Air Density Factors

[Lowlight]

Article Posted

https://www.snipershide.com/density-shooting/

Air Density Factors

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Field Shooting where Air Density Matters

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 :

• Shape
• Area
• Mass
• Velocity
• 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
• Temperature
• Humidity

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.

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Humidity can be ignored or set to 50%

Okay, back to the important factors that go into Air Density. Barometric Pressure

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Barometric Pressure

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.
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Air Density

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.

Temperature

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Air Density in the Field

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.

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Temperature as it effects molecules


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.
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Cadence of Fire, Temperature

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. Density Altitude

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Density Altitude Chart


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.
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Density Altitude in Colorado

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

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Shooting Kneeling


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 …

 

[fdkay]

Lowlight, excellent explanation.
As you know, down here in South East Texas, the density Altitude can shift quite a bit in one day.
I'm about 10 feet above sea level, but I have seen the DA shift from 500 ft below sea level to 2500 above in a few hours.

 

[_Windrider_]

1" of Hg per 1000/ft increase and 2C (3.6F) per 1000 ft are the standard lapse rates. I'm sure you knew that but many don't know what standard conditions are at their location. Standard conditions in Denver would be roughly 24.92 in Hg and 5C. It's a big help to know standard atmospheric pressure and temp at your location. Gives you a good sense of how drastic your DA could be based on current conditions

great write up lowlight.

 

[stwcattle]

Great article Lowlight

fdkay, someone correct me if i'm wrong, every 15 degrees F equates to 1000' change in your DA.

Sean

 

[_Windrider_]

Great article Lowlight

fdkay, someone correct me if i'm wrong, every 15 degrees F equates to 1000' change in your DA.

Sean

If the pressure doesn't change that's about right. But if temp goes up I guarantee the pressure is changing.

 

[winniedonkey]

Tag....thanks for the knowledge

Sent from my HTC6535LVW using Tapatalk

 

[Lowlight]

The offset for Denver is 24.89 as I looked it up specifically so I can include it... this is due to the temperature offset for elevation.

 

[_Windrider_]

sorry, I wasn't disputing your numbers. I was just making a reference to the generally accepted lapse rates. The ones I used are quick reference for ballpark, not exact. they don't take into account the exact temp lapse rate as the chart above does, but you will notice the difference in 24.89 and 24.92 about as much as not accounting for Spin Drift (I'm messing with ya).