How to Work with Density Altitude in Ballistics Calculations
In this video, Bryan Litz of Applied Ballistics talks about Density Altitude and the effect of atmospheric conditions on bullet flight. Bryan explains why you must accurately account for Density Altitude when figuring long-range trajectories.
Bryan tells us: “One of the important elements in calculating a fire solution for long-range shooting is understanding the effect of atmospherics. Temperature, pressure, and humidity all affect the air density that the bullet’s flying through. You can combine all those effects into one number (value) called ‘Density Altitude’. That means that you just have one number to track instead of three. But, ultimately, what you are doing is that you are describing to your ballistics solver the characteristics of the atmosphere that your bullet’s flying through so that the software can make the necessary adjustments and account for it in its calculations for drop and wind drift.”
Bryan adds: “Once you get past 500 or 600 yards you really need to start paying careful attention to atmospherics and account for them in your ballistic solutions”. You can learn more about Density Altitude in Bryan’s book, Applied Ballistics for Long Range Shooting (Third Edition).
General Scientific Definition of Density Altitude
Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air). Here is the formula:
Air is more dense at lower elevations primarily because of gravity: “As gravity pulls the air towards the ground, [lower] molecules are subject to the additional weight of all the molecules above. This additional weight means the air pressure is highest at sea level, and diminishes with increases in elevation”.*
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.
*Source: Miningandconstruction.com
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