How the lake ice melts

[LKunze]

Air Mass with Bill McAuliffe
Startribune.com
March 30th, 2006

Danger: Candles

One January a few years ago I was out on a lake in Texas with some acquaintances from down there. I remarked that back in Minnesota we’d have been walking out there, instead of riding around in a boat. They didn’t know what I was talking about. A frozen lake? Get out!

As Ed Swain of the Minnesota Pollution Control Agency can attest, the sealing and unsealing of lakes is a remarkable phenomenon. Swain has a Ph.D in limnology (the study of lakes) and tracks mercury pollution for a living (ice on lakes has a lot to do with water quality). He explained what happens on lakes between midwinter and the day they’re ice-free again — which is coming soon.

1. In the late fall, the lake loses heat to the atmosphere, and then on a
day or night when the wind is not blowing, ice forms. The ice gets
thicker as long as the lake can continue to lose heat.
2. In most Januaries and Februaries, snow both reflects sunlight and
insulates the lake. With a thick snow layer, the lake neither gains nor
loses heat. The bottom sediment is actually heating the lake water
slightly over the winter, from stored summer heat.

Warming rocks shake off shore ice
3. Around March, as the air warms and the sun gets more intense, the
snow melts, allowing light to penetrate the ice. Because the ice acts
like the glass in a greenhouse, the water beneath it begins to warm, and
the ice begins to melt FROM THE BOTTOM.
4. When the ice thickness erodes to between 4 and 12 inches, it
transforms into long vertical crystals called “candles.'’ These conduct
light even better, so the ice starts to look black, because it is not
reflecting much sunlight.
5. Warming continues because the light energy is being transferred to
the water below the ice. Meltwater fills in between the crystals, which
begin breaking apart. The surface appears grayish as the ice reflects a
bit more light than before.
6. The wind comes up, and breaks the surface apart. The candles will often be blown to one side of the
lake, making a tinkling sound as they knock against one another, and piling up on the shore. In hours, a
sparkling blue lake, once again!

The state climatology office tracks the clearing of lakes across the state here.