Black ice formation: What it is, why it forms, and how to stay safe

"I took this photo around 10 am on my porch -- hail that fell on top of a thick layer of black ice."

Winter storms often bring the threat of ice accumulation, creating dangerous conditions for both driving and walking. One of the most hazardous forms is black ice, a thin, nearly invisible layer of ice that forms on sidewalks and roadways. You usually don’t notice it until you’re already slipping and sliding.

Despite its name, black ice isn’t actually black. The asphalt beneath the layer of ice gives it a dark appearance, allowing it to blend in with the surface. This makes it especially treacherous.

Why does black ice form?

According to the National Weather Service, black ice is most likely to form at night or early in the morning, when temperatures are at their lowest. It often develops when daytime snowmelt refreezes after sunset, once temperatures drop below freezing. It can also form when rain or drizzle freezes on surfaces as temperatures fall below 32 degrees.

Black ice often forms in areas of the road that receive little to no sunlight, such as tree-lined stretches or tunnels. It can also appear more frequently on roads that see less traffic.

Bridges and overpasses are particularly hazardous in cold weather because they freeze faster than regular road surfaces. Since cold air surrounds them from above and below, they lose heat quickly and can reach freezing temperatures sooner. As a result, ice tends to form on bridges well before it develops on roads insulated by the ground beneath them.

Tips for driving and walking on black ice

If you encounter black ice while driving, stay calm. Keep the steering wheel straight, avoid hitting the brakes and gently lift your foot off the accelerator. Ideally, your car will naturally slow down and come to a stop.

When walking on black ice, choose footwear with good traction and move carefully with short, deliberate steps. Keep your hands free for balance or to hold onto railings. Stay alert and watch the path ahead to avoid slipping.