Inside the greenhouse, the radiation, temperature and composition of the atmosphere are modified, and this results in a different micro climate from the one outside. The modifications depend essentially on the nature and properties of the cladding material, the air renewal conditions, the shape, dimensions and orientation of the greenhouse, but also on the plant canopy and the possibilities for evapotranspiration (Berninger, 1989). This microclimate is not uniform and varies from the centre to the borders of the greenhouse, from the ground to the roof and from the limits of the canopy to its interior.
During the daytime, the majority of the solar radiation passes through the cover of a greenhouse and is absorbed by the plants and soil. The plants and the soil are heated and re-emit energy, mostly with wavelengths of 10 mm but ranging from 2.5 to 25 mm (far IR range), according to Wien’s law (because the temperature is about 300 K). This energy re-emitted by the plants and the soil is intercepted by the covering material (as the materials used are usually opaque to IR radiation), which is reheated and re-emits energy in turn outwards and inwards in similar proportions. The greenhouse air is then heated, as it is confined and it is not renewed with outside fresh air.
These phenomena generate a temperature increase that is very evident during the daytime, in relation to the outside. This effect will vary depending on the specific conditions of transmission and absorption of the cover to radiation and depending on the ventilation and airtightness of the greenhouse. At night, the temperature gradient with the outside is the result of a complex balance influenced, mostly, by the sky temperature and the temperature under the cover and the air exchanges between the greenhouse and the outside.
Brief description of the behaviour of the main microclimate parameters of the greenhouse, depending on the weather conditions (adapted from Berninger, 1989)
Greenhouse
|
Types of Sky
|
Outside
|
Day
|
Night
|
|
Clear Sky
|
Large difference between day
and night temperatures. High
solar radiation (especially
direct radiation). Low RH,
especially if it is windy. At night, cold air, ‘cold’ sky
|
High solar radiation (direct
and diffuse). High
ventilation to limit temperature rise and avoid CO 2 depletion. High thermal storage. High evaporation
|
Possible heating to
maintain temperature.
High RH (without
heating)
|
|
Cloudy Sky
|
Stable temperatures. Weak solar radiation, diffuse. High RH. ‘Warm’ sky
|
Weak solar radiation, diffuse.
Ventilation to limit the confinement (high RH, lack of CO 2 ). Scarce thermal storage. Low evaporation
|
Limited heating or may
be unnecessary, except where there is a high plant disease
risk associated with high RH
|
Air Temperature
The air temperature inside the greenhouse is the result of the energy balance of the protection. The greenhouse effect generally has two consequences
- At night, due to the limitation of IR radiation losses, the minimum temperatures are similar or slightly higher (1–3°C higher, depending on the covering material) than the outside. Nevertheless, on clear nights without wind, ‘thermal inversions’ may occur.
- During the daytime, due to the ‘heat trap’ effect and the reduction in the convective exchanges (as the air is confined), the air temperature is higher indoors than outdoors, being possibly excessive when the radiation is high and the greenhouse is not
efficiently ventilated. The measurement of the air temperature must be performed in a representative location of the greenhouse, protected from direct sunlight and below a flow of air.
- The thermal inversions in unheated greenhouses may occur on calm nights, with a clear sky, when the radiation losses towards the atmosphere are larger in the interval known as ‘atmospheric window’ (Rose, 1979), if the greenhouse covering material is permeable to radiation in that interval (Day and Bailey, 1999).
Plant Temperature
A thermometer located (without protection) inside a greenhouse during the night may provide a reading different to the actual air temperature. By approximation, we call it the ‘radiative temperature’ or ‘actinothermal index’. These differences are larger with a normal PE cover than with a glass cover. This ‘radiative temperature’ better represents the plant temperature than the air temperature, during the night.
During the daytime, there are large differences in plant temperature, with respect to the air and also between parts of the plant, depending on the radiation intercepted, the water evaporation and the air movement, among other factors. The temperature of the flowers and the fruits depends greatly on their colour, which influences the absorption of radiation.
Plant temperature has traditionally been assessed on the basis of the air temperature, corrected with the temperature of the greenhouse walls and the ground surface, and on the rate of evapotranspiration (Berninger, 1989). However, now technology is available for the direct measurement of plant temperature and the theory/philosophy of the ‘speaking plant’ in greenhouse crop management is widely accepted (Takakura, 1989; Challa and Bakker, 1995).
Soil Temperature
Close to the surface, the soil temperature follows a very similar pattern of development to the air temperature. The extreme values are buffered with the depth of the soil. The type of irrigation system used influences the soil temperature; on the one hand by the water temperature itself and on the other hand by its effect on water evaporation from the soil and plants, and, therefore, the energy balance (Berninger, 1989).
The soil, as well as the substrate in soilless crops, or the pots in ornamentals, are the heat sink of the greenhouse, that is they are centres of thermal inertia. The crop has little importance with regard to thermal inertia compared with the soil. A 10 cm soil layer has five to eight times more thermal capacity than the mass of a normal crop (Berninger, 1989). During the night the soil returns part of the energy it has stored during the day back to the greenhouse. Use of white mulch, used to reflect radiation, limits the daytime heating of the soil and, thus, reduces the thermal inertia of the soil.