Ice Calculations and Safety
By Lois Hoffman
Frozen ice offers opportunities to still get out and enjoy the outdoors, with a few precautions.
Every season brings its own blessings, and trials, and winter is no different. Here in Michigan and other parts of the frozen Midwest, being on the ice offers its own share of pleasures…and dangers. So many folks partake in ice fishing, ice skating, etc. without really understanding the basics of being on the ice.
It has been a while since I have played on the ice but, when I was a kid, every winter I was out there with my Dad. He loved to ice fish, and I liked to ice skate…a win-win situation. He liked to go to different lakes so he never bothered with a shanty. If the weather was really bad, we just didn’t go.
All lakes and ponds have a routine, so to speak, on how they freeze. A body of water takes longer to cool down and longer to heat up than land. When an entire lake reaches 40 degrees Fahrenheit, the surface water cools further, dropping below that temperature. This water is less dense than the surrounding water, which stays on top and continues to cool. Once the surface water reaches 32 degrees Fahrenheit, it freezes, and this process keeps spreading downward, thus making the ice thicker and thicker.
So, why doesn’t a whole lake freeze? As ice crystals freeze, they float to the surface and as they become thicker, they act as insulators, preventing the cold air coming above the water from removing heat from the unfrozen water below. Because of this process, a lake never freezes solid from top to bottom. Water is shallower at the lake’s perimeter, thus it freezes faster on the outer edges, and the whole lake freezes from the shore to the center.
Rivers and streams are a whole other story. The energy in flowing water is constantly being converted to heat energy that resists freezing on the molecular level. For this reason, it has to be exceptionally cold for rivers and streams to freeze. Glaciers are prime examples of frozen rivers.
So, what is considered a safe thickness of ice to be on the lake? That depends on how much weight you are planning to take on the ice. The Old Farmer’s Almanac recommends 3 inches of ice for a person on foot, 4 inches for a group walking in single file, 7.5 inches for a passenger car with a 2-ton gross and 8 inches for a light truck with a 2-1/2 ton gross. The almanac also stresses that slush ice is only half the strength of regular blue ice, and the strength of river ice is 15 percent less than lake ice. Also, the thickness of a lake’s ice is different at different points on the lake, so it is recommended that you check it every 150 feet.
How long it takes a lake to make the varying degrees of ice thickness depends on what is known as “freezing degree days.” The formula for this is quite simple. First, take the average temperature over the last 24 hours and subtract that number from the freezing point of 32 degrees. Ice will increase at a rate of 1 inch/15 freezing degree days. For example, if the average temperature over the last 24 hours was 25 degrees, subtract that from 32 degrees which will give you 7. Put 7 over 15 like a fraction, 7/15 equals about a 1/2 inch of ice over a 24-hour period.
Keep in mind that these are only guidelines. This formula is based on having a slight to moderate wind speed, no snow on the ground, and clear skies. These all help to pull heat out of the water and accelerate the growth of ice. Just because the thermometer says 32 degrees, does not always mean that water will freeze. The University of Utah chemistry department shows that water can get to -55 degrees before it must freeze.
Frozen lakes and ponds have some phenomena that they claim for their own when they are frozen over. Perhaps the most frightening for anyone who spends any time on the ice is the cracking noise. My Dad always told me, “It’s just making more ice when you hear that noise.” It never made me feel any better, but it was essentially true. Ice expands or contracts when the temperature changes, thus causing cracks to form in the ice.
This same action of expansion and contraction causes lines on the frozen lake’s surface. We were recently down to our friends who live on Union Lake in Michigan and noticed these lines leading out into the lake, with no particular pattern or reason. They are referred to as pressure ridges. Even when a lake is completely frozen, it is not stagnant; it still expands and contracts as it warms and cools. When it warms during the day, it expands, causing a collision between both sides of a crack and causing the ice to buckle up at that pressure point.
Ice heaves and ridges are caused by the pushing action of a lake’s ice sheet against the shore. When lake ice cracks, water rises into the cracks and freezes, gradually expanding the heave.
Frozen lakes offer opportunities to enjoy our water resources in the winter as well as summer. However, it offers its own set of risks. Besides the thickness of the ice, always be aware of those that have been out before you. Specifically, I am talking about holes that ice fishermen have drilled and abandoned. You can easily step into one of these and trip…and a fall on the ice is a lot harder than on land! These holes also pose risks for ice skaters. I found out the hard way what happens when a skate catches an open hole.
As careful as one can be, ice is unpredictable, and there is always that chance that if you play on a frozen lake, you can find yourself plunged into its icy perils. If the dreaded does happen and you do fall through thin ice, the first thing to remember (and this is easier said than done!) is not to panic. It is true that the physiological response to cold water shock and hypothermia is pretty quick, but, by keeping a level head and knowing what to do, you can save yourself.
Immediately put your arms and legs out to slow your descent. The first reaction is to try to claw your way out but, by doing this, your body weight and wet clothing may pull you back down. Many folks also say they cannot get a deep breath, only because when they panic, they forget to exhale first.
Use your behind to lift your lower body and then bring your legs up and extend behind you. If you have skates, snowshoes, and even in some cases, boots, kick them off to dispose of added weight and anything that might “catch” on the ice when trying to pull yourself out.
Kick your legs softly to launch yourself forward on the ice, then kick hard to propel yourself horizontally out of the water on your stomach. Do not stand, but rather, use your arms to pull yourself across the ice to where it is thick enough to support your weight, then get on all fours and crawl towards shore.
Hopefully, you will never find yourself in this scenario, but it is always good to know what to do, just in case. Lakes in winter offer a beauty and solitude that are not present in the other seasons. You are truly missing out if you don’t grab yourself some “nice ice” time on our lakes and ponds.
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