Fog systems are used for two distinct applications in horticulture: humidification and cooling. The action of the fog is the same in both applications, but controlled by different methods.
All air in a greenhouse contains a certain amount of moisture that is available to plant material at all times. Environmental conditions change the available water by heating, ventilation and solar effect.
Plants (like humans) normally thrive within a "comfort zone" of humidity and temperature. Stomata on the leaves open and close depending on the ambient conditions, and will become more active when in their particular "comfort zone".
Generally, plant material below 40% humidity tends to transmit excessive water (the pressure in the air is less than the pressure in the leaf) and it is unable to receive enough from the roots. Therefore, the plant’s natural defenses come into play, slowing (or halting) the growth process in order to accommodate for the excess loss.
At high humidities (generally over 90%), the pressure differential is so slim that little or no transpiration takes place. As there is no transpiration, there is no movement of nutrients through the plant; the fruit is therefore starved.
By providing environmental conditions around the plant that maintain its comfort zone, plant growth and quality is maximized to produce superior crops and higher yields. However, a plant that lives and thrives in a constant environmental "cocoon" is far more susceptible to atmospheric changes around it. When removed from the "cocoon", or when the system is inoperative, plant heath suffers at an alarming rate