Nerd stuff · How it works
How does Wrist Dope know the weather where you stand?
Three numbers, three different sources, and one rule: whatever you type wins.
Quick answer
Your firing solution needs three numbers out of the air: station pressure, temperature, and altitude. Wrist Dope reads station pressure from the barometer in your Apple Watch or iPhone, altitude from GPS, and temperature from live local weather at your coordinates. Any field also takes a number you type, from a handheld weather meter or anywhere else, and a typed number always wins.
This is the question people ask before they buy: will it work on my watch, and where do the numbers actually come from? Here is the whole chain, field by field, including what happens when the weather service is unreachable and what happens when you disagree with it.
The three numbers
A solver does not want the weather. It wants the air.
Forecasts, fronts, and the little cloud icon on your phone have nothing to do with a trajectory. What a ballistic solver needs is how thick the air is between your muzzle and the target, because that is what slows the bullet down. Air density comes out of three measurements, and Wrist Dope gets each one from a different place.
Station pressure
24.89
Barometer on the device
The single biggest lever on density, and the one nobody can look up for you.
Temperature
44 °F
Live local weather
Forgiving enough that a few degrees of error costs about a tenth of an MOA at 600 yards.
Altitude
5,010 ft
GPS on the device
Sets your place in the air column and corrects a sea-level pressure report back down to you.
Humidity is the fourth field, and it is the smallest one by a wide margin. It comes from the same weather feed as temperature. The app folds all four into density altitude, the one number that says how hard the air is pushing back, and shows it live on the environment screen.
From sensor to firing solution
Two of the three numbers never leave your wrist. One is looked up. All four fields accept a number you enter instead.
Pressure
The one number a weather app cannot give you
Ask any weather service for the pressure and it will hand you a number corrected to sea level. That convention exists for good reasons: it lets a forecaster compare Denver with Miami and see a weather system instead of a topographic map. It is also the wrong number for a bullet. Your bullet does not fly through a correction. It flies through station pressure, the honest weight of the air standing on the spot where you are standing.
Reported pressure vs the pressure your bullet flies through
One sea-level report of 29.92 inHg, read at different altitudes. The gap is not a rounding error: by 10,000 feet the report sits more than 9 inHg above the pressure you are actually standing in.
Type a weather app's pressure into a ballistic solver at a mountain range and you have told it you are at sea level. Wrist Dope skips the problem by reading the barometer in the watch or the phone, which reports station pressure by construction. No correction, no nearest-airport guess, no elevation lookup. If the device has no barometer, the app falls back to the sea-level pressure from the weather feed and corrects it back down to your GPS altitude, and if nothing has arrived at all it holds the standard 29.92 inHg as a labeled default rather than presenting a fabricated reading.
Your hardware
Does your Apple Watch have a barometer?
Almost certainly yes. Apple has shipped a barometric altimeter in every Apple Watch since Series 3, and an always-on version since Series 6 and the SE. GPS has been in the watch even longer. So the two numbers that have to be measured on site, pressure and altitude, are measured on site on any watch you are likely to be wearing, and on any iPhone since the iPhone 6.
| Device | Station pressure | Altitude | On-board temperature sensor | Air temperature the solver uses |
|---|---|---|---|---|
| Apple Watch Series 3 to 5 | Barometric altimeter | GPS | None | Live local weather |
| Apple Watch Series 6 and later | Always-on altimeter | GPS | Series 8 and later: wrist temperature, sleep only | Live local weather |
| Apple Watch SE, all generations | Always-on altimeter | GPS | SE 3: wrist temperature, sleep only | Live local weather |
| Apple Watch Ultra, all models | Always-on altimeter | Dual-frequency GPS (L1 + L5) | Wrist temperature, plus water temperature | Live local weather |
| iPhone 6 and later | Barometer | GPS | None | Live local weather |
Wrist Dope itself requires watchOS 11 or later and iOS 18 or later. The iPhone app is a complete solver on its own, using the same code against the phone's own barometer and GPS, so the story on this page is identical whether you own a watch or not.
Temperature
Your watch has thermometers. They are pointed at you.
Apple Watch Series 8 and later, every Ultra, and the SE 3 carry two temperature sensors: one on the back crystal against your skin and one under the display. They exist to track your wrist temperature overnight, sampled while you sleep and reported as a change from your own baseline for health and cycle tracking. Apple is explicit that the feature is not a thermometer and cannot provide measurements on demand, and there is no way for an app to ask them for the air temperature.
Even if there were, a sensor sandwiched between a 90 degree wrist and a jacket cuff is measuring you, not the range. So Wrist Dope takes temperature and humidity from Apple's weather service for your GPS coordinates, and refreshes them as conditions and your position change.
Why this split is the right one. The two inputs have very different tolerances. Pressure has to be measured where you are, because the reported version is corrected to a sea level you are not standing at, and being wrong by an inch of mercury is being wrong about the air itself. Temperature is forgiving: a few degrees of error costs about a tenth of an MOA at 600 yards, which is well under half of one quarter-MOA click. Wrist Dope measures the unforgiving number and looks up the forgiving one.
