What it is
Density altitude is the height in the standard atmosphere where the air would be as thin as the air at the track right now. An engine burns the oxygen it can pull in, so thin air means less power, less downforce and less cooling. A sea-level track on a hot, humid afternoon can make an engine feel it is in the mountains.
Three things set it: the air temperature, the pressure and the water in the air. Hot air is thinner. Low pressure is thinner. And water vapour is lighter than the dry air it pushes aside, so humid air is thinner too.
What the numbers look like
Worked out with the same formula as the track pages, at a sea-level track with a normal barometer:
- A cool evening, 50 °F (10 °C) with a 39 °F (4 °C) dew point: about -500 ft. Below sea level, which is why the night air runs quick.
- A 95 °F (35 °C) afternoon with dry air: about 2,280 ft.
- The same afternoon with a 75 °F (24 °C) dew point: about 2,650 ft. The humidity alone is worth a few hundred feet.
Add the track's own elevation on top: a strip at 5,000 feet starts there before the weather has its say.
How we work it out
Each hour of the forecast gives us the air temperature, the dew point and the station pressure (the real pressure at the track, not the sea-level number on the TV map). From those we work out the air density, then the altitude in the standard atmosphere with that density, with the National Weather Service formula. The pit box on every track page shows it for right now, the hour strip shows it hour by hour, and on drag strips "Best air" picks the hour with the lowest number in the racing hours.
It is a forecast, not a sensor. A weather station on your trailer reads the air where you are parked; ours is for planning the day and the weekend before you get there.
Questions
Is density altitude the same as elevation?
No. Elevation is how high the track is. Density altitude is how high the air feels to the engine, after the temperature, pressure and humidity of the day. A hot day pushes it well above the elevation; a cold one can take it below sea level.
Why does density altitude fall at night?
The air cools after sunset, and cooler air is denser. Unless a front brings warm, humid air in, the best air of the day usually comes late at night or early in the morning.
What is a good density altitude?
Lower is better, and what counts as good depends on the track: a few hundred feet below sea level is quick air at a coastal strip, while anything under 4,000 feet is good at a track a mile up. Compare the number with the same track on other days rather than with other tracks.
Does humidity raise or lower density altitude?
It raises it. Water vapour is lighter than the nitrogen and oxygen it replaces, so humid air is thinner and the engine has less oxygen to burn.
Where does the pressure come from?
From the forecast's surface pressure at the track, which is the real pressure at its elevation. The barometer reading on the page is the sea-level number, the one that shows a front coming.