Sunday, February 6, 2011

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?


Hendy said...
This comment has been removed by the author.
Hendy said...

How could the answer be any clearer? The footprints in the sand... I mean sidewalk... are where Jesus carried you when you couldn't make those six laps on your own!

Obviously kidding. Unfortunately I'm stumped so I'm hoping this is some kind of riddle you have answered and will come back to quell my intellectual anguish.

Just to be clear...
- You walked on dry, bare cement on your outing?
- The air temp was never above freezing, but only fluctuated between below 20F and lower?

The only wonderings that came to mind had to do with your boots leaving some kind of residue on the pavement for snow to agglomerate onto. This is, indeed, weird!

D'Ma said...

"The footprints in the sand... I mean sidewalk... are where Jesus carried you when you couldn't make those six laps on your own!"

That's really funny, Hendy. That was my answer as well! LOL

prairie nymph said...

We often get ice footprints up here in the land of the northern lights. It happens so often I've never wondered about it.
Although there isn't usually so much time between the walking and the footprints.
Your boots would have heat, but I am not sure of the carrying heat capacity of the cement. Perhaps it held the heat long enough to melt the ice when the snow fell. The snowflakes would have to be very light and there would have to be little wind.

DagoodS said...

Did you walk through any ice melting compound? (i.e. salt, anti-freeze, etc.) while on your walk?

HeIsSailing said...

Yeeshshshshs - Well I intended on getting back to this the next day.. but you know how hectic life can be. Anyway, this physics puzzler hardly matters any more, since it is now nearly 70F outside right now. The extreme cold lasted about two and a half days. Another day of extreme cold would have wrought disaster on El Paso. The city issued mandatory water restrictions up through early this week since so many water mains broke throughout the city - mostly in newer residential areas wouldn't ya know? Unbelievable...

DaGoodS asks...
"Did you walk through any ice melting compound?..."

salt is a possibility.

Prairie Nymph suggests:
"Your boots would have heat, but I am not sure of the carrying heat capacity of the cement. Perhaps it held the heat long enough to melt the ice when the snow fell."

This is the only idea that makes sense to me - it was the one running through my mind as I was snapping the photos. Some small bit of snow that landed on the spots where I stepped had to remain liquid slightly longer than the surrounding snow. I do know that IR imagery of the desert ground almost looks wakes in the ocean after a boat passes. The ground retains heat. Now I have only done heat capacity experiments in the desert, not on sidewalks or pavement. As a matter of fact, when we took the dogs for a walk in the desert, the ground was covered in snow, except on the well worn bike paths and running trails - as cold as it was, they were mostly clear of snow - again, the ground retains the heat for a long time!

Someday I will figure out a way to measure the heat-capacity of a cold sidewalk as a function of IR wavelength, and then I think I will have the answer. I do have the equipment to do this, but being lucky enough to get that equipment out on a cold (approx 5-10F) sidewalk might be a bit ... tricky!

HeIsSailing said...

Hendy suggests:

"How could the answer be any clearer? The footprints in the sand... "

Now why in the world didn't I think of that??

Prairie Nymph, forget all that stuff I said about infrared energy and heat retention. We have our answer right here!

Anonymous said...

Cold? Cold is -40. I want to see your IR results!

Hendy- of course you are right - I forgot Jesus was the answer to every question :)