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.
http://www.youtube.com/watch?v=czmQ2_ymaOo
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
Lake Mendota beginning to
as favorable ice nuclei are present.
freeze
Some of these nuclei in the lake come
from snow, dirt, and even cold rain.
•In the heterogeneous case columnar
• Once ice crystals start forming on a
crystals are formed. These ice crystals
lake, eventually this will lead to the
take on a colorful tint when viewed in
formation of more ice crystals.
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?
•
Sample of what ice through polarized light
looks like. The colors were modified a bit by
Peter Wasilewski to create art work.
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."
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 Fig. 2 The freezing surface of a lake
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. 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.