What is up with my hairr?

So I've been trying to nurse my hair back to good health. I used to blow dry and straighten every day. Now I don't do anything to it, except cut a small fraction off here and there to get split ends and breakage off. My hair was coming around to something I was satisfied with, until the temperature outside hit 100+ degrees. When it was cooler outside only the wind messed with my hair, so is the temperature outside really that damaging? How do I keep my hair looking and feeling healthy without having to hide it under a scarf or hat? What kind of treatments should I use? What would you say is the best shampoo/conditioner, and hair serum?