Air Density Factors
A question that comes up in the Sniper’s Hide Forums, especially in the Ballistic Calculator section revolves around atmospheric conditions. Aside from the normal question, “do I use Absolute (Station) Pressure or do I use Barometric Pressure”, people tend to misunderstand how air density effects the bullet’s flight.
I teach a lot of classes around the world, and there are huge swings in conditions so you have to simplify this information so people in Alaska can understand it just as easily as people in Argentina. So let me break down air density factors as easily as possible.
How fast a bullet slows down is based on :
- Air Density
For the typical shooter this is expressed by the Ballistic Coefficient, combined with the Muzzle Velocity we then factor in the Air Density.
Air Density for the purpose of shooting is a combination of:
- Barometric Pressure
Today, we only need to worry about Humidity as a factor of Density Altitude. For 99% of our shooting we can set Humidity to 50% and ignore it. Just to confirm for those who might not understand, Higher Humidity reduces the air density. When comparing 100% vs 0% humidity, air that has 100% humidity is less dense. The difference between the two as far as the bullet is concerned is very, very small. So I just want to get humidity out of the way.
Okay, back to the important factors that go into Air Density.
When using a kestrel combined with a Ballistic App you want to use the Station Pressure or as many Ballistic Apps word it, Absolute Pressure.
Barometric Pressure is the weigh of the air at a defined point. The force exerted down by gravity represented in millibars or inches of mercury. Sea Level is 29.92 here in the US.
As a Shooter, Altitude is a factor of barometric pressure that is why we often post very low numbers. It’s comical to see people on YouTube respond to my barometric pressure numbers as impossible. They are not factoring in altitude like we do.
As a factor of altitude you lose 1” of pressure for every 1000ft of elevation you gain. If sea level is 29.92 here in Denver I use 24.89 as a standard. (There is a temp component)
The reason apps have a check box for Absolute pressure is because of altitude. If you use Station Pressure and then include the Altitude you are essentially doubling the elevation. So we turn off altitude and use just the pressure. I often need 25.10″ here in CO.
Why this is important for the shooter is drag, the denser the air, the more drag the bullet experiences during flight. This slows it down faster. Here in Denver we see a lot of elderly people using oxygen because the air is thinner making it harder to breathe.
When you don’t have access to a Kestrel, or to Station Pressure you then can default to Barometric Pressure. This is corrected for sea level and needs to be combined with your altitude.
JBM for example has a check box that asks if the Pressure is Corrected ? Corrected Pressure is what the local TV weather person uses. Doesn’t matter if you watch the local news in Denver or New York, they will give you the same Barometric Pressure as corrected for sea level. The difference is, here in Denver we all know we are a Mile Higher than NY.
Unless a front is coming through, you can pretty much set the Barometric or Station Pressure and not worry about it for that day. It will barely change.
This is your biggest changing factor when it comes to atmospheric conditions and shooting. Temperature swings effect not only the bullet but, potentially the powders being used.
Temperature as a factor of air density is all about the molecules of air. Are they all jammed together or are they excited and spread out.
Cold Air packs the molecules in tight, causing more drag, while hot air spreads them out.
Depending on the time of year and the current conditions you can be making a change to your software every hour, or at least every other hour. It’s not uncommon to start the day around 55 degrees and end the day’s shooting at 95 degrees out west here. That is a significant change. So that means keep up with the temperature changes as your day goes on.
You can break temperature down into powder temperature variations, the cadence of your shooting, and other factors but it’s far too complicated for this article. It is important to understand how temperature changes effect your shooting as the old school rules of thumb don’t really apply anymore. A decent powder might have a .3fps change per degree, but honestly I would be wrong to say exactly what your changes might be. It’s up to the shooter to determine the effects.
All three air density factors combined, create what is called Density Altitude. It’s a good, 1 number to tell us where the bullet thinks it’s traveling. We get density altitude from pilots as they use it mainly for flying.
You maybe living at sea level, have a very high humidity day, with temps in the 90s and the bullet will believe it is traveling at 3000ft DA.
Here my range is 4500ft, and my average summer density altitude reading is 8000ft. I have a place I shoot at 9000ft and even in October the DA is closer to 12,000ft.
Because I have a Kestrel I use the actual Air Density factors, Barometric Pressure, Temp, and Humidity (35% for CO) and only record my DA in the log book. Using the Impact Databooks Shooter’s Diary Page I asked Tony G to remove Humidity and replaced it with Density Altitude.
It’s worth knowing and understanding Density Altitude as it’s a quick reference to use when considering Air Density.
Wrapping it All Up
What the bullet likes is :
- Low Barometric Pressure / High Altitudes
- High Temperatures
- High Humidity
This is down and dirty to help you understand that air density matters, and to answer that often repeated question, “How does it effect my 100 yard zero” it doesn’t. You need time for it to work on the bullet. So that means this stuff starts to kick in around 350 yards and beyond. Of course the farther out you go the bigger the swings. By my Colorado Zero remains the same even when traveling to Florida.
To be honest, we are effected more so than the rifles are. We add clothes, or remove them, we become uncomfortable in high humidity and rain bothers us more so than the bullet. When we are uncomfortable we make mistakes.
Remember, Knowing is Half the Battle …