The amount of moisture in the air is often expressed as relative humidity or RH. Relative humidity is indeed a relative value, as it expresses the actual amount of moisture in the air as a percentage of the maximum amount of moisture that air of that temperature can hold. The way it is traditionally determined (before the use of electronic relative humidity sensors) is by comparing the temperature of the air (the so-called dry bulb temperature) with the temperature reading of a thermometer that is kept wet. A wet thermometer will evaporate water which has a cooling effect and will therfore have a lower reading (the so-called wet bulb temperature). Less water in the air means that more water can evaporate from the wet bulb thermometer, resulting in a lower temperature reading. The difference between wet bulb and dry bulb temperature is therefore a measurment for the relative humidity, and with a table or Mollier diagram the actual relative humdity can be determined.
But this can sometimes lead to confusion, as relative humidity is not always expressed in the same way. Especially in incubators the relative humdity is often not expressed as a percentage, but as wet bulb temperature. This works well as incubators are always running on a more or less constant dry bulb temperature, so a wet bulb temperature reading gives a more or less constant indication for the actual relative humidity level. Especially in the time where the recordings were done manually, it was easier to just register the wet bulb temperature than convert it every time in a relative humidity value. On top of that, traditionally hatcheries work with Fahrenheit as temperature scale, where nowadays machines often use the SI system and with it use degrees Celsius, or have the option to use Fahrenheit or Celsius.
Why does this sometimes lead to confusion? If we have an incubator running at 37.5oC dry bulb temperature (which is 99.5o dry bulb temperature in Fahrenheit) a relative humidity level of 50% corresponds with a wet bulb temperature of 29.4oC (which is 84.9o wet bulb temperature in Fahrenheit). So if we ask a hatchery manager what the relative humidity in a machine is, some will say 50 (meaning a percentage), where others will say 29.4 (wet bulb in Celcius) and a third group will say 84.9 (wet bulb in Fahrenheit). All of them mean the same, 50% relative humidity, but the way of expressing that value sometimes leads to confusion.