Spraying water on a hot horse to cool it off promotes convection cooling and assists the horse in lowering its core temperature. The reason you spray the legs and belly is because the blood vessels are closer to the skin in those locations, and it promotes faster cooling of the horse’s core temperature by carrying the cooler blood to the heart.
Another important part of cooling out horses is evaporation. After the horse has been sprayed off, it is very important to scrape the water off. This is because once the horse is sprayed, the water absorbs the horse’s heat and becomes warm. In order for evaporation to occur effectively, this warm water must be removed. This process can be repeated until the horse’s temperature comes down (i.e. spray then scrape, spray again then scrape again, etc.). If the water is not scraped off, it could act as an insulating layer and actually make the horse hotter than when you started.
In extreme circumstances, ice can be added to water for sponging to increase the speed of cooling the core temperature. It is commonly thought that ice will be a shock to the horse’s system and could cause tying-up (muscle cramping); however, with extreme heat and internal body temperatures this is not the case. If a horse is prone to tying up, it may be recommended to not directly apply the ice to the large gluteal muscles in the hind end, but focus on those key areas where the blood vessels are more superficial (the belly and inside of legs and thighs).