The winter season is well underway already with snow, cold, and lakes freezing over.  So, it’s a great time to review some of the physics of ice strength as well as general ice safety information.

Because there is a lot to cover, this is post one of two. This post will concentrate on the physics of ice and the strength (or lack thereof) of ice and other factors which can influence ice strength along with basic thickness guidelines.  Our second post will cover general ice safety information, self-rescue techniques, what happens to the body in cold water (yes, we know about the shrinkage factor…but there are other things too), and more.

Myself and others enjoying the water as part of our Ice Rescue Instructor Class

Let’s first start with how much ice is needed to support you, a snowmobile, a UTV, a car, and more.  Like many things in life, it depends.  Ice is NEVER 100% safe.  As an ice rescue instructor for the past ten years, I have learned that there are dozens of factors that that can and will influence the strength of the ice you are heading out on and in many cases, it’s very unpredictable. These factors include temperature cycles; sunlight exposure; wind, snow coverage; depth of the water; springs, currents, and other underwater obstructions; cracks and heaves, and more.

Some interesting ice strength facts:

• Clear Ice Is Good Ice.  This holds true not only for your brandy old fashioned, but also for lakes. Clear, blue/black ice is the strongest ice, while ice that is snow-covered, cloudy (with air bubbles/impurities or the result of a refreeze) or slushy ice is much weaker and only has about half the strength of fresh ice.
• Snow is not great for ice strength or formation. It not only insulates the ice, slowing the freezing/thickening process, but also adds weight to the ice, reducing the weight that the ice sheet can support. As snow thaws, it creates cloudy/white ice which is much weaker than clear/new/black/blue ice.
• All other factors being equal, bad ice (cloudy, white, slushy) only has half the strength of newly formed ice. So, you may need to at least double the safety thickness guidelines based on the ice type.
• Honeycomb ice typically happens in spring and while nice to look at, has very little if any strength and should be avoided at all costs.
• Ice heaves and pressure ridges are dangerous and can cause serious injuries if you do not see them or attempt to jump over them. These form when the air temperature drops well below freezing and the ice contracts and causes cracks to form. The lake water then seeps up into those cracks and freezes, filling the gaps. When the air temperature rises, the ice needs to expand but can no longer fill out the gaps. The pressure from the expanding ice typically causes one sheet of ice to slide on top of the other, creating large heaves in the ice.
• Rain is horrible to any ice sheet. Quick trivia question: What does one inch of rain on one acre of ice weigh?  Give up?  Well, it may seem like just an inch of water but it’s one inch over 43,560 square feet which would weighs over 200,000 pounds just on that one acre of ice.  This weakens the entire ice sheet drastically!

Keeping all these things in mind, here are some general ice thickness guidelines for new, clear, and consistent thickness ice.  These guidelines should be doubled for white/cloudy ice. You should know and are responsible for checking ice thickness on your own or with local establishments to identify thickness, strength, and areas of concern before going out and know the weight of what you are taking onto the ice before you go.

General Ice Thickness Guidelines- Valid for New, Clear, Consistent Thickness Ice

Under 4”         Stay Off!

4”                    Ice fishing or other activities on foot

5-7”                Snowmobile or small ATV

7-8”                 Larger ATV or Smaller UTV

9-10”               Larger UTV, Small car or smaller SUV

11-13”             Medium SUV or Medium Truck

16” +               Heavy duty trucks

Watch for our follow up post where we will cover ice safety tips on what to pack and do before you head out onto the ice, what to do if you should fall through, the 1-10-1 principle, and how to safely help others who may have fallen through.  Stay tuned, get prepared, be safe, and have fun.