Up HortResearch Publication - Humidity in Greenhouses
Part 1 - Terminology in Relation to Humidity
Elly Nederhoff - HortResearch, Palmerston North

Table of Contents

Humidity in Greenhouses

  1. Terminology in Relation to Humidity
  2. How to Use RH and Other Humidity Measures
  3. High Humidity Control
  4. High Humidity and Plant Diseases
  5. The Best Way to Measure Humidity
  6. Humidity and Plants
  7. Coping With Low Air Humidity
In greenhouse climate control, nothing is so complicated as control of humidity. Greenhouse air humidity is determined by various factors and controlling those is virtually impossible. A greenhouse operator can only adjust air humidity and try to avoid extremes. Moreover, humidity has various ‘faces’: relative humidity, vapour pressure, VPD, dew point, which all require different control settings. Even when everything is understood it is not easy to decide which level of humidity should be targeted in the control, as that depends on the weather and plant conditions.

In this series of articles we will discuss these aspects, as well as the relation of humidity with transpiration, plant growth and disease development. The last articles will deal with venting, heating, fogging, misting and roof sprinkling as means to adjust humidity. This first article explains the terminology in relation to humidity. The next article will show how one unit can be converted to another unit. The alphabetic list of terms below is meant to be used as a reference.

Absolute (air) humidity: humidity in gram water vapour per m3 air. Conversion to relative humidity (RH) can only be made when the temperature is known. For instance 15 g/m3 at 17.5 oC equals 100 % RH, while at 25 oC it equals 65 % RH, and at 30 oC it equals 50 % RH (see table in next article of this series).

Boundary layer: thin layer of air and water vapour close to the leaf surface. The boundary layer resistance is a barrier for transpiration. It is reduced by air movement (wind).

Condensation: transition of water vapour to liquid water. Water vapour condensates on glass or leaves that are cold (below the dew point).

Conductance (opposite of resistance): ’ease’ with which water flows through a barrier. When leaf pores are wide open, the resistance for water vapour to go out is low and the conductance is high.

Conductivity, conductivity factor (CF): concentration of mineral salts in water (term used in hydroponics).

Dew point: temperature below which water vapour will condensate (indication for humidity of air). For instance air with 10 g/m3 has a dew point of 11 oC, air with 20 g/m3 has a dew point of 22.5 oC.

Evaporation: transfer of water vapour from a wet surface or from the soil to the air.

Evapotranspiration: transfer of water vapour from soil and plants to the air.

Guttation: water leaking from leaves, caused by high root pressure, mostly at night.

Osmosis, osmotic potential: pressure in water (or in plant sap) caused by salts. The more salts in the plant sap, the higher the water pressure in that plant.

Resistance: strength of hindrance for water flowing through a barrier. For instance when leaf pores are closed they are a strong resistance for transpiration.

Transpiration: transfer of water vapour from a plant to the air. Transpiration has two effects: cooling the leaves during sun shine and internal transport of nutrients.

Relative (air) humidity (RH): humidity in % of maximum humidity at a given temperature. For instance at 20 oC air can hold 17.3 g of water vapour per m3 at maximum. If it holds only 13 g/m3 at 20 oC, it has a relative humidity of 75 %.

Relative water content (RWC) of plants: water content in % of maximum water content.

Root pressure: strength with which the roots press water upwards to the shoots. This is the mechanism that causes water uptake at night, while at day transpiration is the driving force.

Stoma / stomata: pore / pores in the leaf surface.

Stomatal conductance: ease with which water vapour can pass the leaf pores.

Stomatal resistance: strength of hindrance to water vapour by the leaf pores.

Saturation: saturated means filled to the maximum (with water).

Saturation deficit: a measure of air humidity: difference between actual and maximum vapour content (e.g. in gram water per m3 air).

Specific humidity: water content of air in kg water vapour per kg dry air.

Turgor: pressure of plant cell walls to hold water.

Vapour deficit: can be vapour pressure deficit (VPD) or saturation deficit.

Vapour pressure: pressure caused by a gas or a vapour. It normally ranges from or 1 to 5 kPa or from 10 to 50 mbar.

Vapour pressure deficit (VPD): difference between actual and maximum vapour pressure, normally in the range 0.1 kPa (very humid) to 3 kPa (very dry air), or 10 to 30 mbar.

Water potential of a plant: water content expressed in units of energy or pressure.

Water uptake: the amount of water taken up by the plants. It equals the transpiration plus the water used for plant growth.

Water stress (drought stress): shortage of water in the plant, leading to wilting etc.

Water status: condition of the plant in terms of water content.

Wet bulb temperature: temperature indicated by a thermometer covered with a wet wicking. When the humidity is 100% or when the wicking is dry, the wet bulb temperature equals the dry bulb temperature. Otherwise the wet bulb temperature is a few degrees below the dry bulb temperature.

First published in Commercial Grower Vol 52. No.1 February 1997.
Copyright © 1998 The Horticulture and Food Research Institute of New Zealand Ltd. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of The Horticulture and Food Research Institute of New Zealand Ltd is prohibited.