During incubation, the growing embryo produces heat, and that heat needs to be removed from the egg to keep the egg shell temperature at the required level. This so-called heat loss or heat transfer from the egg to the air is influenced by the difference in temperature between egg shell and air, by the air velocity of the air over the egg and by evaporation, the so-called latent heat loss.

It is often assumed that also relative humidity plays a role in the heat transfer. In other words, a high relative humdity would help the eggs with their heat loss. This is not the case, relative humidity plays no direct role in the efficiency of the heat transfer from the egg to the air. However, there is an indirect relation. For instance the work of Van Brecht et al (2005: Poultry Science 84: 353–361) shows very nicely that with a lower relative humidity, there is a slightly lower difference in temperature between egg shell and air, suggesting that there is a relation. The difference is very small, about 0.05oC lower difference between egg shell and air at a relative humidity of 20% compared with a relative humidity of 90%, but there seems to be a difference.

But as the authors correctly explain, this is not caused by a difference in heat transfer, but by a difference in latent heat loss. At a lower relative humidity, the eggs will lose slightly more moisture, and the evaporation of that water has a small cooling effect. Therefore at a lower relative humidity, the difference between the egg shell temperature and the air temperature will be slightly lower as the cooling effect of the evaporation will be slightly higher.

But where does that widely spread assumption that a high relative humidity is beneficial for the eggs losing heat originates from? In the past, we used machines with much less cooling capacity. To remove the heat that the embryos produced, water needed to be sprayed so that evaporation of water (latent heat loss) would help to cool the eggs. But with the evaporation of the water, the relative humdity went up. So when the spraying of water was not sufficient, the relative humidity did not go up and the cooling by evaporation was not sufficient, resulting in overheated eggs. And therefor the hatchery managers had the experience that if the relative humidity was too low, the results were poor as the eggs were overheated. Nowadays the modern machines have much more cooling capacity and do not need the cooling by evaporation, but the assumption that relative humidity affects heat loss is still there.