Relative humidty calculation, small chambers and commercial greenhouses.

Here tropical plant growers will find an interesting calculator of humidity.
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html#c4

At 20 Celsius, 1 cubic meter of air would need 17.25 gm of water vapor to saturate ( "saturate" translates : a 100% humidity, the maximum water vapour that the air can hold )

If the hygrometer of the greenhouse indicates 80% of relative humidity at this temperature the actual quantity of water vapor is about 14 gm per cubic meter.


Tropical greenhouse built in 1880, Parc de la tete d' or Lyon

If the temperature rises to 35 Celsius 1 cubic metre of air needs 39.75 gm of water vapour to saturate and 80 % of relative humidity translates 32 gm per cubic meter. 

When the fogging system of the greenhouse is set to maintain 80% of relative humidity , in order to do its job at 35 Celsius it must multiply by 2.28 ( that 's a lot ! ) the quantity of water vapour at 20 Celsius :  14 x 2.28 = approx. 32 gm of water vapour.

When tropical plants acclimatize in a home in New York, Paris, Moscow or Cape Town, restart or root under a plastic chamber before being progressively released to the temperate climates, foggers may not be indispensable : the volume of the air in the plastic chamber is usually small as the plastic is close to the plants, there is a lot of plant material containing water for this volume of air and the water is put in a large plate ( the larger the plate the larger the evaporation ). Also the rise in heat is better controlled in a house than it is outside.

The simple plastic bag - chamber apparatus works well in homes but for greenhouses set outside only foggers coupled with humidity sensors can maintain smoothly the high level of humidity required to acclimatize tropical plants a few weeks, because :  1) the volume of air in relation to the quantity of plants is much higher and 2 ) from June to August heat can rise and fall high and fast ( December to February in southern hemisphere ).

The calculator of Humidity above can help evaluate in terms of gm of water vapor per cubic metre of air the gaps in Humidity percentages : one can see what a 20% humidity deficit  means at 20 C and at 35 C . 

Hygrometer and thermometer give an essential information that cannot be replaced with the subjective perception of heat : the heat index

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