Relative humidity is the the percentage of water dissolved in the air over how much could possibly be dissolved in the air at that temperature. In other words, the air is saturated with water when the relative humidity is 100% and the air is hold half as much water as it could when the relative humidity is 50%. In mushroom cultivation, relative humidity needs to stay high in order to ensure that surface mycelium and substrates do not become dehydrated. Some species use evaporation as a pinning trigger while others will pin the best under as high a humidity as possible.