There really is no sure answer. 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.
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 more information on ice safety contact the MN DNR and ask for our free ice safety publications, which include the brochures, "Danger, Thin Ice" and "Cold Water Kills" and the wallet-sized reference card and 11X14 posters titled, "Minimum Recommended Ice Thicknesses." Metro (651) 296-6157 or toll free outside the metro area 1-888-646-6367 or email the Information Center at firstname.lastname@example.org.
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