To figure the effective temperature, add the temperature in degrees Fahrenheit and the percent relative humidity. Then subtract the wind speed in miles per hour to get effective temperature. Below an effective temperature of 130 degrees the thermoregulatory mechanism works, at 150 degrees it is compromised and at 180 degrees it does not work. Horses cool mainly by evaporation. In higher humidity areas, when they just can't sweat anymore, you have a problem.
Temperatures are normally lowest from sundown through early morning.
It's also better to haul horses when it's coolest, if at all possible. Find shaded areas when it's possible. This is a good test of a horse's body water status. Pinch up a section of skin on the horse's neck or shoulder. If the pinched section doesn't recoil in less than 10 seconds, the horse is in some degree of dehydration and needs water.