The temperature inside the egg (the so-called embryo temperature) is determined by the balance between:

- Heat production of the embryo
and
- Heat loss of the egg to the environment (heat transfer)

The heat production is determined by
- the genetic line or breed used,
- the size of the egg (grams of metabolic mass)
- moment of incubation (grams of metabolic mass)
- actual temperature (determines metabolic rate)
- oxygen supply (determines metabolic rate)

The heat loss is determined by:
- the difference in temperature between air and egg (large differences increase the heat transfer)
- moisture loss of the egg (evaporation of water creates a constant cooling effect, in practice about 0.3oF)
- the size of the egg (big eggs have less shell surface per gram of egg)
- the air velocity (at a given temperature difference, high air velocity will create higher heat transfer)
-the heat capacity of air (moist air has a larger heat capacity than dry air [minor influence])
- the evaporation of water (water sprayed in the machine will take up heat from the eggs to be evaporated)