Read the can. They're usually pretty accurate. Here in the Piedmont, humidity can be a witch and sometimes even the can doesn't apply 100%. My best bottom line on humidity is no higher than 55% on either paint or primer. Temps on the other hand can vary considerably. Not knowing that fluorescent colors aren't as heat friendly, I botched an entire paint job because it was too hot outside. Fortunately, it's much easier to sand fluorescent stucco than it is paint stucco.