For me it's not the heat, its the sun that causes the most damage. My hair and skin is pretty sensitive to the sun so I have to be pretty careful especially since I've been working outside these past few weeks. I love being in the sun, but my hair dries out and lightens pretty fast with minimal sun exposure. Also my hairline is the first place that burns. To combat this, I mix a sunscreen with minimal ingredients in my leave-ins and wear a sun hat for as long as I could stand it.
ETA My hair did dry out somewhat when I visited family in the Southwest despite the fact I kept my sunhat glued to my head the entire time. It definitely was from the dry climate.