Nerd stuff · Field science
What environmental conditions affect bullet drop the most?
Measure one, model one, forget one.
Quick answer
Air pressure, and it is not close. Taking the same .308 from sea level to 8,000 feet drops station pressure 260 hPa and moves a 600-yard impact 10.8 inches; a strong weather front adds another 2.7 inches. Temperature is second at 6.8 inches over a 40 degree swing, counting both thinner air and faster powder. Humidity is worth 0.9 inches, and it works backward from what most shooters expect.
Everybody knows the list: pressure, temperature, altitude, humidity. Almost nobody knows the order, and the order is what decides which number you have to get right in the field. So we put one .308 through our ballistics engine and moved each condition on its own, by an amount you would actually see, and measured where the bullet went.
10.8in
Vertical error at 600 yards from driving to an 8,000 ft range and nothing else.
6.8in
From a 40 degree temperature swing, counting the air and the powder together.
0.9in
From humidity going bone dry to fully saturated on a 95 degree day.
The ranking
Every condition on one scale
Here is the whole comparison in one chart. Each bar is one variable moved by itself, by a swing a shooter meets in normal life, with everything else pinned to a standard 59 °F sea-level day. The bar length is how far the bullet lands from where you aimed at 600 yards if you never account for that change.
What each condition costs you at 600 yards
Vertical error from one realistic swing of one variable. Reference load, 100-yard zero.
See the numbers
| Variable (realistic swing) | At 300 yd | At 600 yd | At 1,000 yd | 600 yd in MOA |
|---|---|---|---|---|
| Altitude, sea level to 8,000 ft (station pressure falls 260 hPa) | 1.0 in | 10.8 in | 76.2 in | 1.71 |
| Ammo temperature, 40 °F swing (powder at 1 fps per °F, air held standard) | 0.8 in | 4.0 in | 16.1 in | 0.64 |
| Air temperature, 40 °F swing (39 to 79 °F, ammo held at 2,650 fps) | 0.3 in | 2.8 in | 20.8 in | 0.44 |
| Pressure, a strong front (1013 hPa plus or minus 30 hPa) | 0.3 in | 2.7 in | 20.8 in | 0.43 |
| Humidity, bone dry to saturated (0 to 100 percent on a 95 °F day) | 0.1 in | 0.9 in | 6.5 in | 0.14 |
Two of those bars are the same variable. Altitude at the top and the weather front in fourth place are both station pressure, just at different scales: terrain moves it 260 hPa, weather moves it 30. Add the two and pressure owns this chart outright.
The winner
Pressure is the number to measure
Air density is what your bullet actually fights, and pressure is the biggest lever on density. Push more air molecules into the same cubic foot and drag goes up, the bullet slows sooner, and gravity gets a longer turn. For this .308 at 600 yards the rate works out to about 4.5 inches per 100 hPa, or roughly an inch and a half for every inch of mercury on a barometer.
Weather alone swings station pressure by 30 hPa or so between a deep low and a strong high. That is a quarter of an inch at 300 yards, 2.7 inches at 600, and 20.8 inches at 1,000. The front that ruined your weekend is worth close to two feet of elevation error at 1,000 yards, on a rifle that never left the parking lot.
Station pressure, not the weather app. Aviation and weather services report pressure corrected back to sea level, so a 6,000-foot valley still reads near 1013 hPa on your phone. A solver fed that number thinks it is at sea level. What the bullet responds to is the raw station pressure where you stand, which at 6,000 feet is closer to 810 hPa.
Altitude
Altitude is pressure wearing a different name
Elevation gets talked about as its own force. It is not. Climbing 8,000 feet does exactly one thing to your bullet: it takes 260 hPa of pressure off the air. That is why the altitude bar dwarfs the weather bar. A strong front is a 30 hPa event, and a mountain drive is an eight-front event.
Sea level to 8,000 feet with temperature held steady is 1.0 inch at 300 yards, 10.8 inches at 600, and 76.2 inches at 1,000. That last one is over six feet. Real mountain air is also colder, about 28 degrees cooler at 8,000 feet on a standard day, and the denser cold air gives a little back: the same trip works out to 9.4 inches at 600 and 67.7 inches at 1,000.
The practical version: a solver never needs to know your elevation if it knows your pressure. Feed our engine 753 hPa and it does not care whether you typed 8,000 feet or zero, because the answer is identical either way. Elevation is only useful as a way to guess at pressure when you cannot measure it.
Temperature
Temperature counts twice
Temperature is the one variable on this page that reaches the bullet through two separate doors. Warm air is thinner, so drag falls. Warm powder burns faster, so the bullet leaves quicker. Both push the impact the same direction, which is why the combined bar is bigger than either piece and why cold mornings surprise people twice over.
