The Weather App in Windows 8 Metro start screen displays temperatures in Fahrenheit. This obsolete scale is popular only in the United States, and maybe in some other countries; the rest prefer to use the standard Celsius temperature scale. So if you want to change the scale from Fahrenheit to Celsius in the weather app, here is how to do it.
The following method is for Windows Consumer Preview and Release Preview. If you are using Windows 8 RTM, scroll to the bottom of this article.
Open the weather app from the Metro start screen and press Win+C to bring up the new start menu officially called the ‘Charm’ and click (or touch) on Settings. Alternatively, you can move the mouse pointer to the bottom-left corner of the screen to evoke the Charm menu.
Upon clicking on Settings, the Weather setting pane will slide open on the right. (Tip: use this trick to access additional settings on every Metro app)
Now click on Weather Preferences and change the scale from Fahrenheit to Celsius.
Easy when you know it, isn’t it?
Update: In Windows 8 RTM, the method to change Fahrenheit to Celsius and vice versa has been made a lot easier. Open the Weather app and right click on the screen. You will find the "Change to Celsius" button at the bottom of the screen as shown below.