When is ice safe? There really is no sure answer. Ice should never be considered safe. You can’t judge the strength of ice just by its appearance, age, thickness, temperature, or whether or not the ice is covered with snow. Strength is based on all these factors — plus the depth of water under the ice, size of the water body, water chemistry and currents, the distribution of the load on the ice, and local climatic conditions.
Some facts about ice
New ice is usually stronger than old ice. Four inches of clear, newly‑formed ice may support one person on foot, while a foot or more of old, partially‑thawed ice may not.
Ice seldom freezes uniformly. It may be a foot thick in one location and only an inch or two just a few feet away.
Ice formed over flowing water and currents is often dangerous. This is especially true near streams, bridges and culverts. Also, the ice on outside river bends is usually weaker due to the undermining effects of the faster current.
The insulating effect of snow slows down the freezing process. The extra weight also reduces how much weight the ice sheet can support. Also, ice near shore can be weaker than ice that is farther out.
Booming and cracking ice isn’t necessarily dangerous. It only means that the ice is expanding and contracting as the temperature changes.
Schools of fish or flocks of waterfowl can also adversely affect the relative safety of ice. The movement of fish can bring warm water up from the bottom of the lake. In the past, this has opened holes in the ice causing snowmobiles and cars to break through.
For New, Clear Ice Only
2″ or less – STAY OFF
4″ – Ice fishing or other activities on foot
5″ – Snowmobile or ATV
8″ – 12″ – Car or small pickup
12″ – 15″ – Medium truck
White ice, sometimes called “snow ice,” is only about one-half as strong as new clear ice so the above thicknesses should be doubled.
How to check the ice thickness? One way is to use a cordless drill with a 5/8 inch wood auger bit. It won’t take long to drill through the ice to check the depth. Use a tape measure.
Other things to keep in mind when checking ice.
Ice is seldom the same thickness over a single body of water. It can be two feet thick in one place and one inch thick a few yards away due to currents, springs, rotting vegetation or school of rough fish.
Vehicles weighing about one ton such as cars, pickups or SUVs should be parked at least 50 feet apart and moved every two hours to prevent sinking. It’s not a bad idea to make a hole next to the car. If water starts to overflow the top of the hole, the ice is sinking and it’s time to move the vehicle!
(Data sourced from the Minnesota Department of Natural Resources – I figured that those in MN certainly ought to have good information about this!)