Why it matters
Grip lives in the surface. Tyres come in and go off by surface temperature, set-ups are chosen for it, and a track that is fast at night can be slick at two in the afternoon. The air temperature alone does not say how hot the surface is: the sun does most of the work.
How we estimate it
For every forecast hour we start from the air temperature, add the heat of the sun on the surface (the forecast's solar radiation), and take some away for the wind, which carries the heat off. At night the surface sits at or just under the air.
The surface matters. On a 86 °F (30 °C) afternoon in full sun with a light breeze, the estimate puts:
- Asphalt about 39 °F (22 °C) above the air.
- Dirt about 33 °F (19 °C) above it.
- Concrete about 29 °F (16 °C) above it: it is paler and gives back less.
A stiff breeze cuts those numbers; at 18 mph the asphalt estimate drops to about 25 °F (14 °C) over the air.
When to trust it less
Real surfaces vary with their colour, age, rubber and moisture, and with shade from the grandstands. Passing clouds come and go faster than an hourly forecast can show. Use it to plan when the track will be hot and when it will cool; use a pyrometer for the number you set the car up by.
Questions
How hot does a race track get?
In strong midday summer sun, asphalt commonly runs 20 to 30 °C (35 to 55 °F) above the air, so a 90 °F day can put the surface well past 130 °F. Concrete runs cooler, and every surface comes back to the air temperature after dark.
Why is the track temperature higher than the air?
The sun heats the surface directly, and a dark surface absorbs most of it. The air above is only warmed second-hand.
Does it apply to dirt tracks?
Yes, with a dirt surface factor, but dirt is the surface where the crew matters most: watering, working and the moisture in the clay change it hour to hour. The dirt surface hint on the track page (dry, tacky or wet) says what the sky gave the crew to work with.