Emily, you might just try running in the warmer part of the day once or twice, just to see. My experience is that I'm much more likely to experience heat stress if I run in the early morning when the humidity is so much higher, even though the temperature is lower. Once the humidity is even above 85% - and typically in early morning it's closer to 95% - but anyway, beyond 85% sweat really doesn't evaporate at all, meaning my body can't cool itself whatsoever. The main source of core heat is the body's own heat generation, not the ambient temperature. Once the humidity drops to 75% or 80% by midday, even though the thermometer might show a much higher figure, I can run the same distance and pace with a lot less chance of dizziness and nausea by the end.