Each winter, lakes and rivers in the northern U.S. eventually succumb to the numbing cold of the season by freezing up. As winter's grip continues, the ice thickens. Eventually, the ice becomes strong enough for folks to venture out on foot, and even by car and truck. Ice fishing, snowmobiling, and cross-country skiing are all popular past times that entice folks to get out onto the ice. In the far north, ice stays thick enough for several weeks, as some folks tow ice houses onto the ice and drag them over to their favorite fishing spot on the lake. In order for all of this to happen successfully, an understanding of ice physics is of utmost importance.
One thing to understand is that ice is never completely safe. There are many factors that can cause ice to be unsafe, even when ice has thickened to an 'acceptable' range. The image below gives the basic safety guidelines for how thick the ice must be in order to support the many activities that entice folks onto it.
Ice is not deemed safe to walk on until it has become at least 4 inches thick. At that point, ice needs to thicken further before any extra weight can be supported. Snowmobilers can begin to venture out on the ice when 5 inches thick, while those with trucks will need to wait until there is at least a foot of ice present. Folks should always keep in mind that these designated thickness requirements are only applicable when the ice quality is sound, and ice quality will vary from year to year.
There are a few signs to look for, when judging the integrity of ice. In general, there are two types of ice that form, clear ice and white ice. Clear ice tends to be newer ice, and as the name suggests, you can see through it. On the other hand, white ice is ice that has many air pockets that have frozen within it, which compromises the strength of the ice. Snow (which naturally has air pockets in it) oftentimes mixes into the ice as it is forming, resulting in white ice. Another factor that can pollute the purity of the ice is an inconsistent freeze/thaw pattern that can be present while the ice is forming. Clear ice, on the other hand, forms when a consistent period of subfreezing temperatures occurs during a period of little to no snowfall. Ideally, by the time the lake has cooled enough for ice to form, light winds and bitter cold temperatures occur. When these conditions are present during freeze up, a beautiful scene can transpire. This video shows a scene on a lake where this occurred in Manitoba.
Clear ice, as shown in the video above, is twice as strong as white ice. Therefore, in order to walk on white ice, you would want to have 8 inches of ice present, compared to the 4 inches needed in clear ice conditions. Generally, you want to double all the thickness guidelines when white ice is present. The other color that ice can take on is an opaque grey. This is usually present when ice is in the process of thawing or melting. Grey ice is particularly dangerous, as a layer of slush is typically present, making the amount of solid ice underneath unknown.
Whether you have clear ice or white ice present, ice thickness will vary from one point to another across a lake or river. This is due to varied snow depth over the ice, the existence of underwater currents or springs, and even impacts that commonly traveled areas by schools of fish can have. Snow depth impacts ice thickness by insulating the ice from the cold. Snow's insulating properties are driven by the fact that air pockets are present in snow pack. The heavier the snow pack, the less ability the ice below has of thickening. Underwater currents and lake bottom springs create thin areas of ice, due to the friction of moving water below the ice. Habits of the fish population can also create this same effect. If a particular spot in the lake is a common travel lane for schools of fish, their constant movement will create a current, thereby lessening the thickness of the ice above. Due to all these factors that oftentimes create varying ice thicknesses over a lake, it is always best to talk to folks familiar with the lake before venturing out (often bait shop owners or lake side resort staff are a good source of information). One can also measure the ice thickness themselves, but remember ice thickness will vary from place to place.
Due to the variances in ice thickness and quality that are common each winter, it is a good idea to take precautions when venturing onto the ice. Screwdrivers and ice picks are simple but potentially lifesaving tools to bring with you. If you have the misfortune of falling through the ice, you can use these tools to grip the edge of the ice that is still intact around you (it's best to swim back to the area of the ice you were on before you fell in). Without them, it can be difficult to grip any ice around you enough to pull yourself out of the water. Once you are out of the water, roll away from the hole until you are far enough away to be on solid ice. If you stand up too soon, your weight will be less spread out and raise the chance that you will fall back in again. Once off the lake, get to a warm and dry place and remove wet clothes and wrap yourself in dry clothes or blankets. If you aren't suffering too heavily from hypothermia, doing some jumping jacks or push ups is also a good way to increase blood flow and warm the body back up. If you feel disoriented and have uncontrollable shivering or any other signs of severe hypothermia, call 911 to get medical attention.
It is always important to understand the basic principles of how to best judge ice quality and thickness before venturing out onto ice. Clear ice is the preferable type of ice to find, as it is the strongest type of ice that can form. When white ice is present, the thickness guidelines double, due to its lack of structural integrity. Remember that ice thickness can change in the matter of a few feet, so don't venture far from where you are confident as to how thick the ice is. Bringing ice picks or a rope (to help pull someone out) can be lifesaving tools to have with you. No one ever plans on falling through the ice, but be prepared, just in case the worst happens.