Barometric pressure is also known as sea level corrected pressure, and is what the weather station and airports report because it’s useful for pilots and making weather assessments. Barometric pressure is not the actual air pressure where you are, rather it’s a number that’s corrected to sea level. In order to determine the actual air pressure where you are (which is what the ballistics program cares about), you have to account for the effects of altitude. However if you have a handheld weather meter like a Kestrel, you can measure Station Pressure directly which is the actual air pressure where you are. This is the preferred method of inputting pressure data because it’s one less input and relies on only one measurement instead of two.
A common error is to mistake station pressure for barometric or vice versa. The consequence of this error is that the wrong air density gets applied which degrades the accuracy of trajectory predictions. This error is increasingly more severe the higher up you are above sea level.
Refer to the image on the right for proper set-up of the atmospheric pressure inputs. Note the reference altitude is set to 0 ft in the Kestrel which indicates it’s displaying uncorrected station pressure, and the Pressure is Absolute box is checked in the program indicating it’s using station pressure.
To further clarify the output from the Kestrel, here is an excerpt from the Kestrel user’s manual: “Some final notes – If you wish to know the actual or station pressure for your location (such as for engine tuning), simply set the reference altitude on the BARO screen to “0”. In this case, the Kestrel Meter will not make any adjustment and will display the measured value. (Engine tuning and ballistics software sometimes refer to atmospheric or station pressure as “absolute pressure.” These applications are concerned with the actual air density, as opposed to pressure gradients relating to weather, so barometric pressure is less useful.”