Nerd stuff · Field science
Do weather conditions matter when zeroing your rifle?
Same rifle, same ammo, twenty feet apart.
Quick answer
The atmospheric conditions on the day you zero barely matter, as long as you zero at 100 yards: across the two harshest shooting environments on Earth, the air moves a 100-yard zero just 0.03 to 0.04 inches, under one scope click. The conditions at the moment of the shot are a different story. The same rifle's correct 1,000-yard aim point moves 20 feet between those environments.
To settle how much the weather at zeroing time actually changes things, we ran one .308 hunting rifle through our ballistics engine in the two most extreme environments on the planet, zeroed it in each, and compared everything: the zero itself, and the aiming solution out to 1,000 yards. Here is the whole picture.
0.24in
How far a 100-yard zero drifts between Earth's extremes. About one scope click.
20ft
How far the correct 1,000-yard aim point moves between the same two places.
2×
How much thicker the air is at the cold site than the hot one. Drag doubles with it.
The experiment
One rifle, two worlds
Both of these are real places where people shoot every winter and every summer. We zeroed the same rifle at 100 yards in each location, then asked the engine inside Wrist Dope for the aiming solution out to 1,000 yards.
Arctic river bottom, January
Interior Alaska or Siberia, sea level
- Temperature
- −40°
- Pressure
- 1040 hPa (strong high)
- Air density
- 1.55 kg/m³
- Feels to the bullet like
- 8,400 ft below sea level
- Cold-soaked ammo
- 2,551 fps at the muzzle
High desert, August
Mountain West or the Altiplano, 10,000 ft
- Temperature
- 95 °F
- Pressure
- 697 hPa (thin air)
- Air density
- 0.79 kg/m³
- Feels to the bullet like
- 14,300 ft above sea level
- Warm ammo
- 2,686 fps at the muzzle
Finding 1
The air decides how far your bullet falls
Gravity is identical at both sites. What changes is drag. Thick, cold air slows the bullet harder, the bullet takes longer to arrive, and gravity gets more time to pull it down. Thin mountain air does the opposite. The gap starts invisible and grows with every yard.
How high you must aim above the target
Same rifle, zeroed at 100 yards. Inches of holdover by distance, in each environment.
See the numbers
| Yards | Arctic (in) | Standard (in) | Hot + high (in) |
|---|---|---|---|
| 100 | 0 | 0 | 0 |
| 200 | 4.9 | 4.2 | 3.8 |
| 300 | 17.5 | 15.1 | 13.4 |
| 400 | 39.5 | 33.7 | 29.4 |
| 500 | 72.9 | 61.5 | 52.5 |
| 600 | 120.5 | 99.9 | 83.4 |
| 700 | 186 | 151.2 | 123 |
| 800 | 274.1 | 217.8 | 172.3 |
| 900 | 391.6 | 303.1 | 232.4 |
| 1000 | 545.9 | 411.2 | 304.5 |
At 1,000 yards the arctic shooter holds 45 feet over the target; the mountain shooter holds 25. Same rifle, same box of ammunition.
Finding 2
Ignore the weather and here is your miss
Suppose you learned your drops in one environment, wrote them on a card, and trusted the card in the other. The chart below is how far off your bullet lands. Every bar is a clean miss on any game animal alive.
Vertical miss when you use the other environment's numbers
Worst case between the two sites, in inches, by distance.
See the numbers
| Distance | Miss (inches) | Miss (feet) |
|---|---|---|
| 300 yd | 4.1 | 0.3 |
| 400 yd | 10.1 | 0.8 |
| 600 yd | 37.1 | 3.1 |
| 800 yd | 101.8 | 8.5 |
| 1000 yd | 241.3 | 20.1 |
You do not need to fly between hemispheres to feel this. A rifle doped on a hot summer weekend and hunted with in cold late-season mountains sees a quarter to a third of these bars, and a quarter of the 600-yard bar is already the height of a deer's vital zone.
Finding 3
What actually moves the bullet
Break the 600-yard shift into its causes and the ranking surprises most shooters. Air density leads, as expected. But the temperature of the ammunition itself is nearly as strong, because powder burns slower when it is cold. And humidity, the thing everyone argues about at the range, barely registers.
Where your bullet lands at 600 yards, one cause at a time
Inches high or low versus a standard 59 °F sea-level day, with everything else held equal.
See the numbers
| Cause (one at a time) | Effect at 600 yd |
|---|---|
| Dense arctic air (−40°, high pressure) | 10.2 in low |
| Ammo cold-soaked to −40° (−99 fps) | 9.3 in low |
| Thin air at 10,000 ft on a 95° day | 13.9 in high |
| Ammo baking at 95° (+36 fps) | 3.1 in high |
| Humidity: bone dry → fully soaked | 0.9 in high |
The humidity myth, retired. Humid air is actually thinner than dry air, because water vapor weighs less than the nitrogen and oxygen it displaces. Going from bone dry to fully soaked on a hot day moves a 600-yard shot less than one inch. It is the one condition you can safely ignore.
