Drag is proportional to air density. Denser air, more drag, more drop and more drift. Thin air, less of everything. Every atmospheric input a solver asks for exists to answer one question: how dense is the air the bullet is flying through?
What sets air density
| Input | Effect on density | Effect on the bullet |
|---|---|---|
| Higher temperature | Lower density | Flatter, less drift |
| Higher pressure | Higher density | More drop, more drift |
| Higher altitude | Lower density (via pressure) | Flatter, less drift |
| Higher humidity | Slightly lower density | Very slightly flatter |
Humidity surprises people by going the 'wrong' way. Water vapour is lighter than the nitrogen and oxygen it displaces, so damp air is slightly less dense than dry air at the same temperature and pressure. The effect is small — typically well under one per cent of drop even at extremes — which is why it is the last input worth worrying about.
The one number that summarises all of it
Density altitude is the altitude at which the standard atmosphere has the density your air actually has right now. It is a translation, not a measurement: a hot day at 500 m might behave like 1,800 m of standard atmosphere.
Its value is that it collapses three inputs into one comparable figure. Two sessions with the same density altitude will produce nearly the same trajectory, whatever combination of temperature and pressure got them there. That makes a DOPE book portable: label a card with its density altitude and you know when it applies again.
Station pressure, not the airport number
This is the classic trap. Weather services publish pressure corrected to sea level so that maps are comparable across terrain. A solver needs station pressure — the actual pressure where you are standing.
If you enter a sea-level-corrected pressure *and* your altitude, you have counted the altitude twice, and the solver models much denser air than exists. At 2,000 m the resulting error is large enough to miss with.
- A phone barometer or a weather meter reads station pressure. Use it, and set altitude to your real altitude.
- A METAR altimeter setting or a TV weather figure is sea-level corrected. If you use it, the solver needs to know that, or you need to enter zero altitude and let the pressure carry the information.
- Safest habit: measure station pressure at the range, enter your real altitude, and stop thinking about it.
How much does it actually move the impact?
Under 300 m, very little — usually less than your group. The effect scales with time of flight, so it grows fast past 600 m.
A useful mental model: going from a cold sea-level winter morning to a hot mountain afternoon can easily change the drop at 1,000 m by a distance comparable to the height of a torso plate. That is not a rounding error, and it is the reason a DOPE card written in one season betrays you in another.
The honest way to know your own numbers is to run the solution twice — once with your coldest realistic conditions, once with your hottest — and look at the difference at the distance you care about. If the gap is smaller than your target, one card covers your year. If it is larger, you need a card per season, or you need to enter conditions every time.
Getting conditions into the app
- Take temperature in the shadeA thermometer in the sun reads the thermometer, not the air. Give it a minute out of your pocket, too — body heat is worth several degrees.
- Read station pressure at the firing pointNot the forecast for the nearest town, which may be hundreds of metres lower.
- Enter altitude honestly, onceIt is a property of the place, not of the day. Only change it when you move.
- Leave humidity approximateAnything within twenty per cent of reality is close enough for its contribution.
- Re-enter when the day changesA morning card and an afternoon card can be genuinely different in summer.
The app can pull current conditions for your position when you have a signal, and lets you type them in when you do not — which is the normal case on a range in a valley. Either way the numbers land in the same solution.
Frequently asked
Do I need density altitude if I enter temperature, pressure and altitude? +
No. Entering the raw conditions gives the solver everything it needs. Density altitude is a convenience for comparing and labelling conditions, not an extra input.
Why does my rifle shoot flatter in summer? +
Two effects stack. Warm air is less dense, so drag is lower; and warm powder usually burns faster, raising muzzle velocity. Both push the impact up.
Is humidity worth entering at all? +
It is worth entering roughly, and not worth measuring precisely. Its effect on drop is a small fraction of what a ten-degree temperature change does.