Weather Explainers

Weather Words: Surface Temperature

Surface temperature is the temperature on the ground level, which often determines when roads freeze, helping explain why icy conditions can form suddenly even when the air temperature is above 32 degrees.

When it comes to winter weather and icy roads, surface temperature often matters more than the air temperature you see in a forecast. Surface temperature refers to the actual temperature of the ground: roads, bridges, sidewalks and runways.

On clear winter nights, surface temperatures can drop rapidly as heat escapes into the atmosphere, even if air temperatures remain near or slightly above freezing. This is why roads can ice over when your car thermometer reads 33 or 34 degrees. Pavement loses heat quickly, especially when skies are clear and winds are light, creating ideal conditions for freezing on contact when moisture is present.

Surface temperature plays a major role in winter precipitation impacts. Snow may melt initially on warmer pavement during the daytime hours, but if surface temperatures drop below freezing late in the day or overnight, that meltwater can quickly refreeze, leading to slick conditions.

The Kuskulana Bridge is a one lane, 525 foot bridge in Alaska that can many times become icy in cold weather conditions.
(NPS)

The same is true for freezing rain and drizzle, which can turn roadways into skating rinks if surfaces are already cold. Bridges and overpasses are especially vulnerable, as cold air circulates above and below them, allowing their surface temperatures to drop faster than surrounding roads.

Understanding surface temperature helps explain why icy conditions can develop suddenly and unevenly. A road may appear wet in one area and dangerously icy just a mile away due to differences in elevation, shading or pavement composition. That’s why winter driving hazards often linger longer overnight and during the early morning hours, long after precipitation has ended, as cold surfaces continue to refreeze any remaining moisture.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.