Right now, Lake Mendota is mostly frozen over – its only open water way out in the middle of the lake where winds are keeping the surface too riled up to freeze. But, yesterday morning around 9:00 am, Lake Mendota was still mostly in its liquid form despite temperatures in the teens. A gentle, but persistent, easterly wind was enough to push small waves past Hasler Lab toward University Bay.
And we couldn’t help but notice – the water looked cold.
Obviously the lake was wreathed with pancake ice in the shallows along its shoreline and everyone in Hasler Lab had arrived at work completely bundled up against the frigid air, but the lake itself seemed sluggish.
We wondered if we were projecting our own frozen thoughts on Lake Mendota’s open water from our perch on its shore. Does cold water really look any different than warm water?
To find out, we asked Nimish Pujara, an expert in fluid mechanics and assistant professor in the UW-Madison department of Civil and Environmental Engineering. It turns out, we were likely seeing a difference in the cold water.
While Nimish is currently traveling and unable to observe the lake first-hand, he did write back a note that at least proves were weren’t completely off target.
After cautioning that there are all sorts of temperature, stratification and atmospheric condition variables that may be at play, Nimish wrote:
I would say that your initial guess is a pretty good one. The kinematic viscosity of water (the property that is used to determine how “dynamic” water movement is in any given situation) does indeed almost double between 20 degrees Celsius (68 Fahrenheit) and 0 degrees Celsius (32 Fahrenheit). So colder water is much more viscous.
Viscosity, essentially, describes a fluid’s resistance to flow. Glue is viscous. Beer is not. As water cools, its molecules slow down and move closer together. It becomes a little more like glue.
Nimish went on to explain that any “partial freezing” or small ice crystals in the water increase the lake’s viscosity even further.
The result, at least from our view out of the windows of Hasler Lab, were waves that seemed lethargic. They didn’t crest in small white caps as they often do in summer winds of similar speeds and the surface of the lake itself seemed somehow reluctant to move, stuck right on the edge of liquid water and solid ice. (Fun fact: cold water also sounds different than warm water – this piece from NPR explains).
Within only a couple of hours, the winds had died down and, from the view outside our windows, Lake Mendota’s surface became flat and glossy.
It was then that we didn’t need an expert in fluid mechanics to explain what we were seeing – the water looked cold because it was frozen solid!