Why is some of the ice outside very slippery and some of it not?

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Why is some of the ice outside very slippery and some of it not?

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The short answer is surface texture. Some ice can be smooth like glass, like that found on a frozen pond or black ice on the road. Some ice can have a rough texture like sandpaper. The rough ice has jagged bits that dig into your shoe, reducing how easy it is for your shoe to slip sideways.

Fresh ice (especially the ones due to snow) is rougher than old ice. This is because fresh ice is formed when snow melts and freezes again. So it could be rough. But the longer it stays, the more opportunities it has to melt and freeze over again.

When the sun comes out, it melts the ice, it flows around like water, becomes smoother in texture and then freezes over again. This is how you can find smooth ice.

Another way you can find smooth ice is, if ice forms slowly over a long time (stairs), or if there is generally no disturbance in the surface where it is forming (lake).

It depends on the surface water.

If the ice spends lots of time around and below the freezing point, some ice will melt from the sun and freeze in the shade or as temperature drops.

Traffic also affects the surface. Lots of shoes and cars going over the glass creates pressure and friction that polishes and melts a little bit of ice and that water once again fills the bottom of those little rough patches and the freezes smoothing everything out.