As temperatures get colder ice begins to form over water. But just because ice forms over water, it doesn't mean it's safe to walk on.
Ice is very susceptible to breakage.
Here are some guidelines:
5 cm thick- the ice is very susceptible to breakage and is not safe to walk on.
10 cm thick- it should be ok to stand, skate, and ice fish on the surface.
12 cm or greater- the ice should be able to withstand most snowmobiles.
20 and 30 cm- the ice should be able to withstand the weight of a small to medium-sized car.
Anything above 30 cm- should be able to support medium-sized trucks.
These are guidelines only!
If you see open water near the ice, it is not safe to be on the ice. Clear ice is twice as strong as white ice. Avoid cracked ice. Cracks compromise the integrity of the ice sheets, so check for cracks before taking your next step. If you are on the ice and notice it cracking, immediately get off the ice.