Finding 4
Meanwhile, your 100-yard zero didn't move
Here is the quiet good news in all of this. At 100 yards the bullet is only in the air for a tenth of a second, so the atmosphere never gets a real grip on it. Take a rifle zeroed on a standard day to either extreme and fire at 100 yards, and this is the entire damage:
Where the extremes land on a 100-yard target
Rifle zeroed once on a standard day, sights untouched. Rings are 1, 2, and 4 inches across.
See the numbers
| Fired at 100 yd in | Impact shift |
|---|---|
| Arctic air only | 0.03 in low |
| Thin hot air only | 0.04 in high |
| Arctic air + cold-soaked ammo | 0.24 in low |
| Thin hot air + warm ammo | 0.11 in high |
Every impact stays inside a circle smaller than a quarter. No rifle groups tightly enough for you to even see the difference. That is why a 100-yard zero is the one you can trust anywhere on the planet, and why the smart division of labor is: you supply a clean zero, and let the solver carry the weather.
What to do
Three habits, no math
- 1
Zero at 100 yards, on a calm day.
Short range keeps weather and wind out of your sights, so the zero travels with the rifle anywhere. Note the temperature that day; it costs nothing.
- 2
Know what your ammo does when it's hot or cold.
Cold powder at −40° cost this rifle 99 fps and 9 inches at 600 yards on its own. Chronograph near the temperatures you hunt in, or pick a temperature-stable load.
- 3
Give your solver the air you're standing in.
Current pressure and temperature at the shot, every time conditions change. This is exactly the job Wrist Dope does from your wrist, so the twenty-foot problem in these charts simply never reaches your trigger finger.
The takeaway
Zero conditions are noise. Shot conditions are feet.
Everything on this page points at one division of labor. The weather on the day you zeroed is worth hundredths of an inch, so nothing needs to remember it. The weather at the moment of the shot moves impacts by feet, so something has to measure it, where you stand, when you press the trigger. Guessing does not work: station pressure falls with every thousand feet you climb, the swing from a summer zeroing session to a cold late-season morning moves a 600-yard impact about the height of a deer's vital zone, and the weather app on your phone reports conditions from an airport, not your ridge.
That means some instrument at the firing point has to read the air: temperature, station pressure, and altitude, the three numbers that set the density your bullet actually flies through. A handheld weather meter does the job. A watch does it without being asked. Wrist Dope reads the barometer on your wrist for the true pressure where you stand, GPS for your altitude, and live local weather for temperature, then rebuilds your firing solution every time the air changes. Your clean 100-yard zero plus a sensor on your arm covers the atmosphere for you, with nothing to type and no card to reprint.
Common questions
- Do atmospheric conditions from zeroing day affect my ballistic solution?
- Almost not at all, if you zeroed at 100 yards: the air's fingerprint on a 100-yard zero is 0.03 to 0.04 inches across Earth's extremes, so the zero transfers anywhere. What must track conditions is the firing solution itself; give your solver the pressure and temperature where you are shooting, at the time of the shot.
- Do I need to re-zero my rifle at a different elevation?
- Not if you zeroed at 100 yards. Across the most extreme realistic swing on Earth, the air alone moves a 100-yard zero 0.03 to 0.04 inches, far less than one scope click. Keep the zero and let a ballistic solver correct the longer holds for the local air.
- Does altitude affect bullet drop?
- Yes, and strongly at distance. Thinner high-altitude air makes less drag, so the bullet drops less. Our engine puts the same .308's 1,000-yard holdover at 546 inches in dense arctic sea-level air versus 305 inches on a hot day at 10,000 ft. Inside 200 yards the difference is a fraction of an inch.
- Does temperature affect my rifle's zero?
- The air's share is negligible at 100 yards, about 0.01 inches season to season at the same range. What actually moves winter impacts is the ammunition: cold powder produces lower muzzle velocity, worth about 9 inches at 600 yards for a temperature-sensitive load taken from summer chronographing to a −40° hunt.
- Does humidity affect bullet drop?
- Barely, and backward from intuition: humid air is thinner than dry air because water vapor weighs less than the nitrogen and oxygen it displaces. Going from bone dry to fully saturated on a hot day is worth under one inch at 600 yards and about 0.6 MOA at 1,000.
Want the full tables, the zero-range analysis, and the math? Read the white paper: Do atmospheric conditions matter when you zero a rifle?