density-altitude-fdac

Density Altitude

In Blog by Rich2 Comments

We’ve talked a lot about Density Altitude here on AccuracyTech, today we’re going to talk about it in depth! Specifically, why it’s so useful, and why you should be using it in lieu of the atmospherics that it’s comprised of. There are many definitions of density altitude. I’m going to keep it more common sense and less academic so it’s not even more confusing. The idea here is to use a variable that represents what the bullet has to deal with in flight, that you can plug into a ballistics calculator ahead of time, and keep your life simple when it’s time to pull the trigger. Work smarter, not harder!

Density Altitude

For the purposes of long range, precision shooting, density altitude is the altitude which the bullet “thinks and behaves” as though it’s flying through. That may sound confusing now, but it get’s easier to understand as we delve into the subject. As altitude and temperature increase, air density decreases, and the opposite is also true. As your altitude drops, and the temperature drops, the air density increases. Think of the old boiling water analogy, hot water boils and all the molecules are moving and spaced farther apart than cold water. As water gets colder, the molecules pack tighter together, eventually it freezes. As the saying goes, the wind is like water!

How does this apply to shooting? Well, for starters, it’s one variable to work with. Most ballistic calculators, whether application or computer software, will give you two options for entering your atmospheric data. You can either enter everything, one unit of measure at a time, or you can use density altitude. Now on a square range where time and opportunity allow for it, you can enter all the variables before asking for a firing solution. You can enter you Temperature, Altitude, Barometric Pressure, Humidity, etc. However, in dynamic environments, time is of the essence. It’s much easier to use a single number before asking for a firing solution. This is why density altitude is so useful!

Density altitude takes all these factors into account. So instead of printing dope cards for every five or ten degrees of temperature change, every few points of barometric pressure change, every few hundred or few thousand feet of elevation, you can just use one number…the density altitude where you’re standing. You have to understand, that when you fire the rifle, the bullet doesn’t really care whether you’re at sea level in cold temperatures or sea level in warm temperatures. Pilots use density altitude because it more accurately determines the condition of the air the plane is moving through. You might be flying 200 feet above sea level, but if it’s really cold, the air could be so dense the plane would require a lot more power to maintain air speed and flight than you think.

The Projectile Doesn’t Care

When you print your dope cards up, make density altitude cards. Don’t base your cards on a set of individual atmospheric conditions. As the different conditions change, it will throw off the data on the card. By using density altitude, you’re printing cards up using a more gross metric than any of the individual metrics we talked about earlier. Most people print density altitude cards up for every 1000ft or 2000ft of change in the density altitude. That’s a lot easier than trying to come up with firing solutions, ahead of time, for individual temperature changes or variances in barometric pressure as a storm system comes through.

density-altitude-wrist-coach

I have these cards printed for 5000ft and 7000ft DA with my projectile and it’s muzzle velocity, why 2000ft of difference? You have to shoot past 800yds for there to be more than .1 MIL difference in impact, so how much value does printing cards for every 1000ft of DA change really provide?

I’m not discounting the merits of using live data for firing solutions. If you can do it, then do it, but how you go about it is going to be a factor in the time necessary to compute a firing solution. I think this is where the Applied Ballistics Kestrels really shine. They compute firing solutions based on live data and the Kestrel is constantly monitoring all the metrics. That’s a relatively new thing. Years ago you had to read all that atmospheric data, and either enter the variables individually, or read the density altitude and tell the computer what the current density altitude is before asking for the firing solution.

Don’t Make It Difficult

I’ve seen many newer shooters at a match reading environmental data off a kestrel, inputting the information into an app on their phone, and trying to compute firing solutions on the spot. It’s your life, do as you wish. I did the same thing when I got started in this sport. However, if I could go back in time and talk to myself, I’d start with a slap across the back of the head. Print density altitude cards ahead of time, laminate them, and just periodically keep an eye on what the Kestrel is saying your current density altitude is. Then read your adjustments off the cards. That’s the most affordable, low tech, fail safe, always works advice I can give you.

If you really want live data and down to the second accurate firing solutions then you need to invest in the gear to monitor and compute it all so you aren’t stuck trying to enter it by hand before they call your name at a match. Obviously this works for the MIL/LE crowd too. Anywhere the shooting is dynamic, stressful, timed, bad guys shooting back, whatever. If time is a factor, either pay for the gear, or print the density altitude cards ahead of time. If you’re sitting there entering the DA and ranges and getting individual firing solutions for each target on two different devices, you’re doing it wrong!

Wrapping Up

Remember, this article is meant to highlight why using density altitude is more preferable than trying to manually enter each environmental condition independently of the others. Print your cards and your DOPE out ahead of time, then all you have to do is flip to a different card, if there’s even that much change during the day. Don’t get me wrong, here in Colorado you can see some huge swings in density altitude in the same day. Definitely keep an eye on your density altitude. I have an Applied Ballistics model Kestrel that will do all the calculations and pull all the data from the sensors it needs to do the calculations, but you know what? I still bring a wrist coach with density altitude cards! You never know, batteries die, screens break, mechanical devices fail all the time. Paper doesn’t care about the time, temperature, etc. Though as you can see from the photo above, laminate your cards, water can get into the smallest of places!

Owner and Proprietor of AccuracyTech, LLC. Rich is a Firearms Enthusiast, Precision Rifle Competitor, and Writer. He is committed to bringing readers quality reviews and articles related to the Precision Shooting Sports. If you have any questions for him, please use the contact form on the site.

Comments

  1. That’s an excellent idea Rich. What ballistic application do you like to use to create the DA Cards?

    1. Author

      I really like the Applied Ballistics app for Android smart phones. One reason is because this app allows you to email drop charts in spreadsheet format. Then it’s easy to copy, paste, and format the numbers. Print it off and stuff it in your wrist coach as a backup.

      The app also saves your rifle and bullet profiles to “the cloud” so when you wind up with a new phone you can download everything you already worked on.

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