The air humidity (or relative humidity) expresses the saturation of the air with water vapour as a percentage.
In fact, the air can only hold a limited amount of water vapour. If humidity is at 100%, no more water can be stored in the air in gaseous form, meaning that some of it becomes liquid, forming dew.
It is useful to know the humidity level for a number of reasons:
The right humidity is a prerequisite for the efficacy of phytosanitary treatments. A humidity level above 60% limits evaporation and maximises absorption of the active ingredient by the plants.
In combination with the wet bulb temperature, humidity is a useful piece of data in preventing freezing, as it helps farmers to anticipate drops in temperature (see article What is the wet bulb temperature?).
The humidity level during harvest is critical to maximising yields (avoiding breakage in protein crops, for example) and especially for improving the storage life of products.