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What causes black ice, the dangerous condition that causes cars and people to slip?

What is black ice?
What causes black ice? 02:02

Black ice is a well-known weather slang term for patchy ice that cannot be seen on roadways or other surfaces. In its typical form, black ice is completely transparent, only called "black" because it takes on the color of the asphalt underneath it.

How does black ice form?

Black ice can form under a number of different circumstances.  

  1. A very light rain falling on a frozen surface, otherwise known as freezing rain
  2. Mist of fog, again occurring in subfreezing air at the surface
  3. Leftover moisture from rain or snowmelt that doesn't evaporate and temperatures then fall below freezing
  4. Something as innocuous as moisture from car exhaust that freezes on the pavement

Black ice is not typical when you have a heavy, pounding rain or strong winds. This is because upon freezing, ice in these instances can usually be seen due to air bubbles or takes on a slushy formation. Leftover moisture on roadways will also dry up much quicker when there are strong winds present.

Can black ice form when it's warmer than 32 degrees outside?

Black ice can often be very thin and even form when the thermometer reads above 32 degrees. This is because the actual temperature of the pavement may be slightly colder than the air temperature above. This often happens early in the morning before ethe sun has a chance to heat the ground and melt the ice.

Where to watch out for black ice

While black ice can be found anywhere, there are some more frequent danger areas.

  1. Surfaces/roads where the sun doesn't shine including tunnels and underpasses
  2. On bridges and overpasses where there is no insulation underneath
  3. Back roads without a lot of traffic that perhaps, have not been treated by the town

As a final note, black ice isn't just a problem on roadways. In fact, most accidents involving black ice occur on your front steps, driveway and sidewalks. So, as you head outdoors on the next cold morning, beware that what is underfoot may not be what it seems.

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