Thickness in Inches (cm)
|1-3/4" (4.5cm)||One person on skies|
|2" (5cm)||One person on foot or skates|
|3" (7.5cm)||One snowmobile|
|3" (7.5cm)||A group of people walking single file|
|7" (18cm)||A single passenger automobile|
|8" (20cm)||A 2-1/2 ton truck|
|9" (23cm)||A 3-1/2 ton truck|
|10" (25.4cm)||A 7 to 8 ton truck|
Never go out on an unknown ice sheet alone, and always probe ahead of yourself with a heavy ice chisel. Consider wearing a personal flotation device, roping yourself to an assistant, and/or wearing snowshoes.
The main thing to determine is the ice thickness. This can be done by drilling holes with an ice auger or a battery-powered drill with a very long (eg 12") bit. Wood boring bits work better than standard twist-drills, as bits intended for metal don't clear ice chips very well and often freeze in place. Note whether the ice is clear (sometimes called black ice) or white (due to air bubbles -- sometimes called snow ice). White ice is only half as strong as clear ice so 2" of white ice is equal to 1" of clear ice.
It is common to find both kinds. For example, there may be 3" of clear ice before a snowfall. The weight of the snow pressing down on the ice forces water up through cracks, creating a few inches of slush beneath the snow. When this slush freezes, it forms white ice. If there is 2" of white ice on top of 3" of clear ice, the net strength is the same as 4" of clear ice.
If you are in doubt, assume it is all white ice and divide the measured thickness by 2.
Take note of the frequency of cracks and whether they are wet or dry. For both rivers and lakes, warm inflows from springs may create areas of thinner ice. Also, the ice thickness near shore may be thinner (due to warm groundwater inflow or the insulating effect of drifted snow) or thicker (due to the candle-dipping effect of variable water levels).
Observe any snow cover as well as variations in its thickness. Obtain the record of air temperature for the past several days, and continue observing air temperatures during the period the ice will be used to support loads. A rapid and large air temperature drop causes an ice sheet to become brittle, and the ice may not be safe to use for 24 hours. If the air temperature has been above freezing for at least six of the previous 24 hours, multiply the required thickness by 1.3 to obtain a larger minimum ice thickness, accounting for possible weakening.
If the air temperature stays above freezing for 24 hours or more, the ice begins to lose strength, and the table no longer represents safe conditions. This becomes the general condition in the spring. Even though the ice may have adequate thickness, the strength is quickly lost the longer the air temperature is above freezing. In all cases of air temperature changes, the effects are greatest on bare ice, and are subdued by increasing depths of snow cover.
An ice sheet must be supported by water. Sometimes, near a riverbank, the water level will drop after the initial ice sheet is formed, leaving the ice sheet unsupported near the shore. This occurrence can be detected by hearing a hollow sound when probing with an ice chisel. Naturally this is not a safe location for loads on the ice.
Finally, respect warning signs and/or flashing beacons. Some people use "bubblers" to protect their docks from ice damage. The bubbles keep the water moving and prevent it from freezing. The resuting currents can result in thin ice for hundreds of feet beyond the dock, so the danger is not limited to the immediate area around the dock.