Across a 40 degree swing, from 39 °F to 79 °F, the air effect alone moves a 600-yard impact 2.8 inches. The powder effect for a load that gains 1 fps per degree, which is typical of a load that has not been built for temperature stability, moves it 4.0 inches. Together, 6.8 inches. Push out to 1,000 yards and the ranking inside the pair flips: air is 20.8 inches and powder is 16.1, because drag compounds with flight time while a velocity head start does not.
The powder half is the one you cannot read off a sensor. It belongs to your specific lot of ammunition, and the honest way to learn it is a chronograph at the temperatures you actually hunt or compete in.
By distance
Nothing here matters inside 300 yards
The same five swings, now plotted against distance. Every one of them is a rounding error up close and a scoring problem far out, because drag has to accumulate over flight time before it shows up as inches. At 300 yards the worst offender on the list is one inch. At 1,000 yards it is over six feet.
How each condition grows with distance
Inches of vertical error from the same realistic swings, 100 to 1,000 yards.
See the numbers
| Yards | Altitude | Ammo temperature | Air temperature | Pressure | Humidity |
|---|---|---|---|---|---|
| 100 | 0.03 | 0.08 | 0.01 | 0.01 | 0.00 |
| 200 | 0.28 | 0.34 | 0.07 | 0.07 | 0.02 |
| 300 | 1.03 | 0.81 | 0.25 | 0.25 | 0.08 |
| 400 | 2.66 | 1.54 | 0.66 | 0.64 | 0.21 |
| 500 | 5.67 | 2.58 | 1.43 | 1.40 | 0.45 |
| 600 | 10.75 | 4.01 | 2.75 | 2.69 | 0.87 |
| 700 | 18.83 | 5.93 | 4.88 | 4.81 | 1.54 |
| 800 | 31.17 | 8.47 | 8.20 | 8.14 | 2.58 |
| 900 | 49.52 | 11.79 | 13.26 | 13.23 | 4.15 |
| 1000 | 76.23 | 16.10 | 20.81 | 20.83 | 6.46 |
Look at the two middle lines. The air-temperature curve and the pressure-front curve sit on top of each other the whole way out and finish 0.02 inches apart at 1,000 yards. A 40 degree change in air temperature and a strong weather front are worth the same thing to your bullet, which is the tidiest proof on this page that all of these conditions are one condition: air density.
Density altitude
One number for pressure and temperature
If pressure and temperature both work through density, they can be rolled into a single figure, and shooters borrowed the aviation one: density altitude. It is the altitude in the standard atmosphere whose air is as thin as the air you are standing in. One number, and your come-up follows it.
How density altitude gets built
A range near 4,800 ft on a hot day flies like 7,772 ft, and the come-up follows.
See the numbers
| Conditions at the firing point | Density altitude | 600-yd come-up (MOA) |
|---|---|---|
| Sea level, 59 °F, 1013 hPa | 0 ft | 15.90 |
| Sea level, 95 °F, 1013 hPa | 2,275 ft | 15.55 |
| About 4,800 ft, 90 °F, 850 hPa | 7,772 ft | 14.58 |
| 8,000 ft, 30 °F, 753 hPa | 8,000 ft | 14.43 |
| 8,000 ft, 59 °F, 753 hPa | 9,815 ft | 14.22 |
The table under that diagram is the useful part. Sea level on a 95 °F day is already 2,275 feet of density altitude. A range near 4,800 feet on a 90 degree day is 7,772 feet, thinner air than a standard day at 8,000. Rank the rows by density altitude and you have ranked them by drop, which is exactly why the number is popular.
It is a shorthand, though, not the whole truth. Two airs with the same density altitude are not quite identical, because temperature also sets the speed of sound, and the speed of sound sets where your bullet sits on its drag curve. Our engine gives 1013 hPa at 95 °F and 950 hPa at 60 °F the same 2,275-foot density altitude, then puts their 600-yard come-ups 0.6 inches apart and their 1,000-yard come-ups 4.6 inches apart. Close enough for a card, not as good as handing a solver the raw pressure and temperature.
Humidity
Humidity runs backward, and it never matters
Muggy air feels heavy, so it seems like it should drag on a bullet. The physics goes the other way. A water molecule weighs less than the nitrogen and oxygen molecules it pushes out of the way, so at a fixed pressure and temperature, humid air is thinner air, and the bullet drops slightly less.
Slightly is the operative word, because warm air holds far more water than cold air does. At 39 °F, saturating the air takes 0.3 percent off the density. At 95 °F it takes 2.1 percent, the largest humidity can ever manage. Here is the entire argument, drawn to the same scale as the conditions that do matter.
