Black ice

At what temperature do you get black ice?

Black ice most commonly forms when air temperatures are at or just below 0°C (32°F). However, it can also develop slightly above freezing if conditions allow the surface temperature of roads or sidewalks to remain colder than the air. This often happens during:

• Early mornings or late evenings, when temperatures drop quickly
• Radiational cooling after sunset, particularly under clear skies
• Cold air drainage in valleys or low-lying areas
• On bridges and overpasses, which cool from both above and below and tend to freeze earlier than regular roads

Importantly, black ice can persist even after sunrise if shaded areas or wind conditions prevent warming.

Why is black ice so slippery?

Black ice is particularly dangerous because of its smooth and uniform structure, which lacks the irregularities or granular surface found in snow or slush. This means:

• Tires and shoes have nothing to grip onto
• There are no visual cues (like frosted or textured surfaces) to alert drivers or pedestrians
• Sudden loss of traction can occur without warning, especially during braking or turning

This makes black ice one of the leading contributors to winter road accidents, especially on untreated or poorly maintained surfaces.

What is the cause of black ice?

Black ice forms when liquid water encounters a freezing surface. This can happen under several conditions:

• Freezing rain or drizzle: Precipitation falls in liquid form and freezes on contact with a cold surface.
• Melting snow: Daytime thaw followed by an overnight freeze can cause meltwater to refreeze as a clear ice layer.
• Fog or dew: Moisture in the air can condense and then freeze on cold surfaces, particularly overnight.
• Vehicle exhaust or water spray: In high-traffic areas, moisture from car exhaust or wet tires can accumulate and freeze on already cold roads.

The most treacherous aspect of black ice is that it often forms without active precipitation, making it easy to overlook during routine weather monitoring.

How to prevent black ice

While black ice cannot always be completely avoided—especially in unpredictable weather conditions—there are several preventive strategies that can reduce its formation and mitigate its hazards, particularly on roads and walkways.

1. Pre-treatment with anti-icing agents

The most effective way to prevent black ice is to apply anti-icing treatments before freezing conditions occur. This typically involves spreading:

• Brine (saltwater solution)
• Calcium chloride or magnesium chloride
• Other chemical deicers formulated for low temperatures

These substances lower the freezing point of water, preventing moisture from bonding to the surface and freezing. Pre-treatment is especially important before anticipated frost, light precipitation, or in areas known to freeze quickly—like bridges and shaded roadways.

2. Monitoring surface temperatures

Since black ice often forms when the road or surface temperature is below freezing—even if air temperatures are slightly above—it’s critical to use tools that monitor pavement conditions, such as:

• Infrared surface temperature sensors
• Road Weather Information Systems (RWIS)
• On-site weather stations

These provide real-time data that helps road crews and facility managers apply treatments at the right time.

3. Improve drainage and surface design

Standing water or lingering moisture increases the risk of black ice. Preventative infrastructure measures include:

• Proper slope and grading of roads and pavements
• Drainage systems to prevent pooling
• Heated pavement systems in high-risk areas (e.g., hospital entrances, pedestrian bridges)

4. Reduce surface shading

Surfaces that receive little or no sunlight during the day are more prone to black ice, as they do not warm up enough to prevent refreezing. Where possible:

• Trim overhanging trees
• Avoid placing structures (like sound barriers) that block winter sun in high-risk areas

5. Target known hotspots

Certain locations freeze earlier and more often due to microclimates or structural conditions. These include:

• Bridges and overpasses
• Underpasses and tunnels
• Rural, shaded, or low-lying roads

Maintaining a map of problem areas and applying treatments preemptively in these spots can significantly reduce risk.