Winter brings new opportunities to experience the outdoors. Take a look at the activities that go on during a cold and snowy day.
Many kids and even some energetic adults venture outside to sled, build snowmen and throw snowballs. Another interesting activity is to walk on the surface of that local lake that has now frozen over.
However, walking on that frozen lake could be very dangerous if you do not understand the science of ice. How lake ice forms, how thick it has to be before you can walk on it and other behaviors of ice are important to know before you venture out.
It Takes a Lot of Energy
It takes a lot of energy to change the temperature of water. Just like it takes quite a bit of time to boil a large pot of water on the stove, it also takes a long time for water to cool. Surely you have noticed a very cold, snowy day at the beginning of winter where a nearby lake still was not frozen. The ground cools faster than water, and the more water in a lake, the longer it will take for the temperature of the water to fall below freezing.
How Long Does it Take?
Now we get back to walking on a frozen lake. When cold air is in place, the top layer of water will begin to cool as it comes into contact with the cold air. If the cold air is around long enough, that top layer of lake water will drop in temperature until it freezes. Ice is less dense than water, so the ice will float on top of the water.
If the cold air stays in place, the ice will grow thicker and thicker. But that top layer of ice insulates the heat from the bottom of the lake, so it is not a fast process.
Scientists have developed a formula for calculating how fast ice will freeze on a lake. Here’s how:
1. Calculate the average temperature of the day by adding low temperature and high temperature and dividing by 2. Use degrees Fahrenheit. Now subtract that average from 32 degrees, the temperature at which water freezes. You have now just calculated what they call “freezing degree days.”
Example: If the low temperature today was 24 degrees and the high was 30 degrees, the average temperature of the day was 27 degrees. Subtract that from 32, and you get 5 freezing degree days for this day.
2. Based on studies, once a thin layer of ice film forms on a lake, it will add 1 inch of ice for every 15 freezing degree days in a 24 hour period.
Example: If we had a day with 5 freezing degree days, you would use the formula 5/15 = .33. On this day you would add .33 inches of ice, or 1/3 of an inch. But remember, this is after the first layer of ice forms.
This shows how long it can take a lake to freeze, even after a cold day.
The Four-Inch Rule
Most agree a lake needs to have at least a 4 inch layer of ice on top for it to be safe to play on. You can use the freezing degree day calculations to estimate when that would happen, but those are only estimates. The amount of sunshine or wind on the lake, the depth of the lake and the source of the lake’s water all can impact how fast its ice thickens.
The only safe way to tell is to drill a small hole into the lake to measure the ice or wait for local officials to tell you it is safe.
Unfortunately, people and animals die every year by walking on frozen lakes where the ice is too thin.
It is best to enjoy the view of the lake from shore unless you are told it is safe. However, when you are told it is safe — go have fun!
