Cooling Pigs: The Humidity Ceiling on Evaporative Cooling of Wetted Skin

Pigs produce almost no sweat, so in hot weather a pig's own skin evaporation does very little, and farms instead wet the skin for it, with wallows, drip lines, or sprinklers. This paper models how much heat evaporation from wetted skin can remove, as a function of air temperature, relative humidity, and air speed, and compares it with the metabolic heat production of a finishing pig of 100.0 kg, computed as 196.1 W. The central result is a ceiling. Even with the whole skin surface kept wet at an air speed of 1.0 m/s, thermal balance fails once the wet-bulb temperature rises above a narrow band, 33.15 to 33.30 °C, across the humidity range studied. In dry-bulb terms the full-wet ceiling falls from 43.25 °C at 50% relative humidity to 36.49 °C at 80% and 34.04 °C at 95%. Real methods fall well short of the full-wet ideal: at 1.0 m/s the practical ceiling is 39.21 °C for a freshly wetted sprinkler pig and 38.80 °C for a wallow pig at 60% relative humidity, and only 34.03 °C for drip cooling, because a drip line wets too little of the animal. A sprinkler cycle is sized for a stated pen: 2.0 minutes on in every 20.0 minutes, which holds balance at the design point of 34 °C, 60% relative humidity, and 0.8 m/s, and costs 25.92 litres of water per pig per day. Above the ceiling no evaporative method works, and the paper states that region at full size: at 1.0 m/s, even a fully wetted pig fails at 14 of the 64 temperature and humidity combinations studied, including a deficit of 268.3 W at 38 °C and 85% relative humidity. In that region only shade, lower stocking density, and shifting feeding to the cooler night hours remain, and none of those removes heat from a pig that is already in surplus. AI disclosure: The calculations, drafting, and document preparation for this paper were carried out with the assistance of Dave, an artificial intelligence system. All results are simulated, not measured. No animals were used in this work.