What is black ice? The science behind the winter threat hidden in plain sight
Nearly invisible and deceptively thin, this ice is one of winter’s deadliest hazards. Here’s how it forms and why it causes thousands of accidents each year.
When the weather is calling for winter storms, drivers know to prepare for roads with slippery sleet and snow. But there is an invisible hazard that can appear even when skies are clear—black ice.
"It can be present when there is no precipitation so it can sneak up on you,” says Michael Muccilli, a meteorologist with NOAA’s National Weather Service.
Black ice creates a dangerously slippery surface where tires can no longer grip the pavement.
“Even basic maneuvers—like turning a corner or slowing down for a light—can trigger an uncontrollable slide because the vehicle's tires are essentially gliding over a thin, frictionless sheet of ice,” says Ernesto Urbaez, pavement engineering expert at Virginia Tech Transportation Institute. “This slipperiness is especially deceptive because it often occurs in patches, meaning a driver can suddenly transition from a high-traction dry road to a zero-traction icy one in a split second.”
What exactly is black ice? Here’s what to know about how, when, and where it forms, along with tips for spotting this hazard and navigating it safely when behind the wheel.
How does black ice form?
Black ice is a thin, transparent layer of ice on a dark asphalt road surface, which gives the phenomenon its descriptive name. Also called clear ice, it freezes without the trapped air that can give other ice a chalky appearance that’s easier to spot.
Most black ice is created by freeze-thaw conditions when temperatures hover around the freezing point.
“When you have water on the roadway from things like rain or melting snow, that then refreezes at night when the temperature drops below freezing,” Muccilli says. This cycle explains why black ice is prevalent after nightfall and in the early morning hours, particularly on stretches of road shaded from sunlight.
Freezing rain can also produce black ice, as the air is warm enough for rain, but it falls on ground that is still frozen.
Moisture in the air, like mist or fog, can also condense and freeze on cold pavement, as can car exhaust fumes: In places like intersections or your driveway, where cars idle at cold temperatures, exhaust vapor can freeze when it comes into contact with the pavement.
Can black ice form in above freezing temperatures?
The nature of black ice means it forms less often in very cold temperatures, and more often when the mercury is fluctuating below and above freezing. Muccilli says you need some kind of liquid, whether that's rain, snowmelt, or even fog droplets, for black ice to manifest.
So even when your car's thermometer tops 32°F, black ice may still be lurking where the pavement temperature remains colder than the air.
Black ice may become more or less common as global warming impacts the number of days hovering around these key temperatures. Experts say it all depends on location.
“In colder areas, where average temperatures have been rising over time, you may have more melting and refreezing which could lead to more black ice,” Muccilli says. “But in other areas, where temperatures might not get to freezing as often, you may have less.”
Where is black ice most common?
While black ice is more familiar in colder regions, it can occur anywhere temperatures drop below freezing overnight. Drivers should be especially alert in some key areas.
Bridges and overpasses are prone to black ice accumulation because cold air flows beneath them, enveloping the structure and lowering the temperature of the road surface to cause freezing. Underpasses shade roadways from the sun and prevent them from warming. In fact, all tree-lined or otherwise shaded roads tend to hold black ice longer as temperatures rise, as do low-lying areas that trap moisture.
While black ice is frequently seen as a driving danger, it is also a problem for pedestrians. Black ice regularly forms on sidewalks, steps, decks, and other areas where those on foot are susceptible to slips.
How to recognize black ice
Anytime the roads are wet and temperatures are hovering around the freezing point, experts say drivers should assume black ice is possible.
To spot black ice, look for areas that appear a bit shinier or wetter than the surrounding pavement or reflect headlights differently at night. However, don’t count on being able to spot every patch of black ice. The best way to stay safe when driving in this winter weather is to slow down, leave plenty of space between cars, and avoid distractions while driving. If you do encounter black ice, don’t panic and avoid sudden movements.
“If you feel your car start to slide, do not slam on the brakes or jerk the steering wheel, as this will cause you to lose control completely,” Urbaez notes. Instead, keep the wheel pointed straight and take your foot off the gas to let the car slow down naturally. If you must brake, and your car is equipped with an anti-lock braking system (ABS), apply steady pressure; otherwise gently pump the brake and try to remain calm.
“Remember that black ice usually appears in small patches, so if you stay composed and keep your wheels straight, you will likely regain control within a few seconds,” says Urbaez.
