Moisture content of the air is normally specified by its relative humidity (RH), ie. the moisture content of the air at a certain temperature expressed as a percentage of the amount that would be necessary to fully saturate the air at that same temperature.
The relative humidity of the air at a given moisture content, therefore, varies greatly with the changing temperature. Thus, relative humidity is far from ideal as a precise measure of water vapour in the air – however, because of common usage, we are stuck with it.
The humidity of greenhouse air can be reduced by opening the ventilators, and increased with the use of foggers. These operate at extremely high pressures of 5,500 to 7,000 kilopascals (kPa) or 800 to 1,000 pounds per square inch, and produce droplets of less than 10 microns in diameter (one micron is one millionth of a metre, or 10-6m). 

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