Physics Puzzler - Ice Footprints

On Tuesday evening, 1 Feb, I took my dogs for a walk around the block. I usually take them outside of town to run around in the desert, but nightfall was coming on, and I could tell a cold front was moving in. It was at least 30 degrees F colder than usual, so I wussed out and took them for about 6 laps around the block. As the sun went down, it got biting cold. I could feel my cheeks and nose start to get numb. It rarely gets below freezing here in El Paso, and the forecast was warning us to prepare for cold of historic proportions. Not a soul was on the street with me, and as I completed my 6 laps with my dogs, the sun set and the bitter cold evening began.

I woke up the next morning to this:





The temperature remained at about 10F all day, and the snow that fell was light, dry and powdery. I brought the dogs indoors, and my panicked tropical wife and I spent Wednesday indoors doing our best to keep warm despite intermittent power failures. El Paso is ill-equipped to deal with so much snow and ice-covered streets, and nearly every business, school and government office was closed. Everybody was advised to stay indoors and off the streets. When I left the house to check on my elderly neighbors, I noticed no footprints on the snow-covered sidewalks. Everybody heeded the advice and remained huddled indoors.

On Thursday, the temperatures were slightly, but not much higher. However the skies had cleared and a strong wind had blown most of the dry snow off the sidewalks. RoseMary and I were getting cabin fever, so we decided to brave the streets and drive out to the desert and let the dogs play in the snow. When it snows in El Paso, enjoy it quickly because it will not stay on the ground for long - I don’t care how cold it gets!

As I looked up the sidewalk in front of the house, I noticed a most curious thing:



The snow was drifting off the sidewalks, and leaving behind sets of footprints formed of ice! They were definitely the same pattern and size of the boots I wore while walking the dogs out the night before the snow fell, and they went all the way down and around the end of the block.



My baffled wife had no idea why I was shooting so many pictures of the sidewalk. She yelled at me to get in the truck, since she was freezing her tropical ***** off, so I climbed in and off we went for a snow hike in the desert.

OK science nerds, here is your brain teaser: Explain why footprints encased in ice formed on the sidewalk. Here is what we have:


· Tuesday night, I walked outside before any snow fell. The temperature was roughly 20F. The temperature dropped to about 5F that night.

· Wednesday was cloudy with a high of about 10F. Light, dry snow fell off-and-on all day and covered everything. I noticed no footprints in the snow-covered sidewalks. Wednesday night was also about 5F.

· Thursday was sunny, and slightly warmer but still well below freezing. The wind was a bit stronger, and blew the snow off the roads and sidewalks. That is when I noticed my footprints, left before the snow fell, encased in ice.

Any ideas?