Why pavement stays hot after sunset
The sun goes down, the air cools, the walk feels comfortable — and the asphalt is still hot enough to hurt. This catches out careful dog owners more than almost anything else about summer heat, because every instinct says the danger went down with the sun.
It didn't. It's still leaving.
Pavement is a battery, not a mirror
A surface in sunlight doesn't just reflect heat back; it stores it. Asphalt is dense, dark, and deep, and over a long summer afternoon a road absorbs an enormous amount of energy into the material itself, not just its top millimetre.
When the sun sets, that stored energy has to go somewhere. It leaves the way it arrived — slowly — radiating to the night sky and bleeding into the air above it. The road is warmer than its surroundings, so it keeps losing heat until it finally equalises, typically well into the night.
The rate at which it does that is set by how much heat the material holds and how fast it can shed it. For asphalt, that works out to a time constant of roughly three hours: about the time it takes to lose ~63% of the gap between its current temperature and the air's. One time constant is not "cooled down". After three hours it is still meaningfully warm; after six it's most of the way there.
Why a simple model gets this wrong
The obvious way to estimate ground temperature is to balance the energy arriving against the energy leaving, and solve for the surface temperature that makes them match. That's a steady-state model, and it's a good description of a surface that's had time to settle.
Its blind spot is exactly this situation. With the sun down, the incoming solar term goes to zero, and a steady-state model concludes the surface is at roughly air temperature — because that's the only answer consistent with "no sunlight". It has no memory of the afternoon.
That is a comfortable answer and a wrong one, and it's wrong in the dangerous direction: it under-warns, at precisely the hour when people take their dogs out.
What we do instead
Our surface model is transient: it integrates the energy balance forward through the recent past rather than solving a single instant in isolation. It carries about a day of hourly weather — air temperature, solar radiation, wind — and steps the surface temperature forward through them, so the ground you're standing on at 9 p.m. still reflects the sun that fell on it at 3 p.m.
Practically, that means the evening estimate comes out above air temperature, by an amount that depends on how strong the day's sun actually was and how much wind has been carrying the heat away since. A hot still evening after a blazing day stays hot much longer than a breezy one.
What to do with this
- Don't treat sunset as the all-clear. The air cools much faster than the ground does.
- A hot, still evening is the worst case. Wind is what actually strips heat off the surface; without it, the road stays warm for hours.
- Check the number rather than the sky. The paw safety check uses the surface estimate, not the air, so it keeps warning after dark when the ground warrants it.
The full derivation — the energy balance, the damping depth, the constants for asphalt and concrete — is in the methodology, and the limitations are listed there honestly.
These are modelled estimates for open, level surfaces, not measurements at your location. Shade, surface age and colour, and recent rain all change the answer.