Part of training is to get used to running in all kinds of weather conditions. Since running is mostly an outdoor sport, you will probably encounter all kinds of weather running races. It may be hot, it may be cold, it may be windy, it may be rainy. If you train in those conditions you will be used to running in them. Also, if you live in an area that frequently has less-than-perfect weather and you wait for the perfect weather to run you may end up running half the time if that.