The three drivers, to scale, at 600 yards
Pressure and temperature against the biggest humidity swing physics allows.
See the numbers
| Driver | Swing | Error at 600 yd | In MOA |
|---|---|---|---|
| Air pressure | sea level to 8,000 ft | 10.8 in | 1.71 |
| Temperature | 40 °F swing, air and ammo together | 6.8 in | 1.08 |
| Humidity | 0 to 100 percent on a 95 °F day | 0.9 in | 0.14 |
That sliver is the whole humidity debate. It is 0.9 inches on the hottest, wettest day physics allows, about a third of what a passing weather front does at the same distance. At 1,000 yards it grows to 6.5 inches, still under a third of the front.
| Air temperature | Density lost, dry to saturated | At 600 yd | At 1,000 yd |
|---|---|---|---|
| 39 °F | 0.30 % | 0.14 in | 1.14 in |
| 59 °F | 0.64 % | 0.29 in | 2.23 in |
| 79 °F | 1.26 % | 0.54 in | 4.11 in |
| 95 °F | 2.10 % | 0.87 in | 6.46 in |
What to do
The hierarchy, in order
- 1
Measure pressure.
It is the biggest lever and the one that changes without telling you, both when the weather turns and every time you gain elevation. Use station pressure from a barometer at the firing point, not the sea-level figure a weather service publishes.
- 2
Model temperature.
Read the air temperature where you stand and let the solver handle the density half. The powder half belongs to your ammunition, so chronograph your load hot and cold, or buy one built to be temperature stable and stop thinking about it.
- 3
Ignore humidity.
The most it can ever do to a 600-yard shot is 0.9 inches, and it does that in the wrong direction from what everyone expects. Enter it if your solver asks. Never let it occupy a second of your attention on a hunt.
One thing that is not on this list: the conditions on the day you zeroed. Those are worth hundredths of an inch at 100 yards, which is why this whole page is about the air at the shot rather than the air in your logbook.
The takeaway
Measure one. Model one. Forget one.
The ranking on this page is a shopping list. Pressure earns a real measurement at your position, because it is the biggest lever and the one weather services report wrong for shooters. Temperature can be modeled, because a few degrees of error costs almost nothing. Humidity never mattered.
That is exactly how Wrist Dope divides the work: the barometer on your wrist reads true station pressure where you stand, GPS supplies altitude, live local weather fills in temperature, and the firing solution rebuilds itself every time the air changes. The whole hierarchy above runs on your arm without you touching anything.
Common questions
- What environmental conditions affect bullet drop the most?
- Air pressure, by a wide margin, because pressure sets air density and density sets drag. Taking the same .308 from sea level to 8,000 feet drops station pressure 260 hPa and moves a 600-yard impact 10.8 inches; a strong weather front is worth another 2.7 inches. Temperature comes second at 6.8 inches over a 40 degree swing, and humidity is worth 0.9 inches.
- Does air pressure affect bullet drop?
- Yes, and it is the single biggest environmental input. Higher pressure means denser air, more drag, a slower bullet, and more drop. For a .308 at 600 yards our engine puts the rate near 4.5 inches per 100 hPa, so an ordinary high-to-low swing of 30 hPa moves the impact about 2.7 inches, and 20.8 inches at 1,000 yards.
- What is density altitude in shooting?
- Density altitude is the altitude in the standard atmosphere that has the same air density as the air you are standing in, so it folds pressure and temperature into one number. A range near 4,800 feet on a 90 degree day has a density altitude of 7,772 feet. Our engine drops the 600-yard come-up from 15.9 MOA to 14.6 MOA between those two airs.
- Does humidity affect shooting accuracy?
- Almost never, and it works backward from intuition: water vapor is lighter than the air it displaces, so humid air is thinner and the bullet drops slightly less. Going from bone dry to fully saturated on a 95 degree day drops air density 2.1 percent and moves a 600-yard impact 0.9 inches, well under one scope click at that distance.
- Does temperature affect bullet drop?
- Yes, twice over. Warm air is thinner, which is worth 2.8 inches at 600 yards across a 40 degree swing, and warm powder pushes the bullet faster, which is worth another 4.0 inches for a load that gains 1 fps per degree. Together a 40 degree change moves a 600-yard impact 6.8 inches and a 1,000-yard impact 37 inches.
Want to see how far this goes at the edges? Do weather conditions matter when zeroing a rifle? runs the same rifle through the two harshest environments on Earth and finds a 1,000-yard aim point twenty feet apart. For the zero side of the question, with the full tables, read the white paper on whether atmospheric conditions matter when you zero a rifle.