The curious case of footprints in the snow.

[lakedude]

I'll post links to pics when I get a chance...

After a relatively warm period the mercury took a dive and we got about 6 inches of snow. A couple of days later there was still several inches of snow on most undisturbed surfaces including the grass however on the sidewalk and on the driveway all the snow melted in unshaded areas where the snow was undisturbed. The sidewalk was bare cement except where a lone boy had walked. His footprints along with a small uncompressed area around his footprints were still snow or ice covered. The driveway told a similar story. My side which had been driven on by my AWD was solid ice while my wife's side which hadn't been driven on was completely clear down to bare cement.

I've showed pictures of this phenomenon to several people and they have typically said some nonsense about the sun and something about the snow being compressed into ice and the ice being harder to melt.

I have 3 key pieces of evidence that refutes the common thinking on this phenomenon.

1) There is an uncompressed area of unmelted snow around the footprints.

2) Only the snow over concrete melted. Snow on the grass did not melt away.

3) I have a second picture of footprints in grass taken a few days later. The compressed icy footprints melted first leaving footprint shaped holes in the snow down to bare grass. Undisturbed areas in the grass were all covered by several inches of snow. Only the trodden areas were melted, the exact opposite situation as what happed on the concrete.

After giving this far more thought than I'd like to admit, I've come up with a plausible theory which I'll post after y'all get a chance to take a stab at it.

 

[glugglug]

The compressed snow gets hit by less wind since it is closer to the ground. Especially while the surrounding snow is blocking wind as well, so the parts of the snow pushed down will be the last to start melting.

Also, sunlight hitting the compressed snow is more likely to make it to the surface below instead of being dispersed within the snow earlier.

 

[lakedude]

So this is the footprints on the concrete:

http://imageshack.us/a/img801/4382/mihp.jpg

 

[lakedude]

And this is the footprints in the grass:

http://imageshack.us/a/img843/9916/fjsl.jpg

 

[lakedude]

Notice that they are opposite! No way wind and sun explain both. I think something else explains both pictures. One thing explains both...

 

[pandemonium]

I believe the answer lay within the relation of geothermal radiation, aerated water form, and receptability of solar energy.

The pressed footprints on the grass receive a higher conductivity of geothermic energy transfer than areas with unpressed snow. The unpressed snow on the grass is aerated and has higher insulation from geothermic radiation.

Concrete generally has better geothermic conductivity than grass, however, higher receptability from solar radiation. The geothermic radiation is likely at the point that the concrete isn't radiating much heat and the sun is the primary source for heat here. This causes the footprints on the concrete to receive less energy from the earth and more from the sun. The aerated effect of snow leaves it susceptible to being evaporated faster than the footprints that are now ice, and the insulation from the footprints extends to the snow around them.

 

[lakedude]

Pandemonium, I believe you are on the right track. IMHO insulation is the key to understanding the footprints on the concrete. The concrete was warmed by the sun before the snow fell but areas in the shade were cooler having received no warmth from the sun. The grass would have far less thermal mass to store heat and a higher insulation value as well. On the concrete the leftover heat from previous days starts melting the snow from the bottom. The insulation of the snow keeps the concrete realitively warm but where there are footprints the lower R-value lets the heat escape, leaving cooler concrete in the area in and around the footprints.

It seems to me that if deep geothermal heat was the primary source of melting energy the areas in the shade would be melted in a similar way as areas in the sun.

I'm less clear on the bare footprints in the grass. Compressing the snow would give it better contact with the ground underneath and the compression would let more solar energy to reach the ground as well. Undisturbed snow would insulate the grass from solar warming but the footprints would let the sun warm the grass from above more due to the lower R-value in the footprints.

 

[Zorba]

Snow in the grass will have a layer of air under it, protecting it from the warmth of the ground, but if you step on it you push it closer to the ground decreasing the insulation of the air. You also compress the snow, meaning there is less air in the snow, increasing its conductivity, so the heat from the ground warms the entirety of the snow move evenly.

Not sure about on the sidewalk, I'll have to think about that some more. I notice this a lot with tire tracks on driveways as well.

 

[dkozloski]

Moose tracks in the yard demonstrate the same phenomenon.