There are two units of temperature that are commonly used around the world: degrees Celsius (ºC) and Kelvin (K). For some strange reason, Americans also use degrees Fahrenheit (ºF).
While you can’t set your iPhone to use Kelvin (they’re more popular with scientists than the general public), you can swap it between Celsius and Fahrenheit. Siri, iOS’s Weather app, and any other app that pulls the default weather data from your phone will use your chosen units.
This makes life a little easier if you’re an American who’s come to their senses and wants to join the rest of the world, or a European in America who wants to use the local lingo. Here’s how.
Go to Settings > General > Language & Region.
Select Temperature Unit and then change it to either Celsius or Fahrenheit, whichever you want to use.
Now your iPhone will use that unit by default whenever it needs to show the temperature. Note that many third party apps don’t use iOS’s weather service to get data, so they might have their own preference settings.