its usually a temperature thing but humidity plays a bit of a factor as well as how hot your fuel is ie. percentages of nitro .

higher temp is supppose to equal to cooler plug (not cold but medium) but high humidity can fill the combustion chamber with unburnable moisture drawn from the air which in my findings it has been found that keeping with a medium hot will help ignite the fuel better as well as keeping with the cooler plug and lowering deck heighth (shimming) .

cooler temps is supposed to equal hotter plug , usually stick with the medium hot and have no issues . 97t fits the bill in most all situations that is why it is highly recommended .