What is Black Ice?

The most ironic weather forecast of the morning: "Be on alert for black ice during your commute." It's a bit tricky to be cautious about something so elusive, Mr. Meteorologist.

It often masquerades as a harmless puddle, fooling drivers into a false sense of security. Then, they drive over it, lose control, and skid straight off the road, potentially off a cliff.

When the car loses control, it tumbles end over end, crashing into the bottom of a deep ravine, where it erupts into a fiery explosion at least six stories tall. But wait! The driver is clinging to the side of the cliff by his fingertips!

What were we talking about again? Oh, right, black ice. In reality, it's a perilous hazard; though, it probably won’t send you spinning into a dramatic action movie scene.

Black ice usually forms overnight when temperatures drop, making chilly, sunless winter mornings the most common time for drivers to encounter it. While there are plenty of helpful guides on how to drive on black ice — like this one — it mostly comes down to easing off the gas, resisting the urge to slam the brake pedal, and keeping the steering wheel steady until you regain traction.

Of course, that’s easier said than done. It’s also wise to avoid using cruise control if you suspect black ice is nearby, as it gives you more control over the car if you start to slip.

Knowing how to respond (or not respond, as the case may be) when driving over patchy ice is practical; but if you've ever wondered what black ice is, how it forms, and why it’s so dangerous, we’re here to help.

How Black Ice Forms

Black ice is nearly transparent. It has no trapped air bubbles or swirls (referred to as occlusions, if you’re using the technical term for bubbles and imperfections within ice).

White ice, as we know, is either white and translucent (letting some light pass through) or opaque (not letting any light through) because of occlusions. Black ice lacks these imperfections, making it perfectly clear. It appears black simply because the pavement underneath is visible.

Fun fact: Before paved roads, drivers never had to worry about black ice! Ice existed, of course, but it was the color of whatever surface it formed on, like dirt. The term "dirt ice" or "macadam ice" never really took off, though.

To achieve this level of clarity, a few conditions must align. First, there needs to be water, and it should fall gently — fog, mist, light rain, or even exhaust condensation all qualify. If it were a downpour, bubbles and swirls would form, turning it back into plain old white ice. Slow-melting snow also contributes when it seeps onto the pavement and refreezes. Second, it must be calm. Wind would create bubbles and swirls, making it white ice again. On a sunny, cold day, the sun can warm the road enough to melt any slush, and overnight, that melted snow can refreeze into black ice.

The way water gently settles on the pavement leads to a thin layer of freezing, which makes black ice nearly invisible. It might appear as just a wet patch — until you drive across it. Black ice can even form when the air temperature is above freezing. If the road surface is 32°F (0°C) or colder, black ice can form even if the thermometer says it's above freezing. This is especially common in the morning when the winter sun hasn’t warmed up the pavement, and you’re still half asleep, rushing to work, making it even more dangerous.

Black Ice and Its Habitat

Certain areas are more prone to black ice formation — Wisconsin, for instance, compared to southern Florida. You’ve probably already figured that out from regional weather patterns.

You’ve likely seen signs warning of bridges freezing before the roads. That’s true — frozen water on a bridge is often black ice. Cold air and wind flow above, below, and all around a bridge, while the absence of ground beneath it prevents the pavement from maintaining a steady temperature. Any moisture on the bridge can quickly freeze into smooth, bubble-free black ice.

Black ice also loves shady spots where the sun doesn’t reach. In urban areas, this means tunnels and underpasses. Last night’s commuters likely left a fine mist of moisture from their exhaust pipes, which can freeze into black ice for your morning drive. Thanks a lot, guys!

For those driving in rural areas, black ice is often found under trees with branches hanging over the road. And anyone with a shaded driveway should be cautious of black ice before even heading out.

Rural motorists should remain vigilant as icy conditions can persist on less-traveled roads. While vehicles in bustling areas benefit from the heat of their engines and tire friction, which quickly dry up and thaw the streets, remote roads might not see enough traffic to warm up and melt the ice.

It's important to remember that official temperature readings, such as those from the National Weather Service, are measured about five feet above the ground. So, while the air temperature might register as 37°F (2.8°C) on the thermometer, the road surface could still be at or below freezing. Watch out for black ice!