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A Simple Frost-damage Box Utilizing Natural Radiative Cooling

1979; Society of Agricultural Meteorology of Japan; Volume: 35; Issue: 3 Linguagem: Inglês

10.2480/agrmet.35.165

1881-0136

Juro Hanyu, Seiichi SANO, Shoichiro YAMANAKA, Daizo IGARASHI, Shigehisa SUGIYAMA,

Plant Physiology and Cultivation Studies

The present paper deals with a box which can easily cause frost damage to plants by utilizing natural radiative cooling.One or two sheets of polyethylene (PE) film were spread on the top of the box, while the four sides and the bottom of the box were insulated with insulator boards (5cm thick). The inside surfaces of the box except the top were covered with aluminium foil (17μm thick) in order to reduce long-wave radiation emitted from them. The box was set up at 40cm-height above the ground. The size of the box could be changed according to the size of plants being tested. For maximizing the cooling effect of the box, however, it is necessary to make the bottom area large enough compared to the height of the box. As shown in Fig. 1, a microscope slide glass was used as a model leaf. A difference in air temperature between outside and inside the box was observed before sunset, and it reached an average of approximately 7°C at one and a half hour after sunset (Fig. 2, Table 2). The model leaf temperature was approximately 2°C higher than the air temperature in the box. When the effect of a new aluminium plate covering the bottom of the box was compared with that of an aluminium plate painted black, the former resulted in a higher temperature, i. e. 4°C at the bottom surface and 1°C in the air (Table 3). A comparison was also made between polyvinyl chloride (PVC) film and PE film with respect to its effect on the interception of the long-wave radiation, and the result is presented in Table 2.Data obtained with a Beckman-type atmospheric radiometer as well as the physical values of the materials of the box which were given in Tables 4 and 5 were used to calculate the heat balance on the model leaf surface. The results are shown in Table 6. The heat transfer coefficient, h, obtained from the heat balance calculation was compared with the free convection coefficient, hf (Gate and Benedict, 1963), and was discussed for the error. The mean range of h values obtained was 3 to 6×10-2cal·cm-2·min-1·°C-1.Items of special notice from the use of this box are as follows:(1) A box with double PE layers reduces, more effectively than that with a single PE layer, the variation of the difference in the air temperature between outside and inside the box. Therefore, the former is more superior than the latter.(2) The greater the long-wave emissivity of the bottom surface, the cooler the box is.(3) Wind should be blown on the PE film from the outside of the box to prevent the condensation of dew or the sublimation of frost on the film.(4) The plant should be taken out of the box or shaded by an automatic shading sheet fitted on the box after sunrise, since the air temperature in the box may rise quickly due to solar radiation in the morning.