Atmospheric inputs exist to tell the solver one thing: how dense the air is. Get them right and the model handles the rest. Get one of them subtly wrong and the error is invisible in the interface and visible on the target.
The station pressure trap
This is the most common atmospheric error in shooting software, and it comes from a reasonable habit: reading a pressure off a forecast.
Weather services publish pressure corrected to sea level, so that maps are comparable between a coastal city and a mountain town. A solver needs station pressure — the actual pressure where you are standing, which at 1,500 m is substantially lower.
| You have | Enter as pressure | Enter as altitude |
|---|---|---|
| A reading from a phone barometer or weather meter | That reading (station pressure) | Your real altitude |
| An airport altimeter setting or TV forecast figure | Not directly usable with your altitude | Set altitude to zero, or convert to station pressure first |
| Nothing but an altitude | Leave at standard | Your real altitude |
The habit that avoids all of it: measure station pressure at the firing point, enter your real altitude, and stop thinking about it.
Temperature
Air temperature drives air density, and it is easy to measure badly.
- In the shade. A thermometer in direct sun reads the thermometer, not the air, and can be many degrees high.
- Away from your body. A phone or meter just out of a pocket is warmed by you. Give it a minute.
- Away from the ground on a hot day. Air just above dark ground is not the air the bullet flies through.
Air temperature and powder temperature are two different inputs and they diverge exactly when it matters — ammunition on a hot bonnet or in a cold pocket. If your load has a meaningful temperature coefficient, enter the ammunition's temperature separately. See powder temperature.
Altitude
A property of the place, not of the day. Enter it once for your usual range and change it only when you move. It works with the pressure reading rather than replacing it.
Humidity
The least important of the four. Water vapour is lighter than the air it displaces, so damp air is slightly *less* dense — the opposite of most people's intuition — but the effect on drop is a small fraction of what a ten-degree temperature change does.
Anything within twenty per cent of reality is fine. Do not buy an instrument for it.
Fetching versus typing
The app can pull current conditions for your position when you have a signal. That is convenient at a range with coverage and useless in a valley, which is the normal case — so every field is typeable.
| Situation | Approach |
|---|---|
| Signal, and no instruments | Fetch. Better than guessing |
| Instruments on hand | Type your own readings. Local beats regional |
| No signal, no instruments | Type your best estimate of temperature and leave pressure at standard for your altitude |
| Building a DOPE card for a trip | Type the conditions you expect, and label the card with them |
A fetched reading comes from the nearest station, which may be a different altitude and a different microclimate. It is a good default and a poor substitute for a meter at the firing line.
How often to re-enter
For shooting inside 400 m, once a session is plenty. Past 700 m, a summer morning and a summer afternoon can be genuinely different conditions, and re-entering takes fifteen seconds.
The way to find out how much it matters for your own rifle is to solve the same distance twice, with your coldest and hottest realistic conditions, and compare. If the difference is smaller than your target, you can relax about it.
Frequently asked
What is the difference between station pressure and barometric pressure? +
Station pressure is the actual pressure where you are. Barometric pressure as reported by weather services is normally corrected to sea level so that maps compare. A solver needs the former.
If I enter station pressure, do I still need altitude? +
Yes — enter both, honestly. Station pressure and altitude describe different things and the solver uses both. What you must not do is combine a sea-level-corrected pressure with a real altitude.
How much does humidity really matter? +
Very little. It is worth entering roughly and not worth measuring precisely; its contribution is a small fraction of the temperature effect.