To shoot accurately at long range, you need to know what the environmental conditions are…specifically what the Density Altitude (DA) is because the DA is the true representation of what the bullet is flying through.
Now, having a Kestrel is great and all, but for some people its just to much $$$ to lay down in an already expensive hobby.
So…in this situation, the next best thing is to use DOPE cards that are referenced to density altitude.
These can be produced at home on your computer, printed and laminated ready for use.
But there is one slight problem…how do you calculate DA without a Kestrel?
Well, thats what this tool is for.
Density Altitude Card by Skill at Arms.
Our DA card is available in an Imperial version (Altitude in feet and Temperature in °F) and in a Metric version (Altitude in meters and Temperature in °C) and the download link is at the very bottom of this article.
Here are the steps to use the DA card PROPERLY*:
- Ascertain Station Pressure using a watch or mobile phone. Many modern watches and phones will have this function. Alternatively you can use a couple of formulas** that will give you an “approximate” Station Pressure so you at least be “in the ballpark”.
- Reference the Station Pressure reading you got from your watch or formula to the nearest Station Pressure reading on the Station Pressure to Pressure Altitude Chart to establish your Pressure Altitude.
- Follow an imaginary line up from the bottom of the chart in line with the current temperature at your location until you reach the approximate Pressure Altitude. If your Pressure Altitude is “between the lines” you simply judge an approximate point between the two lines. For example, if your Pressure Altitude is 500ft, you would track up until you were halfway between 0ft and 1000ft.
- Now that you have established a point where you temperature and pressure altitude intersect, simply track horizontally until you reach the Density Altitude Axis and read off you DA from there.
That’s it! You now have your approximate DA and can reference your DA Dope cards so you can make a shot!
*By PROPERLY, I mean don’t skip the step where you ascertain your Pressure Altitude!
I see guys using similar cards who simply read the Pressure Altitude as their ACTUAL altitude/elevation.
This is WRONG and although it might be close (by mere chance) on occasion, it is likely you will end up with an incorrect DA figure more often than not.
**Now, I mentioned a couple of formulas that you can use so that in a “worse case” scenario (i.e. you have NO way of getting an accurate SP using a watch, mobile phone or other electronic device.
These formulas will give you an APPROXIMATE Station Pressure and so they aren’t perfect but they are better than nothing.
Station Pressure Formula 1…
In this formula, if you know the sea-level barometric pressure (what the weather channel gives you), you can approximate your Station Pressure.
For Imperial users…
Sea-level Barometric Pressure (inHg) – (Physical Altitude (ft) ÷ 1000) = Approximate Station Pressure
For example, the weather man tells you that the barometric pressure near you is 30.14 inches and you know that you are at an altitude of 3,500 feet.
30.14 – (3500 ÷ 1000) = 30.14 – 3.5 = 26.64inHg
Now you just reference 26.64inHg to the Station Pressure to Pressure Altitude Chart to get your Pressure Altitude.
For Metric users…
Sea-level Barometric Pressure (hpa) – (Physical Altitude (m) ÷ 9) = Approximate Station Pressure
For example, the weather man tells you that the barometric pressure near you is 1020hpa and you are at an altitude of 1000 meters.
1020 – (1000 ÷ 9) = 1020 – 111 = 909hpa
Now you just reference 909hpa to the Station Pressure to Pressure Altitude Chart to get your Pressure Altitude.
Station Pressure Formula 2….
This formula is a “worse case” scenario for when you have NOTHING to tell you what the atmospheric pressure is AT ALL!
For Imperial Users…
29.92 – (Physical Altitude (ft) ÷ 1000) = Approximate Station Pressure
For Metric Users…
1013 – (Physical Altitude (m) ÷ 9) = Approximate Station Pressure
This is definitely a “worse case” scenario formula.
If you can get Station Pressure from a more reliable source…use it.
However, if you have NOTHING else, use this formula in the knowledge that it isn’t going to be precisely accurate because of the two assumptions it makes…
- That the barometric pressure at sea level is always 29.92 inHg / 1013hpa.
- That atmospheric pressure declines at the rate of 1 inch of mercury per thousand feet of altitude gain OR approximately 28hpa per 250m of altitude gained.
But…it’s better than nothing.
That’s it…easy right?
These cards are a great option if for whatever reason you don’t have access to expensive ballistic solvers, weather meters, or just need a “solid” backup in case for some reason you can’t use those things.
Of note, the DA Graph was produced with humidity set to 50%. As we know, humidity doesn’t have a significant effect and so 50% is a good “middle ground”. Also, if you are interested, I used this Density Altitude Calculator and this Pressure Altitude Calculator.
Finally, I would like to thank Lindy Sisk for helping to clarify the Station Pressure formulas for me. I was compiling the metric data and wanted to align it with the imperial data he had previously published.
He has some great information at his website. It’s definitely worth taking a look.