Does Climate Affect Horse Racing?

Yes it does. Horse racing is a spectator sport and even if mild weather may not affect the performance of the thoroughbred race horses it may affect visibility of the audience or patrons of the sport.  

As we find it hard to walk in snow so do horses, and no jockey is going to risk his ride, worth in all probability at least a couple of million pounds, to slip and break a leg. Heavy rain may act as a curtain and is sure to mar visibility; it will also dampen the soil and affect performances. Most race courses cover their tracts with plastic sheets if it rains too heavily.  

A light rain may not affect visibility much ands events are carried on under such weather conditions, but horses like humans do suffer the putting down affects of constant dullness.

Conversely extreme heat can also harm the animal. Racing a fine animal in Arabia in the middle of the afternoon in all probability is impractical. A sunny day in England perhaps is ideal as horses can grip the ground and spectators can get an excellent view of what's occurring.