and sunlight affect the snow depth.

The snow here at the GLOBE Program Office was so heavy we were told to go home. It was a “snow day”! I love snow days because they free you from your normal work or school. You can make new plans. And what can possibly be more fun than playing in the snow! So, instead of writing my promised blog on hard-to-identify clouds, I’ll describe what I learned about our recent snow.

The snow started the morning of 20 December. By mid-morning, it was quite heavy. Once we got home, we shoveled snow off the sidewalks, and started to measure the snow depth.

As we do for the GLOBE Solid Precipitation Protocol, I measured snow depth in relatively open areas where there wasn’t much drifting. Since the snow was so deep, I didn’t have a ruler long enough. So I recorded each measurement by putting a mark on a long, straight stick, and then measured the distance from the end of the stick to the average of the marks using a tape measure.

The figure shows the results of my measurements. The first reading, on the early afternoon of 20 December (big white dot), was 36 cm. Three hours later, when we went out to shovel more snow, I measured about 47 cm depth. Where we had shoveled, there was 15 cm of new snow (thick “+” sign). Two hours later, the snowpack was up to 55 cm, with 13 cm more on the sidewalks where we had shoveled. Just before bedtime, the snow depth was 58 cm, with 11 cm more having fallen on the sidewalks. The snowfall was still heavy when we went to bed, so I was curious as to how deep the snow would be in the morning.

Figure. Snow depth as a function of time. Big white dots and thick line: Actual snowpack; Thick + symbols: New snow on sidewalk after shoveling; x symbols connected by thin white line: Depth adding “new” snow totals to first snowpack measurement.

On the morning of 21 December, we had a little over 11 cm of new snow on the shoveled sidewalks, but the total snow depth had not increased — it was still 58 cm. When we checked with our neighbors, we got two other snow depths — both 58 cm.

On the graph, the x’s connected by a thin white line represent the snow depth if we just add the depth of new snow on the sidewalk to the first snow amount. This total is much bigger than the actual snowpack. Even more interesting, the snowpack on 22 December was smaller, down to around 43 cm, even though the temperature remained below freezing for most of the day. On later days, the snowpack continued to shrink.

What was happening? If you waited until 21 December to shovel the snow, it was a difficult job, because the snow was heavy. This was not the case the night before — the snow was quite light. The snow had become much denser overnight. That is, the snow was squashed by the weight of the snow above it. Also, the snow crystals change in such a way that they can crowd together. If there is water present, even if the snow doesn’t melt, both processes speed up.

After 22 December, there was some snowmelt, but the snow was still becoming denser. On 27 December, it’s almost possible to walk on the snow.