Take this for what it's worth:
If I was naked outside, even on the warmest day, and a soft breeze blew against me, it would be cool (in temperture). Wind crossing your naked body tingles the senses and tickles the hairs on your skin. I would imagine even for the most casual nudists, there is still a thrill associated with being completely naked in public that causes goosebumps on their skin. It's a thrill equal to, if not fueling the passion of what I presume follows next. Out of curiousity, does this scene take place in the summer?
The excerpt is taken out of context, so I don't know Mel or Heather, nor do I know HOW into one another they are; The conversation seems flat WITHOUT the description of their body language. Are they holding each other? Is Heather playing "hard to get?" Is she shy / uncertain? Does she cover herself while he's trying to relax her (which I assume explains why she had to reassure him that she was comfortable being naked by the use of her example from her past...)?
Also, the dialouge seems a little disjointed-- they don't seem to be responding to one another. For example, Mel said, "“Can’t do this in Chicago,” to which Heather relates the moment to (in my opinion a surprising) event in her past, to which Mel almost seems unphased by saying, "“I like being under the open sky naked. It does feel free." Either he didn't care about her experience from the past or he was just rushing to the grand finale-- both are bad examples of the chemistry needed to fuel the reader's passion for what comes next (even if you end the scene as you did, the reader instantly races away with imagination).
To sum up-- you did a very fine job tying the physical description of the environment to the passion of the scene, but I would focus more on the unspoken dynamic in the scene; the chuckles, the "shy" foreplay, the "catch me if you can," the wonder, the trust necessary to WANT to be naked in public together, the chemistry, etc...
Hope that helps... I'm horrible with love scenes by the way. Anyway, I'd be interested to know what's going on with Mel and Heather from Chicago that they're naked in the wilderness somewhere... haha. Good luck... MP