Humidity affects horses more than temp. as it inhibits a horse's ability to stabilize it's body temperature which is done mostly by sweating [75%]. Pre-race anxiety and perspiration (washing out) before a race will affect nervous horses far more than calm ones and often leads to electrolyte imbalances leading to all sorts of muscle and breathing problems during a race.

For me, I'll be ignoring 2 or 3yr. old races over the next few days!