I've been told that it's the yardage that matters most. 5 yards or less for hot climates, and more yardage for cold climates.

In my experience humidity plays the largest part. So in the low humidity of Phoenix, you should be quite comfortable in a 5 yard heavyweight or middleweight kilt. That 7 yard heavyweight from SWK may be somewhat uncomfortable though. Whenever I wear my 7 yard heavyweight in North Texas in the summer (hi temp & hi humidity) it's been very uncomfortable.

So I guess I'm saying that temperatures are relative.