This article explains how to activate and add the temperature filter to your snaps.
Steps

Step 1. Open Snapchat
If prompted, enter your username and password, then tap "Sign In".

Step 2. Tap the Capture button, which is the white circle at the bottom of the camera screen

Step 3. Swipe your finger left or right

Step 4. Tap Enable Filters

Step 5. Tap OK, this way Snapchat will have permission to access the device location and activate the filters in the settings
Once enabled, filters (and therefore location access) will remain so until you disable it in Snapchat settings

Step 6. Swipe left or right to add the temperature filter
At the time of sharing, a filter will appear on the photo that will show the temperature of the place where you are.