Same data, so no forecast argument
This comparison is unusual in that there is nothing to argue about on accuracy.
Apple Weather draws on national meteorological data processed through Apple's own forecasting, delivered to apps through the service Apple calls WeatherKit. Thermometer uses WeatherKit too. Same source, same location, same number.
Which rules out most of what comparison pages normally claim. Neither app can be more accurate than the other about the current temperature, because it is the same reading. If you want to know how good that reading is in absolute terms, that is is Apple Weather accurate.
So the honest question is not which is more accurate. It is which presentation suits how you actually check the temperature.
What each is for
| Apple Weather | Thermometer | |
|---|---|---|
| Data source | Apple Weather | Apple Weather, via WeatherKit |
| Current temperature | Numeral | Analog mercury dial |
| Both scales at once | No, one unit at a time | Yes, numbered °C and °F side by side |
| Rain, radar, maps | Yes | No |
| Severe weather alerts | Yes | No |
| Air quality, UV | Yes | No |
| Hourly and 10-day | Yes, in detail | Yes |
| Widgets | Yes, several kinds | Small, medium, large, with numbered scales |
| In-app language choice | Follows the device | System plus 19 languages |
| Cost | Included with iOS | Free on the App Store |
The shape of that table is the point. One column is broad; the other is narrow and deliberately so.
Who should keep using Apple Weather
Stated plainly, because a comparison that recommends itself to everybody is not a comparison.
Keep Apple Weather and skip this if what you check is rain rather than temperature, if you rely on severe weather alerts, if you want radar and maps, if you track air quality, or if one weather app on the phone is enough for you. Apple Weather is good, it is already installed, and it covers far more ground.
Thermometer is worth a look if you check the outdoor temperature specifically and often, if you think in one unit but live somewhere that reports the other, or if an analog scale is genuinely easier for you to read at a glance than a numeral.
The two real differences
Both scales at once. Apple Weather shows one unit and you change it in settings. Thermometer's dial carries a numbered Celsius scale and a numbered Fahrenheit scale side by side, so there is no conversion and no toggle. If you moved countries, or work with people who use the other scale, this is the thing that matters.
The widget behaviour. Thermometer's widgets render from a snapshot the app writes each time it fetches, so they draw instantly, work offline and never show a placeholder or an error. The cost is that the value may be a few minutes old until the app runs again. Whether that is a feature or a limitation depends on which failure annoys you more — see iPhone weather widgets.
What neither can do
Worth saying, because it applies to every weather app on the phone.
No iPhone measures outdoor air temperature. There is no outdoor air sensor in the device, so every reading in every iPhone weather app comes from a weather service. And no app resolves your street: reporting for your coordinates is not the same as observing at them, which is why your garden can differ from the forecast by several degrees on a clear still night. That gap is explained in hyperlocal weather.