Humidity definition: 

Humidity is the level of moisture that intervenes in the climate and implies the amount of water vapour that is contained in the atmosphere.

Humidity is fundamental to life on Earth and to the realisation of many natural processes. As such, it is present in virtually everything around us including Firewood thats not been well seasoned. In all living bodies, animals or vegetables, in the clouds, in the earth, in the food we consume, in the cold lawn of the park, in the drops that adhere to the walls of a cold glass, in the smoke of the coffee, or in a kiss.

Interesting article about how wood is dried.