During the colder months, you will often hear our team of meteorologists warning to “be alert for black ice.” The icy glaze can cause accidents and increase your needed stop time when approaching intersections. It can also form without any precipitation having fallen. So, what is black ice and when does it form?

What is black ice?

In simplest terms, black ice is a thin layer of ice on pavement. It is called black ice because it blends in with the pavement. Ice is transparent and a thin layer on a road can be difficult to spot. The road may just look wet when, in reality, the frozen glaze will result in slippery conditions.

Black ice is a thin layer of ice that can form on roads when temps are below freezing. The thin layer is difficult to see due to the dark color of the pavement and transparency of ice.

When does black ice form?

Also making black ice tricky to spot is the fact that it can form on a day when no precipitation has fallen. There are three primary ways black ice forms. One of those is from melting snow. When daytime temperatures rise above freezing or when abundant sunshine helps melt snow on or next to roads, the water may not all evaporate off the pavement. When that melted precipitation remains on the pavement as temperatures dip below freezing, it will re-freeze. The result can be slick travel conditions.

Another way black ice forms is when light rain falls onto the pavement when the pavement temperatures are below 32°. The water will freeze onto the pavement. It can also form after the rain has stopped falling if the rain doesn’t evaporate before temperatures drop below freezing.

Fog can be another contributing factor to black ice developing. Fog is composed of tiny water droplets. When fog develops and surfaces are below 32°, the water droplets can freeze on contact. If you drive through a foggy area and your car thermometer is close to or below 32°, be extra alert for slippery conditions.

There are three main ways black ice forms during the winter: Melting snow, rain/drizzle on the roads as temperatures drop below freezing or water droplets in fog freezing onto surfaces.

Bridges and overpasses can be especially susceptible to black ice

Be especially cautious on bridges and overpasses when temperatures are forecast to drop near to below freezing. Even if the road before or after the bridge isn’t icy, the bridge itself may ice over faster. When air temperatures drop, it will take a little longer for the ground temperatures to catch up. This is especially true if the day is sunny without snow on the ground. The ground can hold onto heat energy from the sun longer than the surrounding air.

When it comes to bridges and overpasses, they won’t benefit from the energy stored in the ground. As the air temperature falls, the colder air will take heat from the upper surface and the lower surface of the bridge. The loss of heat above and below allows bridges to cool a lot faster. On nights when temperatures are only falling to right around the freezing mark, it can be more likely that you encounter black ice on the bridge compared to the pavement with solid ground below it.

Bridges and overpasses lose heat from the upper and lower surface. That causes a faster drop to the surface temperature and can lead to ice formation before surrounding paved surfaces when water is present on them.

Impact of snowy and icy roads on vehicle stopping time and distance

Obviously, it is important to slow down and drive with extra caution when snow, ice or even water is on the road. But did you know just how much longer/farther it takes to stop when roads are not dry or covered in snow and ice? The increase makes it clear how important it is to slow down when approaching traffic signals, intersections and stop signs.

Below is a look at the distance it can take to come to a complete stop from 35MPH and 50MPH when roads are dry, wet, snowy or icy. Slide the slider bar to the right to view the distance at 35MPH and to the left to see distances at 50MPH.

Below is a look at the time it can take to come to a complete stop from 35MPH and 50MPH when roads are dry, wet, snowy or icy. Slide the slider bar to the right to view the time at 35MPH and to the left to see stopping times at 50MPH.