I agree with the alcohol, I definitely sweat a lot less when I'm drunk. I don't think its because I'm less nervous or stressed, however, I seem to think it's something more biological, because I'm not a nervous person. Maybe the alcohol alters the nervous system in some small way, same with if you are tired. This doesn't explain the rain, though...seems counter-intuitive that if there is a lot of water vapor in the air my hands would sweat less, because that's certainly not the case on humid days.