INFLUENCE OF THE EXTERNAL CLIMATE ON THE MICROCLIMATE OF THE GREENHOUSE AT LEVEL OF THE EXCHANGE SURFACE
The difference between the ambient temperature and that on the surface of the canopy:
- increases with the intensity of radiative exchange,
- diminishes with the growth in turbulence intensity in the neighbourhood of the surface.
In a greenhouse, the convective exchanges are always reduced, as the turbulence intensity is generally weak compared to the exterior, and the radiative exchange takes a dominant place. Intensity of this radiative exchange is a function of:
- the characteristics of the external climate, sunshine during the day, balance of the longwave exchange being more or less negative during the night due to the sky conditions,
- heating of the greenhouse.
In a glass or a PVC greenhouse for example, whose cover is opaque to the infra-red radiation, the temperature of the cover is always closer to the temperature of the exterior than to the ambient air inside the greenhouse. This "cold cover" creates a particular situation for the radiative exchange between the surface of the vegetation canopy and the cover (laws of black body radiation).
In a polyethylene greenhouse, the above phenomenon is more pronounced because such a cover is practically transparent (about 80%) to the infra-red radiation of the longwavelength. Again, the radiative exchange between the soil surface and the free atmosphere has to be considered.
The changes observed during the night between the temperature difference of air and the surface of the vegetation canopy are proportional to the heating of the greenhouse as expressed by the difference between the interior and exterior temperatures.
The first results show that the growing techniques (horizontal or vertical growing which favour the convective exchange) have to bring about important changes to the actual