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The amount of water present in the gas phase of a given volume of air increases exponentially with temperature. Humidity is usually lower than the maximum amount of water vapor until dew appears. Usually humidity can be characterized as:

- absolute humidity: saturated water vapor content of the air at a certain temperature,

- relative humidity: ratio between the absolute water vapor content in the air and the saturation current (absolute humidity)

- specific humidity: ratio of the water vapor content of the mixture to the total air content on a mass basis.

In plant sciences the most common way to characterize air humidity is by the measurement of relative humidity (RH) with:

- hygrometers: there is a range of different hygrometers which measure the changes in temperature, air pressure, mass, mechanical or electrical change in a substance as moisture is absorbed,

- infrared sensors: humidity can be measured by infra-red gas analyzers which measure the absorbed infra-red radiation by water vapor in specific water absorption bands.

For the functioning of plants the relevant measure it is the evaporative demand and not the relative humidity. The evaporative demand depends on the difference in absolute and actual vapor pressure of the air (VPDair) at a given temperature. Specifically for plant the vapor pressure deficit between the actual vapor pressure between the leaf and the air (VPDair) is the force that drives transpiration. Here we assume that the relative humidity inside the leaf is 100% at the leaf temperature.

The relative humidity is the most frequently measured parameter in many plant experiments. Therefore it is essential that relative humidity measurement is combined with the accurate air temperature measurement to allow the calculation of VPD (Monteith and Unsworth 2008).

Further reading:

Monteith, J. L. and M. Unsworth (2008). Priinciple of Environmental Physics. Amsterdam, Academic Press