Using Station Pressure For Trajectory Calculations
Station pressure, or absolute pressure, is the measurement of the air pressure with no adjustment for altitude.
In other words, station pressure is the air pressure right where you’re measuring it. Air pressure, temperature, and humidity influence air density. Along with gravity and wind, air density is a major influence on our bullet’s trajectory. We have to accurately account for it and input it to our ballistic solver.
Air pressure is typically measured in inches of mercury (inHg), and is literally the weight of the earth’s atmosphere pushing down on us.
Think of it as a column of air with stuff in it. For that reason, the closer you get to sea level, the denser it becomes. As a general rule of thumb, the air pressure changes about one inHg per 1000 feet of elevation change. For example, if the air pressure at 1000 feet ASL is 28.92 inHg, the pressure at 4000 feet will be 25.92 inHg. That’s assuming the same weather conditions, of course.
Barometric pressure is station pressure adjusted to sea level.
Standard pressure at sea level is 29.92 inHg. Weather forecasters use barometric pressure so that no matter where you are, the pressure change value will reflect what’s going on with the weather. Whether you’re in Denver or Houston, if the weather is the same, your forecaster will give you the same barometric pressure reading. If you wanted to use barometric pressure as the input for a shot correction, you would have to calibrate your Kestrel to the exact elevation where you’re at. That’s why we use station pressure, and here’s how we do it:
To set up your Kestrel and ballistic solver to use station pressure,
You simply set the reference altitude to “0”. Once the Kestrel is referenced to “0” altitude, the number you see on the barometric screen is the actual station pressure. It now thinks you’re at sea level, so any deviation from that standardized number of 29.92 inHg will reflect the actual air pressure of the earth’s atmosphere.
When you input the conditions into your ballistic solver, make sure the altitude reads “0”, then input the station pressure to the pressure field. Most solvers will show a prompt reminding you to do that, a Kestrel with AB function just handles it for you.
Using station pressure is simple, easy, and accurate. That’s why we do it!