Density altitude: the air that slows the bullet, explained simply.
The denser it is, the more the bullet slows down, drops and drifts.
All three make the air lighter.
Density altitude sums them up in one value: at 1000 m, there are 2.17 mil between 0 and 2000 m.
Same rifle, same ammunition, same range. On the plains in winter and in the mountains in summer, the shot does not land in the same place. The reason is the air.
The bullet slows down because it has to push air aside. Denser air means more drag, more time of flight, more drop and more wind drift. Lighter air means the opposite.
Density altitude combines pressure, temperature and humidity into a single number: the altitude at which "standard" air would have the same density. A hot day in the hills can have the density altitude of a mountain.
Note: density altitude is not the elevation of the place. It also depends on the temperature and pressure of the day.
The same rifle, zeroed at 100 meters on the plains, in standard air. Here is the elevation to dial at three different density altitudes:
| Range | Density altitude 0 m | Density altitude 1000 m | Density altitude 2000 m |
|---|---|---|---|
| 600 m | 5.22 mil | 4.97 mil | 4.75 mil |
| 800 m | 8.83 mil | 8.25 mil | 7.76 mil |
| 1000 m | 13.76 mil | 12.57 mil | 11.59 mil |
.308, 175-grain bullet, BC G7 0.243, 800 m/s, 100 m zero (standard air: 15 °C, 1013 hPa), scope 6.85 cm above the bore, 10" right-hand twist. Wind and shooting angle zero.
In lighter air the bullet drops less and needs less elevation. And the difference grows quickly with range:
| Range | Difference between 0 and 2000 m | On target |
|---|---|---|
| 600 m | 0.47 mil | 28 cm |
| 800 m | 1.06 mil | 85 cm |
| 1000 m | 2.17 mil | 2.17 m |
At 1000 meters, shooting in the mountains with your lowland elevation means going more than two meters over the target.
In the app you can enter pressure, temperature and humidity, or density altitude directly, read the pressure from the phone's barometer, and print backup tables for different air conditions.
