Existing knowledge on the relative significance of air temperature, humidity, and velocity in a hot environment for housed pigs and poultry is reviewed and synthesized in an effective temperature (ET) equation. The suggested unit has an easily perceivable scale where ET is equal to air temperature if the relative humidity is 50% and the air velocity is 0.2 ms1. The included method to determine the relative significance of air temperature and humidity is similar to the way it is done in the Temperature Humidity Index.
Several authors have suggested different Thermal Humidity Indices for different categories of animals, but this chapter found no evidence that the relative importance of temperature and humidity is different for pigs than for poultry or for large than small ones. The suggested ET equation includes a separate velocity term, which assumes that the chill effect is proportional to the air velocity or to the square root of the air velocity and that the chill effect declines linearly with increased air temperature until it becomes insignificant as the air temperature approaches the animal body temperature.