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Station Pressure vs Barometric Pressure: The Mistake That Costs a Miss at 800 Yards

The weather report's pressure is corrected to sea level; your bullet flies through the real one. At a 5,000 ft range, mixing them up sends a .308 over a 12-inch plate from 600 yards out. The worked example, the reverse mistake, and how to tell the two apart.

5 min readSolved with the Ballista engineUpdated October 2026

You’re at a range 5,000 ft above sea level. The weather report says the pressure is 30.00 inHg. A barometer on the bench reads 24.96. Both are right, and only one of them describes the air your bullet flies through. Type the weather report’s number into a solver’s station-pressure field and it thinks you’re at sea level: density altitude -20 ft instead of 6,130 ft. At 1,000 yd the .308 then lands 56 inches high.

Two pressures from one barometer

Station pressure is the absolute pressure where you stand: the weight of the air above you. It drops as you climb. A barometer at the firing point reads it directly, and it’s the number air density depends on.

Barometric pressure, as weather reports give it, is station pressure corrected to what it would be at sea level. In aviation it is the altimeter setting, often called QNH. The correction lets forecasters compare stations at different heights and lets a pilot’s altimeter read field elevation. It also removes exactly the thing a ballistic solver needs to know: how thin the air is where you are.

Undoing the correction takes your altitude. Ballista uses the standard-atmosphere relation:

station pressure = altimeter setting × (1 − 0.0065 × h ÷ 288.15)^5.2559

where h is your altitude in meters. At 5,000 ft the factor is 0.832, so 30.00 inHg becomes 24.96 inHg. The old rule of thumb of “about 1 inHg per 1,000 ft” holds well low down: over the first 5,000 ft the pressure falls 1.01 inHg per 1,000 ft. Over 10,000 ft the average is 0.94, so the rule drifts as you go higher.

The mistake at 5,000 ft

The common error is simple. The shooter’s meter or phone app shows the weather-report number, and it goes into the solver as station pressure. Here’s what that does to both rifles.

Out to 500 yd both rifles still hit a 12-inch plate with the wrong pressure. That’s why the mistake survives: the rifle seems to work, and then the first long shot of the day goes high. At 800 yd the .308 is 0.78 mil off, 22.6 inches. The miss is always high because sea-level air is denser than the air at the range, so the solver expects more drop than the bullet has.

For scale, a muzzle velocity 25 fps lower than the solver assumes costs 0.17 mil at 1,000 yd for the 6.5 and 0.24 mil for the .308 in this same air. The pressure mistake is 4.4 and 6.4 times larger. It grows with the height of the range, because the gap between the two pressures does. The velocity versus BC article covers the velocity side.

The reverse mistake

Some shooters know about the correction and apply it twice. They read station pressure from a meter, then enter it in an altimeter-setting field along with their altitude. The solver reduces a pressure that was already low. The air it computes has a density altitude of 12,020 ft, and the shot goes low.

The fix is the same in both cases: know which number you have and put it in the field made for it.

How to tell which number you have

• A barometer at the firing point, a phone’s built-in barometer or a handheld meter’s absolute pressure, gives station pressure.
• A weather report, a forecast site or an airport report gives pressure corrected to sea level. Treat the pressure in a phone weather app the same way unless it says station pressure.
• Check the size. Corrected pressure reads close to 29.92 inHg wherever you are. Station pressure falls with altitude: about 25.0 at 5,000 ft. If you’re well above sea level and your “station pressure” reads near 30, it’s the corrected number.

Ballista’s Air sheet has a separate source for each. “Station pressure” is for a meter’s reading, and its note says it is not the weather-report number. “Weather report” asks for the “Altimeter setting” and “Your altitude” and undoes the correction for you. “Phone sensors” reads station pressure from the phone’s barometer. The “Get weather” row fills in the air from Apple Weather. On a phone with a barometer the pressure comes from the barometer. Without one, Ballista uses Apple Weather’s sea-level pressure only with a GPS altitude good to 30 m, or after you type your altitude. Ballista never saves your location; Get Weather sends it rounded.

A routine for the range

1. Decide which pressure your source gives before you write it down. Write “station” or “sea level” next to it.
2. Prefer a barometer at the firing point. It needs no altitude and no correction.
3. If you only have a weather report, enter it as an altimeter setting with your altitude, never as station pressure.
4. Read the density altitude your solver shows back. If it’s near zero at a mountain range, the pressure is in the wrong field. The density altitude article shows what the right number does to your come-ups.