A lot of the fellows here will disagree with me. But, I think an 11 oz. worsted wool tartan suits a warm weather climate. I have a 5 yarder made from Henderson (ancient shade) and even in this desert (Southeastern New Mexico) which is notorious for the wind, I manage to keep "the mystery."

And, as some of the others mentioned, Poly Viscose is GREAT. I have three casual models from USA Kilts, and adore them.