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What is “Density Altitude?"

What is “Density Altitude” and does it matter to shooters?

By Keith Wood

The Background
As more and more hunters and shooters explore the complexities of long range shooting, factors like barometric pressure, altitude, and humidity have all the sudden entered into the shooting equation. At most hunting distances, none of these things matter: range, angle, and wind are the only real external factors that will cause a miss in the first 300 or so yards.

The Question
I recently had a chance to try out a Kestrel 4000 wind meter (available from Sinclair) that provides the shooter with, among other things, a density altitude reading. As if calculating this stuff wasn’t confusing enough, what in the heck is “density altitude”? 

The Expert Deferral
We reached out to our friend Paul “The Rocket Scientist,” who is literally a scientist who spent his career working on things like missile guidance systems. He’s also a long-range rifle shooter. His verbatim answer:

Density altitude X is the altitude that, on a day with standard pressure and standard temperature, would have a geographic altitude of X. In hot weather the density altitude is always higher (i.e., less dense) than the geographic altitude (from a topo map or GPS) because the temperature is always hotter than a standard day. Conversely for a cold day. Washington, D.C. at 100F has a density altitude of about 2700 ft even though the geographic altitude (the real altitude of the dirt) is very close to sea level. What really matters to the bullet is the actual density, usually measured in pounds per cubic feet. Sea level standard day (59F) is 0.075lb/cubic foot. But for every combination of pressure, temperature and humidity (ignore for practical purposes) there is a unique density and therefore a unique density altitude.

The Layperson’s Translation (I hope)
Density altitude is the altitude that actually matters in the context of shooting. When calculating a bullet’s path, the density altitude should be used in the equation if it is available.

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2 Responses to What is “Density Altitude?"

umesh wrote:
November 19, 2012

hi, your blog is so informative about hunting, nice posting Thanks! http://www.rockhollowhuntclub.com/

Kevin Schwinkendorf wrote:
August 13, 2012

It is true that range to target, uphill/downhill angle, and wind are the most important factors to long range shooting. The other factors, barometric pressure, temperature, altitude, and humidity, are less important, but they will affect bullet drop and wind deflection, but usually by no more than a 10% correction or so. These other "environmental" factors can be are included into the calculation by adjusting the ballistic coefficient, C, using equations given in the Third Edition Sierra Reloading manual. Local, or ambient, temperature may also affect the thermodynamics of the burning powder as well, causing muzzle velocity to increase slightly with temperature, usually around 2 ft/sec per degree of temperature change. The Speed #11 reloading manual has a table that shows this effect. However, some modern propellants are advertised as being temperature insensitive.