Air is comprised of billions of molecules the bullet must push aside on its way to the target. Each molecule the bullet hits robs it of a tiny fraction of its energy, and thus the bullet slows. So if the air is less dense--fewer molecules occupying a specific volume of space--a bullet does not experience as much drag force against it and so slows at a lower rate and is traveling faster than normally expected when it arrives at the target.
This shortens its time of flight, meaning gravity does not have as much time to pull it downward and thus our shot goes higher than we would see in a denser air condition. Therefore, when it comes to long-range competition, it's important to have an understanding of how changes in these conditions will influence your trajectories as the weather changes or as you travel to different areas of the country.
Atmospheric conditions--barometric pressure, altitude, temperature and humidity--influence air density. Storm fronts, cold fronts, warm fronts, temperature changes and so on mean the atmosphere changes continually. This makes it impossible to derive a set of equations that positively states what the density, pressure or temperature will be at a given altitude, yet we need a reasonably accurate estimate of these things.
Because this estimate is important for aviation and weather forecasting, the U.S. Army created a standard atmosphere model Standard Metro model, which it used until the early 1960s and is still employed by the shooting industry to this day because of the wealth of data it generated in its 50 or so years of use.
Pilots have need of this same information as well, since air density affects aircraft performance. Rather than have equations to calculate the actual air density, they have long used a simple graphical chart that gives them a value they call "density altitude."
This is the altitude in the standard atmospheric model at which point the air has the same density as the air that we are flying or shooting in. Put very simply, density altitude is the elevation at which the bullet thinks it's flying.
We now have only one variable to account for all other parameters we normally have to factor in. Let's suppose a shooter wants to sit down in front of his laptop and compute ballistic charts for every situation he could reasonably expect to find himself in--from shooting in shooting in warm humid weather at sea level in Louisiana to freezing cold in the Rockies at 7,000 feet. How many tables would it take to encapsulate all of this data?
We don't need to make a chart for every single degree of temperature change or every single foot of altitude and every percentage point of humidity, but let's say every 10 degrees from 0 to 100 degrees and every 1,000 feet from sea level to 7,000 feet, and every 25 percent change in humidity. That alone would require 440 charts.
If we instead use the concept of density altitude, we will make a ballistic table for every 1,000 feet of density altitude from sea level to, say, 8,000 feet, and put that data in columns on one single sheet. We have just reduced several hundred charts to a single page, and our information is functionally just as accurate.
That simplification saves a huge amount of effort and time. All we need to know then is our density altitude. How do we figure that? Hand-held electronic units such as the Kestrel 4000 are recommended for extreme ease and precision, and you can find density altitude calculators on the internet. I've also developed a new version of the DTAC reticle that takes this atmospheric information into account.
This information is invaluable to me in my travels from my home in Texas to Camp Perry, Ohio, or to Raton, New Mexico. Both these locations produce substantially different behaviors from my ammunition. Likewise, it's essential to be able to estimate and interpolate the atmospheric condition effects at each of those places under different circumstances once I'm there.