What does it mean when we say “Relative Humidity”?

Incubator humidity is a 'moving target'. By this we mean that it depends on two factors: air temperature and air pressure.

If you imagine that the air around us is a giant sponge, you could imagine that it has a certain, critical capacity at which it is ‘full’ of evaporated water and cannot hold any more. Let’s call this state '100% humidity'. If it begins to rain, and the air loses half its moisture, it could be said to reduce to 50% humidity.

In reality, however, air is different to a sponge because hot air can hold more water than cold air. That’s why hot countries tend to feel more humid than cold countries - there is more water held in the air. So 100% humidity at 37.5°C means there is a lot more water in the air than 100% humidity at 10°C. Ever wondered why carpets get damp in a cold basement? There’s your answer. The cold air in the basement cannot hold as much water as the warm air upstairs, so the basement carpets get damp instead.

A similar principle applies with air pressure too. But unless you’re planning to place your incubator on Mount Everest, it’s not likely to concern us too much.

So, in summary, ‘relative’ humidity is exactly that... it's the amount of water that's in the air, as a percentage of the maximum amount of water that the air could hold at its current temperature. This is usually expressed as RH%.