Hi and welcome - It sounds possible that your thermostat could be stuck shut which would stop the coolant from circulating properly and cause the engine to overheat, but if your outside air temperature is really 31c it could just be that. Just to be sure I'd take the thermostat out and put it in a pan of cold water with a thermometer and confirm that by 85c it is fully open (8mm valve lift).
As for what measures the temperature, there is a thermo unit located within the thermostat housing itself, this can be removed and checked but I'd get a Haynes manual or somesuch as it involves water, electricity and risk of personal injury.