Thursday, June 18, 2015

A relative humidity deficit of 20% impacts plants at 35 Celsius much more than at 20 Celsius

Hotter air holds more humidity.

Plants that acclimatize need around them a space of which 80 % is filled with water vapour ( humidity ). If the hygrometer indicated only 60 % there would be then a shortage of 20% ( 80% - 60% = 20 % ). How many grams of water vapour is this shortage of 20 %, hence how fast and by which means it can be filled depends on the temperature. Plant growers should always consider the % of humidity with the room temperature, not the % of humidity only.
At 20 C  about 60% of humidity translates 11 gm of water vapor per cubic meter of air.
In order to target our 80 % at this temperature ( a necessary % during the rooting or the acclimatization phase of tropical plants stressed by international travel ) the water vapor must increase up to 14 gm per cubic meter, which is the amount of 80 % water vapour when temperature is 20 C.

Photo credit: University of Missouri Cooperative Media Group
At 20 C this humidity gap of 20 % ( 80% - 60 % )is 3 gm ( 14 gm - 11 gm ) per cubic meter, what if the air is hotter ?

At 35 C the situation is very different : 60% of humidity is 24 gm of water vapour per cubic metre of air ( more than twice the value at 20 C ! ) and  80% of humidity with these 35 C means 32 gm per cubic metre of air !  Hence the 20 % humidity gap is much wider : 8 gm at 35 C instead of 3 gm at 20 C,  and this gap of 8/3 = 2.66 times will be 3 times at temperatures just above 35 C !

Hygrometer and thermometer are essential during the rooting or acclimatization period until the plants are released to a dryer environment. The subjective perception of heat , the " heat index " is not reliable for an " intuitive measure " of the heat and humidity levels in the greenhouse. 

Why are Geneva and Stockholm better places than Madrid or Las Vegas considering the effect of the relative humidity - temperature relationship on tropical plants ?

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Relative humidty calculation, small chambers and commercial greenhouses.

Here tropical plant growers will find an interesting calculator of humidity.

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

Hoya and other plant collections are for sale on Plant care is available in various posts of this blog. Contact :

Find all Aleyagarden posts on