Or put another way, how thick does ice on a pond or lake have to be before it’s safe to skate on? Do public parks with ponds that freeze over still even allow people to skate, or is it too much of a liability?
Please explain the facts to this California boy.
What I remember:
1 inch - stay away
2 inches - one may
3 inches - small groups
4 inches - OK
Hmmm, http://www.dnr.state.oh.us/watercraft/safetips/ice.htm seems to think that is reckless. Maybe I am misremebering the rhyme.
Brian
Who has fallen through the ice at the creek near where he grew up.
And remember, the ice may be thick enough in one place, but much thinner (or thicker) in others. This is particularly true on rivers.
THat being said, I used to drive across a river daily - it saved about 12 miles from my work commute. I was stupid, and not the only person doing it. I never went in, but came close. In the spring, the ice would start melting from the shore first, and more than once I crossed 3-4’ of open water. Dumb.
This site says, “If you plan to go where the ice is not monitored for safety, make sure that the ice is at least four inches thick and carry safety equipment.”
Another site says, “A minimum of four inches of solid ice over the entire lake is recommended before individuals attempt ice skating.”
Yet one more site says, “The truth is that there is no guaranteed safe ice thickness.” This last site goes on to discuss how safe ice thickness should be contemplated.
My Floridian brother once told me a story about a trip he took to Kansas in winter. Having never seen iced over lakes/rivers, he was having a blast playing on it. But when he went to go on one particular farm pond, our cousin warned him off of it. He said it was “dirty ice” and was unsafe.
What is dirty ice? Is it an ice that forms on a muddy pond? That’s what my brother assumed, but that seems odd to me. Why would ice from muddy water be so dangerous?
One thing could be if it was marginal to start with, the dirt is dark and collects heat better that the white so it has small places or a weak and soft structure. More slush and as the sun climbs higher in the day, it just get worse. It can even have a melting going on while the air temp is below freezing. Dirty ice in marginal conditions = bad news. YMMV
My WAG on dirty ice is that since it’s not pure water, it won’t freeze as solidly. Dirt and most other random crap found on the ground won’t do anything special at freezing temperatures, so you’d end up skating on something that was part ice, and part dirt, dead plant bits, and old chewing gum- all of which will weaken the bonds of the frozen water.
My father’s books on ice fishing match N9IWP’s rhyme.
1" - No.
2" - Maybe.
3" - A few people.
4" - Safe for fishing.
5" - Safe for Snowmobiles.
FYI, you can drive snowmobiles on plain water. I’ve seen it with my own eyes. 
According to the Canadian Red Cross (Ice Safety is a component of their Water Safety program), the recommended minimums are:
a few people - 6 inches (15 cm)
a large group - 8 inches (20cm)
a vehicle - 10 inches (25cm)
Stay away from moving water. Call the local Fire Department for the latest measurements. Never go on the ice without a buddy.
I had heard something to the effect that ‘Dirty Ice’ was ice that formed with a lot of air in it (due to forming on less then a calm surface, I presume). I would guess that the air would make it less dense and, thus more unstable.