Contributed by: Rouhea Thanks to: Nigel_Samhain Submitted: 03-04-2003
The difference between fog and mist is the particle size generated. 
Fog
Any water droplet smaller than 50 microns is considered fog. There is ‘wet’ fog and ‘dry’ fog. Wet fog has a particle size in the range of 10-50 microns. Dry fog is produced by ultrasonic systems and has particles in the range of 2-10 microns.
Dry fogging systems use very little water, but they do require a high quality supply (ie. R.O. or distilled), as they are prone to clogging.
The disadvantage of dry fogging systems is that they are no good for cooling, since the quantity of water available for evaporation is small. Wet fogging systems can be run to excess during summer and the surplus fog can be vented.
Fogging systems require additional watering of the cuttings, unlike conventional misting systems.
Misting systems are the cheapest to set up and run, but fogging systems may give better results with some plant species, for example some Verticordias, Brachycome, lavenders, and many species with hairy or finely divided leaves.
Misting
Misting maintains a fully saturated atmosphere around the cuttings, whereas fogging aims to keep the leaves cool. Reducing leaf temperature reduces the water vapor pressure within the leaf and less water escapes. However, a fully saturated atmosphere will not entirely prevent transpiration water loss from cuttings. If the leaf temperature exceeds the air temperature, then the internal vapor pressure will be greater than the surrounding air, and there will be evaporation from the leaf. To avoid this, shading is necessary to prevent high leaf temperatures.
(Nigel_Samhain) The foggers usually use a diffuser, although they call it a ceramic disc coated with what appears to be brass. These foggers produce vapors in the 2-15 micron range. When applied to the root system, it is comparable to growing your plants within a cloud.
Foggers experience calcification.
Usually it can be cleaned off with a dilute solution of White vinegar. When the EC of the solution is high, the calcification occurs at a much greater rate.
It is recommended that the fog be dispersed in increments, rather than remain constant, to cut back on diffuser wear. This also tends to make the roots strive for faster growth.