(Temperature + relative humidity) - wind speed = Heat Stress Index Based on the sum of the numbers, you can get a general sense of how safe it is to ride. LESS THAN 130: All go. Horses can function to cool themselves assuming adequate hydration.
130 - 170: Caution. A horse's cooling mechanisms can only partially function as intended. Some cooling management procedures will need to be performed.
180 OR ABOVE: Stop. A horse's cooling systems cannot and will not function adequately. All cooling procedures will need to be utilized to keep the horse out of serious trouble. Whenever riding in the heat it's important to also keep in mind your horse's fitness level (and your own) and whether you will be riding in direct sun
EXAMPLE: 93° F + 51% humidity = 144 minus 5 mph winds = 139 CAUTION: Extra cooling measures needed for riding...i.e. cool with water on legs and chest, unsaddle - or ride bareback.