If you ask Siri: "What's the Temperature Now," she will tell you the temperature in Fahrenheit. This at least being the case for iOS devices purchased in the US. I prefer temperature in Celsius; this has been bothering me, and I finally find a solution recently. I am using iOS9 on the original iPad Mini, but the solution should work for most iOS devices.
From Jai_d81 in the Apple Discussion Forum at https://discussions.apple.com/thread/4179183?start=0&tstart=0
Try to change it in the weather app, lower left corner C/F, or try changing the settings>general>siri langauge to English(Australia) or change the settings>general>international region format to australia not sure whether its going to work
I can confirm that this works for an iOS9 iPad Mini purchased in the US (meaning all the localization and region settings are US-based). The discussion thread have a few other suggestions that I did not try.