I noticed that there is no way you can change Siri in iPad to speak in centigrade instead of the bloody Farenheit. But guess what I found from the comment section of a post from kirkville? The simple way to solve it, here’s how.
Open up your Clock app, click the edit on top left corner. At the bottom of the small box you will notice the toggle between F and C, which denotes Farenheit or Centigrade. Just tap on C and Siri will now answers in Centigrade. Good luck!
Credit goes to a commentator of the post, Ovi.