If you are going to clip your foals, and I never do, you have to be able to protect them not just form the wet and cold but also from the sun.
I am not convinced about this being for the horses benefit entirely, as the coat is there for a reason.
The foals I had born at the very beginning of the year have shed most of their coat naturally and just have a protective "blanket" over their back, which they need whatever the weather, the ones I had born five weeks ago , well, there is nothing to clip, they were born in full summer coat.
Clipping with the hair will just take off the baby fluff and allow the horse to not over heat, that is the very most I would ever do, but I have not actually needed to do that yet.
As someone on another board pointed out, sometimes it is the humidity that counts??
We have hot (believe it or not!!) but we do not have the humidity that some states get.