Overrides
Every field takes a number you type
Automatic conditions are a convenience, not a doctrine. Each of the four atmospheric fields carries its own AUTO and MANUAL switch, so you can pin one number and leave the rest live. Tap the field on the watch and it flips to MANUAL, pre-loaded with whatever the sensor was reading so you are editing a real number rather than starting from zero, then the Digital Crown walks it up or down. On iPhone you type it. Your value is then the value, on every solve, and it is saved on the device and synced to your other one.
Environment
• SENSORS ON↻ Refresh
Read it off your meter
type it, and it wins
This is the shipped path for a handheld weather meter: read the instrument, enter the number, keep shooting. Nothing to pair, nothing to charge, no protocol to break in a firmware update. The fields accept anything a real day can produce, from 20 to 35 inHg, −40 to 130 °F, sea level to 15,000 feet.
Reading the air automatically is part of Wrist Dope Pro, which has a free seven-day trial. Entering conditions by hand is free forever, on the watch and on the phone.
Where each number comes from, in order
Resolved per field, every time the solver runs. The first line that has a value is the one your bullet gets.
- 1▼
A value you entered
Switch a field to MANUAL and your number is the number, on every solve, until you switch it back.
- 2▼
The sensor on the device
Station pressure from the barometer, altitude from GPS. No network involved, so a canyon with no bars changes nothing.
- 3▼
Live local weather
Temperature and humidity for your coordinates. If there is no barometer, the sea-level pressure from the same feed gets corrected back down to your GPS altitude.
- 4
The standard atmosphere
29.92 inHg, 59 °F, 50%. Used only when nothing else has arrived yet, and held as a known default rather than dressed up as a reading.
Nothing below line 1 can outrank it. That is the whole contract: the app works hard to fill the fields for you, and stops the instant you fill one yourself.
No signal
What happens when the bars go away
The solver runs on the device, so the math never needs a network. The barometer and GPS do not need one either, which means the two on-site numbers keep updating at the bottom of a canyon with no service at all. Live weather is the only link in the chain that wants a connection. Lose it and the app keeps the last temperature and humidity it managed to fetch and goes right on solving; if it has never fetched one, it uses the standard 59 °F and 50% and lets you type over them. Manual values are stored on the device, so a temperature you entered yesterday is still there after a relaunch in the field.
How often it looks
Weather re-checks about every three minutes, and refetches once you have moved roughly a third of a mile or a quarter hour has passed. There is a Refresh control on the environment screen when you want a new pull right now, after a drive to a different elevation for instance.
Why the numbers hold still
Sensors jitter, and a solution that twitches is a solution you stop trusting. Values only move the solve once they move for real: 0.02 inHg of pressure, half a degree of temperature, 20 feet of altitude. The environment screen still shows you the raw live readout underneath.
Android
Android is in testing
Everything on this page describes Wrist Dope on Apple Watch and iPhone, which is what ships today. An Android version is in testing and not yet released. When it does ship we will publish the same breakdown for Android phones and watches, because the sensor story there is genuinely different and deserves its own page rather than a footnote on this one.
The takeaway
The right source for each number.
Everything on this page reduces to one idea: each input gets the instrument that actually suits it. Pressure wants a barometer at your position, so the watch reads its own. Altitude wants GPS, so the watch asks its receiver. Temperature tolerates a good forecast, so live local weather fills it in, and the error that leaves is smaller than a scope click.
And when you know better than the sensors, you outrank them: switch any field to manual and type what your meter reads. Wrist Dope keeps the whole chain running in the background so the solution on your wrist always reflects the air your bullet is about to fly through.
Common questions
- Does the Apple Watch have a barometer?
- Yes. Every Apple Watch since Series 3 has a barometric altimeter, and Series 6, every SE, and every Ultra have the always-on version. Wrist Dope reads it directly for station pressure, the actual pressure of the air where you are standing, and pairs it with GPS altitude. Apple Watch Ultra models also carry dual-frequency GPS.
- How accurate is the Apple Watch barometer for a firing solution?
- The important part is not the last hundredth of an inch, it is that the reading comes from where you stand. Weather services report pressure corrected to sea level, so at 5,000 feet their number is about 5 inHg away from the air your bullet flies through. If you carry a weather meter you trust more, switch pressure to manual and type its reading in. The manual value wins.
- Where does Wrist Dope get the temperature?
- From live local weather for your GPS coordinates. The temperature sensors in Apple Watch Series 8 and later read wrist temperature while you sleep; Apple states the feature is not a thermometer and cannot give measurements on demand, so no app can read ambient air from them. Temperature is also the forgiving input: a few degrees of error costs about a tenth of an MOA at 600 yards.
- Can I use a handheld weather meter with Wrist Dope?
- Yes, by entering its readings. Every atmospheric field has its own AUTO and MANUAL switch. Tap a field to put it in MANUAL, dial the value with the Digital Crown or type it on iPhone, and that value overrides the sensor until you switch back. Manual values are saved on the device and sync between your watch and iPhone.
- Does Wrist Dope work offline?
- Yes. The solver runs on the device, and the barometer and GPS are hardware that needs no signal, so pressure and altitude stay live in a canyon. Live weather is the only piece that needs a connection. Without one, the app keeps the last temperature and humidity it fetched, and any value you entered by hand survives a relaunch.
Wondering how much the air is really worth downrange? Do weather conditions matter when zeroing your rifle? runs one .308 through the two harshest environments on Earth: the zero survives untouched, and the 1,000-yard aim point moves twenty feet.