Using Density Altitude for Long Range Shooting

I decided to bring this post over from my old blog.  Let's take a look at using Density Altitude for calculating ballistic drop when shooting long range targets.  If you've read some of my previous posts about long range shooting, you'll notice that I generate drop charts based on 1000 foot Density Altitude intervals and use those for my shooting adjustments.  I don't actually calculate the drops for each individual shot or shooting station with an app.  So let's start here:

What is Density Altitude?
Checking DOPE at the NRA Whittington Center
Notice the Kestrel for checking Density Altitude

So what exactly is Density Altitude?  First off, here is the definition according to Wikipedia: 

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. In other words, density altitude is air density given as a height above mean sea level.

So what does that mean in basic terms?  There are several factors that affect how dense the air around you is.  The two biggest factors are your altitude above sea level and the temperature.  The higher in altitude and the higher the temperature, the lower the pressure. 
When you fire a bullet, the ballistic coefficient of that bullet is a representation of how much drag the bullet has as it pushes through the air.  The more dense the air, the more drag there will be.  Conversely, if the air is less dense, there will be less resistance.  So a bullet fired at 1000 yards at sea level on a cold day will need more scope adjustment to hit the target than one fired at 10,000 feet of elevation and 90 degree temperatures. 
What Density Altitude does is simplify those numbers down to one equivalent reading.  It takes into account the temperature, altitude (based on barometric pressure), humidity, etc, and gives you a number that takes all of it into account.  It was originally (and currently) used by pilots to be able to calculate the lift that they would generate for a certain airport altitude and weather conditions. 

How do I calculate Density Altitude?

There is an easy and a long way to calculate Density Altitude.  Obviously you can use the formula to calculate Density Altitude as long as you know each of the required inputs for the formula.  The official NOAA Density altitude calculator can be found at the NOAA site, link under the picture to the right:

In order to use the calculator, you will need to input your temperature, station pressure (not corrected for altitude), and dew point.  It will then calculate the station pressure for you in feet or meters.  I have used this method a number of times to calculate the Density Altitude after shooting, based on my log.  I have also used it in locations were I have cellular service.  Unfortunately, unless you have a weather meter, you may not have correct information.  Most of the time the weather updates that you get to your phone may not be exactly accurate to your location, depending on how far the weather station is from your shooting position.

Some shooting log books have a simple chart that gives you a graph of altitudes and temperatures and lets you look up an approximate DA for your altitude at a given temperature.  While these may not be quite as accurate, they will still get you very close to the right number and may be adequate for short to medium ranges.

The easier method is to use a weather meter, such as the Kestrel, that calculates Density Altitude directly.  After asking other shooters for the DA for months, I finally broke down and ordered a Kestrel from Amazon.  I didn't order the fancy ballistic model, but ordered the newer 5000 model with Bluetooth link.

The Kestrel 5000 is currently the lowest end model that they offer that still provides you with Density Altitude.  You can still obtain readings for wind speed, temperature, dew point, barometer, station pressure, etc, but you can also output a DA number specifically.  This made it much easier in the match I shot at the NRA Whittington Center.  I simply checked the DA at the beginning of the day and a couple times throughout the day and I was able to make sure that I was shooting the correct DA card throughout the match.  This helped me to stay on target, even though I was shooting a match at an elevation several thousand feet lower than where I initially checked the DOPE on my rifle.

Why use Density Altitude?

So why would you want to use Density Altitude when shooting?  The answer is simplicity.  While there a lot of great ballistic programs and phone apps, it is not always convenient or appropriate to rely on an electronic device to generate your shooting drop.  When I first developed a load for my 260 Remington, I trued the drop information out to 1000 yards and printed up cards for DA in 1000 foot increments.  I used those cards in April at the Battle of Breakneck when it was cold and snowing, took the same cards to the NRA Whittington Center during the summer heat and have shot several of the local Cheyenne Mountain Precision Rifle matches using those same cards.  As long as I can get the Density Altitude reading from my Kestrel, or earlier on by asking another shooter, I have been accurate in 30 degree to 90 degree weather and at various elevations. 

While I am certainly not discounting the convenience of being able to pull up ballistic drop to the exact conditions you are shooting, or being able to true the data throughout a day of shooting, using a phone before every stage at a match may not be practical.  During the Battle of Breakneck, the constant rain, wind and snow made it difficult for the guys that wanted to use their phones.  A few were using waterproof cases, but I also saw guys with their phones in Ziploc bags or trying to use their body to shield the phone from the weather.

This can also be very beneficial for hunters.  While I wouldn't shoot as far at an animal as I do at a steel target, the Density Altitude of where I hunt can vary several thousand feet during a day.  Where I live and hunt in Colorado, it is possible to change 2,000-3,000 feet of elevation during the course of a day.  The morning may also be 10-15 degrees, but warm up to 50-60 degrees during the afternoon.  By printing out a DA chart for the distances you are willing to take an ethical shot on an animal, you can laminate it or attach it to your stock and know at a glance what your drop will be for the current conditions.

So do you use Density Altitude measurements for shooting, or do you calculate all of your drops using a ballistic app or program?  I am interested in hearing your thoughts below:

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