Humidity levels are determined by the quantity of water vapor in the atmosphere and the moisture content of the soil. The point at which the air becomes saturated varies according to sunlight, temperature and wind. Atmospheric humidity is usually referred to as the relative humidity and represents the amount of water vapor present in the air expressed as a percentage of the saturation point.
The effects of humidity
Atmospheric humidity varies from area to area, for example in areas with heavy rainfalls the atmospheric humidity is high and in areas with very low levels of precipitations the humidity levels are low.
There are plants that thrive in conditions of exceptional humidity, like ferns and mosses. In a closed space, like a greenhouse, we can control the humidity levels. You can increase the humidity in a greenhouse if necessary, by damping down around plants. This is also of great value when propagating plants to reduce the loss of water by transpiration.
High relative humidity may also have unfavorable effects. Fungal diseases, like grey mould, will flourish in the moist conditions created by high humidity. To prevent this from happening you must ventilate the greenhouse properly and avoid crowding the plants tie close to each other.
