Originally Posted by 79pmooney

I grew up ice skating, both on artificial ice in rinks and pond ice. Black ice is ice that has a very pure and strong crystal structure. Happens as undisturbed water is cooled. But as that ice goes through temperature changes, that very strong crystal structure is lost and the ice turns grey. Rink ice is grey ice. (Perhaps really cold places can make black rink ice but I never saw it in Massachusetts. 5F grey rink ice is very hard but it is not as hard as pond ice.)

Black is is so strong that very thin ice can support a person easily, even though it is so thin that it bends down a half inch or more under their weight. I've skated ice that was well under a 1/2" thick. The danger is that it is so clear it is invisible. 1/4" or 1/8" looks no different. Now, once that ice has seen a day of warmer air, it turns to grey and better be over an inch thick to skate. (And it will continue to degrade with more warm days. But if the nights mean adding thickness, the overall strength may well build. Danger comes at the end of winter as the thickness building stops and the very thick ice "rots".

So, back to ice and how it behaves under skates and bicycle tires. What makes ice so much more slippery than say glass is that under sufficient pressure it melts. Skates place a lot of weight on a thin blade. That amount of pressure melts the ice. (There is a minimum temperature at which skates work. I've skated at 5F. That works. But I am guessing -20F would be disappointing.) Also, ice hardness varies a lot. Black ice is hard. Cold black is very hard. Skates need to be a lot sharper for good skating, turns and stops on new pond ice than rink ice. Skates sharpened for cold black ice are trippers on rinks! We walked the wooden ramps aggressively to dull our skates if they were too sharp to play hockey. Good skate sharpeners know how sharp to to go. Some of us put in special requests.

And bicycle tires. Ice is all over the place. Frozen puddles may well be classic black ice. If cold enough not all that slippery because it isn't melting and relatively hard but snow on it may well be like deep dust on glass. Warmer and you may have water, either from the prevailing temps or your tire's pressure on it. Slippery! There is no rule for how hard grey ice will be. The strongest is maybe 1/3 to 1/2 that of black ice but it degrades much like wood rots, just far faster. (Think slush and that tree that came down 25 years ago.)

So - experience with your local weather and roads counts for a lot. Riding the same old roads all winter sounds boring but knowing where to expect that ice can prevent a lot of falls. Access to thermometers and reliable weather forecasts that tell you what you need to know can be invaluable.