Every deicer decision starts with one number, and it's not the one on your dash. Salt melts ice at the pavement surface, so the pavement temperature, and which way it's heading, is what decides whether your material works.12
Why air and pavement disagree
Air temperature is usually similar across a town. Pavement temperature can vary a lot from lot to lot and hour to hour. It depends on sun exposure, pavement type, what's underneath (a heated building, a bridge deck with cold air below, a parking ramp), and the weather history of the last few days.1 The same 25°F air temperature in November and January often comes with very different pavement temperatures, because November ground still holds summer heat and January ground doesn't.
How to get a real pavement number
- Infrared thermometers are cheap and fast. Make sure yours reads well below freezing; some are built for ovens and only read warm temperatures.1
- Mirror- or dash-mounted pavement sensors give a running read as you drive the route.
- KDOT posts road conditions on KanDrive and uses brine anti-icing on state routes.3
What changes the number on your sites
| Spot | Usually runs | Why |
|---|---|---|
| Sunny, south-facing asphalt, midday | Warmer than air | Dark pavement absorbs sunlight, even on a cold day |
| North side of buildings, under canopies | Colder than open lot | No solar gain all day |
| Bridges, ramps, elevated decks | Colder, freezes first | Cold air above and below; no ground heat |
| Clear, calm night after sunset | Can drop below air temp | Pavement radiates heat to a clear sky |
| Over heated spaces, near building walls | Warmer | Heat leaking from below |
That clear-night cooling is why frost and black ice can form on pavement when the air never hits 32°F, and why refreezing meltwater ambushes lots after midnight.
Next in the guideChoosing a deicer by temperature →