The closest comparison on the site
Most weather apps are compared on features. These two are better compared on subject, because they share an approach and differ in what they point it at.
Mercury Weather is a design-led forecast app built across the Apple platforms — iPhone, iPad, Mac and Apple Watch — with conditions, charts, widgets, storm and hurricane features, trip planning and, in recent versions, the current system design language. It is free to download with a subscription unlocking widgets, the Watch app, historical data and multiple saved locations.
Thermometer is a design-led instrument on iPhone only. One screen: a continuous mercury column read against a numbered °C scale and a numbered °F scale at the same time, with feels-like, humidity, pressure, wind and sky beneath it, a second page for hourly and ten-day forecasts, and three widget sizes rendering the same instrument.
Both are made by people who care how an app looks. That is not the deciding factor, because it is true of both.
Side by side
| Mercury Weather | Thermometer | |
|---|---|---|
| Subject | The forecast | The outdoor temperature |
| Platforms | iPhone, iPad, Mac, Watch | iPhone, iOS 17 or later |
| Temperature display | Numeral, with charts | Analog mercury dial |
| Both scales at once | No | Yes, numbered °C and °F |
| Saved locations | Multiple, on subscription | Current location or one pinned city |
| Trip planning | Yes | No |
| Historical data | Yes, on subscription | No |
| Widgets | Several types, on subscription | Three sizes, included |
| In-app language choice | Follows the device | System plus 19 languages |
| Cost | Free with subscription tiers | Free on the App Store |
Where each one wins
Mercury is the better choice if you want the same well-made app on your Mac and your Watch as on your phone, if you track several locations, if you plan trips around weather, or if you want charts and historical data. It covers considerably more ground, and being across the platforms is a real advantage that Thermometer simply does not have.
Thermometer is the better choice if the thing you check is the outdoor temperature, if you want it legible at arm's length on a Home Screen without opening anything, if you want both scales visible at once, or if you want the app in a language other than your phone's.
The subscription difference, stated fairly
Mercury's widgets, Watch app, historical data and multiple locations sit behind a subscription. That is a normal and reasonable model for an app maintained across four platforms, and the free tier is not the product its users are enthusiastic about.
Thermometer includes its widgets because there is one thing to include. It has no subscription, no advertising, no account and no analytics — which is easier to offer when the product is a single screen than when it is a cross-platform forecast suite. That is scope, not virtue.
If neither sounds right
If you want maximum configurability and a selectable data source, CARROT Weather is the app in that direction. If you want one app that covers everything and is already installed, that is Apple Weather. And if what you actually want to understand is why any of these apps can disagree with the thermometer in your garden, that is hyperlocal weather.