AOS 101 Weather and Climate Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences

Ice on Lakes Ice cover on lakes is more than just a signal of winter. The amount of ice cover and how long it remains on the lakes during the winter season changes from year to year, and long-term changes may occur because of global warming. Studying, monitoring, and predicting ice is important to determine climate patterns, lake water levels, water movement patterns, water temperature structure, and for predicting spring plankton blooms.

Ice Formation Before talking about lake freezing we need to learn about ice formation. The formation of ice crystals is similar to the formation of cloud droplets. Even so, there are a few important distinctions between both. However, just as in the case of cloud droplets, the key to the formation of ice crystals are nuclei.

Formation of ice crystals in the Atmosphere The process of formation of an ice crystal begins pretty similar to the formation of a cloud droplet. – we have nuclei in the atmosphere – water droplets begin to attach to the surface of the nuclei In order to form ice, we need a crystal structure for water droplets to attach to, ice nuclei. So ideally the nuclei would be an ice crystal or a particle with a structure similar to ice. Once we have these ice nuceli, supercooled water attaches to it in order to form larger ice particles.

Lake Freezing Process 1)Spontaneous Nucleation Takes place on cold calm nights when the surface of the lake supercools, that is, the temperature drops below freezing and the ice nucleates -- spreading rapidly across the lake ice surface. Individual crystals can reach meter wide dimensions.

Lake Freezing Process 2) Heterogeneous Nucleation In this case ice nuclei must be present in the lake. Freezing in a lake occurs below 32˚F. At exactly 32˚F liquid and solid are in equilibrium, thus there is no tendency for a phase change to take place. At this temperature water will supercool. Supercooled water will freeze as long as favorable ice nuclei are present. Some of these nuclei in the lake come from snow, dirt, and even cold rain. Once ice crystals start forming on a lake, eventually this will lead to the formation of more ice crystals. Lake Mendota beginning to freeze In the heterogeneous case columnar crystals are formed. These ice crystals take on a colorful tint when viewed in polarized light.

If you're watching the Winter Olympics, you know that snow and ice are an integral part of the sports. But did you know that snow and ice need to be different for each sport? NASA scientist Peter Wasilewski studies ice using polarized light to create colored pictures of the snow and ice, and enable people to see if the snow and ice is "right" for each type of sport. "The ice is softer for figure skaters than it is for hockey players. Figure skaters need to dig in with their toe picks for jumps. Ice hockey players want the hard ice that makes the ice fast and easier to skate on. With a microscopic look at the ice using the spectrum, I'm able to see how the ice differs." Sample of what ice through polarized light looks like. The colors were modified a bit by Peter Wasilewski to create art work.

How does Lake Freezing begin? Lakes freeze from the top down. Ice is less dense than water, which is why ice floats. The density of liquid water is determined by its temperature, and water is most dense at about 40 F. Why is that important? As winter sets in, lakes lose energy to the atmosphere, and water near the surface cools, becomes more dense, and sinks. Warmer, less dense water under the surface will rise to replace this surface water. When the entire lake reaches 40 F, the surface water cools further, dropping below 40 F. Because this water is now less dense than the surrounding water, it will stay on the top and continue to cool. Fig. 1 The cooling of a lake

How does Lake Freezing begin? Once the surface water falls below 32 F, it freezes. The freezing then spreads downward into the lake and the ice thickens. Unless the lake is very shallow, you will find liquid water below the ice. This deeper water is about 40 F; fortunately fish can live in this cold temperature. Freezing first occurs along the shoreline, where the water is shallow. Before ice can form on the surface, the entire water column must first reach 40 F, which is likely to first occur along the shoreline. Fig. 2 The freezing surface of a lake Ice forming around the shore of Lake Michigan

Lake Ice Melting 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. 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. 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. Warming continues because the light energy is being transferred to the water below the ice. Melt water fills in between the crystals, which begin breaking apart. The surface appears grayish as the ice reflects a bit more light than before. The wind comes up, and breaks the surface apart. The candles will often be blown to one side of the lake piling up on the shore. In hours, a sparkling blue lake, once again! Black ice

When do Lake Freezing and Lake Ice Melting occur? Lake freezing can occur anytime during the Fall and Lake Ice Melting can occur anytime during the Spring. The specific month in which these occur can vary. The appropriate conditions must be present for each process to take place.

Freeze-Up and Ice Break-Up dates for some Lakes and Rivers

Lake Mendota Lake Mendota is the northernmost and largest of the four lakes near Madison, Wisconsin. It’s one of the most studies lakes in the US.

Climate change effects on Lake Mendota Ice In Madison, Wisconsin, winter is milder than in years past. Researchers monitoring ice on Lake Mendota find that ice cover on the lake averages 40 fewer days now than it did 150 years ago. Says University of Wisconsin researcher John Magnuson, "Wisconsin is losing winter as we knew it."

Today in the Lab Today you will be examining a dataset of Lake Mendota Ice duration. This dataset consists of 146 years of data. You will be looking for trends in the data.