It’s Friday and time for our weekly shoot-the-shit topic. This week we thought it would be productive to discuss the weather. More specifically station pressure vs. barometric pressure. For precision shooters; particularly long range shooters (i.e. 800 meters and beyond), understanding the influence of environmentals in determining a firing solution is critical to making the shot. As a long range shooter you have two problems to solve; Aerodynamics, which influence elevation adjustments and the wind vector which influences the bullets flight path over the ground.
For some reason, or another, pressure gives people a hard time and if you don’t get pressure right you probably won’t make a long range shot – excluding the effects of wind. But, before jumping into pressure, which can be easily addressed in a paragraph or two, I’d like to talk about density altitude.
The simplest way to view density altitude is as the net effect of three discrete environmental variables: air temperature, station pressure, and dew point. If you found yourself at one end of a tunnel and was asked to run to the other end, but you had to do it by running through wet towels hanging from ceiling to floor you would encounter a significant amount of drag; hence, your speed would go down and you’d use more energy in running the length of the tunnel. If I then remove some of those wet towels (the equivalent of increasing density altitude), you’d experience less drag; hence, increased speed and use a little less energy. This is precisely what a projectile experiences as it moves through the atmosphere. Those of you that fly know that true airspeed (as opposed to indicated airspeed) increases as density altitude increases.
Density altitude is a calculated value not measured. Here’s a little horse to help you get a feel for what is happening with density altitude. When temperature rises DA increases; when it decreases DA decreases. When station pressure increases DA decreases; when it decreases DA increases. When dew point increases DA increases; when it decreases DA decreases. This is just a rule of thumb to keep in the back of your mind; however, all of these parameters interact.
In long range shooting, your projectile will perform differently at various density altitudes and since station pressure is a component used in calculating density altitude if your station pressure measurement is wrong your firing solution will be wrong. It may not be a huge error at 100 meters but it’ll be large when you get out to 800+ meters.
Station Pressure is nothing more than air pressure at any point on the earth, meaning that it is not corrected for terrain elevation. To measure pressure you need a weather meter and Kestrel is by far the industry standard – period. To arrive at station pressure using the Kestrel meter simply go to the BARO page and set your reference to zero. Setting the reference to zero removes the sea level reference from the pressure measurement resulting in station pressure.
Barometric Pressure is the air pressure reported by a weatherman or NOAA weather broadcast. It is adjusted for sea level. To measure the barometric pressure with your Kestrel, you need to know your current elevation, and the best way to do that is with a topographic map. It is by far more accurate than your GPS altitude. If you’re out in the weeds with just a GPS shame on you. Always have a topographic map of the area you’re hiking or hunting.
The picture on the left shows station pressure. Note that I’ve set the reference to 0. The picture of the right indicates barometric pressure. Note that the reference is set to my topographic elevation.
Before leaving the subject and whishing you a safe and relaxing weekend, I want to make sure that I leave you with one last thought.
Most ballistic calculators provide you with an option to use density altitude or discrete atmospheric data for its environmental inputs. When they ask for pressure, they’re asking for station pressure not barometric pressure. Concurrent with that input, they will also ask for altitude / elevation which can be done using your topographic map. When I say “most”please do not interpret my generality as “all.” So, know your ballistic calculator – train with it and use it regularly. For those of you that don’t have a standalone ballistic calculator running on a smart device or PDA, Kestrel makes a meter that includes ballistic software from Horus Vision and Applied Ballistics.
Next week our shoot-the-shit topic will take you through setting up the ALT (altitude page). It’s not obvious and merits some discussion so that you understand the reading and how it’s derived.
All shoot-the-shit rules apply so let us know what you think!
Have a great weekend, get out to the range and have some fun with your buds and family.