Here in southern NV, for example, we get 118-degree heat in the summer. Making a baby afghan, for example, out of worsted yarn, using a smaller hook, like a G or F, would make a very dense and heavy blankie. Poor baby would roast in the summer. Poor baby would also be weighed down by the weight, I suspect.
I just finished a very nice ripple afghan for a friend's new baby, in Red Heart Banana Berry. Instead of using my usual G hook for this one, I went all the way up to a K. I also used a double-crochet instead of a single crochet or half-double. I also crocheted in back-loop only. All these things made a lighter weight product.
The result was a very soft, lightweight, and flexible blankie that works well in this climate.
The reverse is true, also. There are times when I am sending a gift to friends in colder climates. I try, at those times, to make a tighter weave, a heavier, warmer product, by using a smaller hook, if possible. A smaller, tighter stitch also helps. This works well for scarves and slippers.
Gauge is very important at these times.
I also try to make a note on the back of the pattern, or on a sticky note attached to the pattern, showing what I did, and for whom. Sometimes, I really surprise myself when I do something wise - it makes up for all the dumb things I do.