Spark plug tip temperature is primarily a function of engine output, not coolant temperature. The proper tip temperature operating range is 400C to 900C. Below 400 the plug will foul, and over 900 can cause preignition, which can lead to detonation.

Low speed driving in hot weather can cause high coolant temperatures, but plug tip temperature will be at the low end or the range since average engine output is low.

It sounds like your driving conditions are similar to the US, so a heat range "9" would be suitable.

Duke