This requires some common sense as well as the information we will give you. Common sense pertains to how well your horse has been conditioned and how well you can interpret signs of distress and possible heat exhaustion in your horse. Temperature alone is not the guide but the combination of temperature and humidity. Temperature and humidity affect the way a horse eliminates heat during exercise. When the relative humidity exceeds 75 percent, the effectiveness of cooling by sweating is greatly decreased. When the temperature plus humidity is greater than 180, extreme caution is needed in working horses. In other words, you can ride a horse in about any temperature, but when temperature and humidity added together are greater than 180, you need to be extremely careful not to overexert the horse.