HortResearch Publication - Humidity in Greenhouses
A good indication of air humidity would be the absolute humidity in g/m3, but horticulturist usually work with relative humidity (RH) in percents. RH of 100 % means extremely humid conditions and for instance 50 % indicates very dry air conditions. RH is a handy measure and growers are used to it, but RH is not always appropriate. The disadvantage is that RH does not say anything about the amount of water in the air, unless the temperature is given.
Some examples may illustrate this, and it can be seen form the table. If we would use a steamer to bring water vapour into the air, we would see that moisture is absorbed until the maximum water content (or ‘saturation’) is reached. If the steaming continues we would see condensation appear. Cold air can hold much less water than warm air, so cold air is easily saturated. Air of 10 oC can hold 9.4 gram of water vapour per m3 at maximum, air of 20 oC can hold 17.3 g/m3, and air of 30 oC can hold 30.4 g/m3 at maximum. Relative humidity is the percentage of maximum water content at a given temperature. If air of 20 oC holds 13 g/m3, the air has a relative humidity of 75 % (as 13 is 75 % of the maximum amount which was 17.5). In contrast air of 30 oC with the same absolute humidity of 13 g/m3 has a relative humidity of only 43 %.
If air temperature goes down, while the absolute humidity (in g/m3) remains equal, the relative humidity goes up. This continues until saturation is reached, and condensation occurs on glass and leaves.
Spores of Botrytis and other fungal diseases are waiting for condensation on the leaves (‘wet leaves’) to start growing, so growers would like to avoid wet leaves. Unfortunately there are no handy ways to measure condensation, and instead RH can be used to indicate the risk of condensation.
In theory, RH below 100 % means that there is no condensation. When the sensors via the computer tell it is 20 oC and 95 % RH, it means it is 20 oC at the sensor, and absolute air humidity is 16.5 g/m3 (see table). The absolute humidity is the same on all places in the greenhouse, but the temperature can be a few degrees lower on many places in the greenhouse and higher on other places. Therefore the relative humidity is different on different places. Since cooler air can hold a lower amount of water, the relative humidity is higher and condensation may occur on the cooler spots. So in reality, if RH of around 95 % is measured, there will be condensation on many cooler spots, most likely also on leaves. Thus RH can be used as an indication of the risk of condensation, and thus RH is useful in relation to control of fungal infection. When very high RH is measured, let’s say above 90 or 95 %, the control should aim at reducing the humidity.
VPD is the proper unit in relation to transpiration control, as it indicates the ‘drying effect’ of the air. The meaning of VPD can be explained as follows: vapour pressure (VP) is the pressure caused by a gas or a vapour. All gases in the air together make up the air pressure. Vapour pressure of water normally ranges from 10 to 50 mbar (millibar) or in other units, from 1 to 5 kPa (kilo-Pascal). At each temperature there is a maximum vapour pressure of water, and if more water was added water would condense (as described for relative humidity).
Vapour pressure deficit (VPD) is the difference between the actual and the maximum vapour pressure. For water vapour, VPD is normally in the range 0.1 kPa (very humid) to 3 kPa (very dry air), or 1 to 30 mbar. Note that a low VPD means a high air humidity, and vice-versa. The higher the VPD the stronger the drying effect, so the stronger the driving force on transpiration.
Relative humidity (RH), absolute humidity (abs.) and Vapour Pressure Deficit (VPD) are important units in horticulture. The table below shows the conversion from one unit into other units, at various prevailing temperatures. For instance at 20 oC, 80% RH equals 13.87 g/m3 absolute humidity, and 0.47 kPa VPD.
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at 10 oC |
at 15 oC |
at 20 oC |
at 25 oC |
at 30 oC | ||||||
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RH % |
abs. g/m3 |
VPD kPa |
abs. g/m3 |
VPD kPa |
abs. g/m3 |
VPD kPa |
abs. g/m3 |
VPD kPa |
abs. g/m3 |
VPD kPa |
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100 |
9.42 |
0 |
12.86 |
0 |
17.33 |
0 |
23.09 |
0 |
30.43 |
0 |
|
95 |
8.94 |
0.06 |
12.21 |
0.09 |
16.47 |
0.12 |
21.94 |
0.16 |
28.91 |
0.21 |
|
90 |
8.47 |
0.12 |
11.57 |
0.17 |
15.60 |
0.23 |
20.79 |
0.32 |
27.39 |
0.42 |
|
85 |
8.00 |
0.18 |
10.93 |
0.26 |
14.73 |
0.35 |
19.63 |
0.48 |
25.87 |
0.64 |
|
80 |
7.53 |
0.25 |
10.28 |
0.34 |
13.87 |
0.47 |
18.84 |
0.63 |
24.34 |
0.85 |
|
75 |
7.06 |
0.31 |
9.64 |
0.43 |
13.00 |
0.59 |
17.32 |
0.79 |
22.82 |
1.06 |
|
70 |
6.59 |
0.37 |
9.00 |
0.51 |
12.13 |
0.70 |
16.17 |
0.95 |
21.30 |
1.27 |
|
60 |
5.65 |
0.49 |
7.71 |
0.68 |
10.40 |
0.94 |
13.86 |
1.27 |
18.26 |
1.70 |
|
50 |
4.71 |
0.61 |
6.43 |
0.85 |
8.67 |
1.17 |
11.55 |
1.59 |
15.22 |
2.12 |
|
40 |
3.77 |
0.74 |
5.14 |
1.02 |
6.93 |
1.41 |
9.24 |
1.90 |
12.17 |
2.55 |
|
30 |
2.82 |
0.86 |
3.86 |
1.20 |
5.20 |
1.64 |
6.93 |
2.22 |
9.13 |
2.97 |
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RH = relative humidity (%). abs. = absolute humidity (g/m3). VPD = Vapour Pressure Deficit (kPa). | ||||||||||