The incubation temperature of naturally (nest) incubated eggs is controlled by the hen. The recommended temperature within an artificial incubator depends upon the type of incubator being used. If the incubator used has a fan for air circulation, the temperature must be adjusted to 99-100o F.
An incubator without an air circulation system requires a higher temperature. The temperature in this "still-air" incubator is measured using a thermometer with the bulb positioned at the same level as the top of the incubating eggs. The recommended temperature in this type incubator is 102o F.
The reason for different temperatures is that circulating air warms all points around the egg shell while still air temperatures are warmer at the top of the egg than at the bottom. Therefore, increasing the temperature at the top of the egg will compensate for the egg's cooler parts. The same average egg temperature of 100o F can be maintained (for the entire egg) if the higher temperature of 102o F exists at the egg's uppermost point.
Do not allow temperatures to exceed these recommendations, even for only a short period of time. Although it is not recommended, slightly lower temperatures will not kill the chick embryos, but can increase incubation times and produce weakened chicks. Temperatures only a degree or two above the recommended temperatures can kill chicks within 15-30 minutes, depending on how high the temperature is and the stage of development of the chick embryo.