Temperature conditions in the tunnels covered with different plastic films

Abstract

The object of this study was to clarify the difference in the temperature patterns in the tunnels covered with different plastic films. Tested films were PVC ordinary film 0.1mm thick, drained (non-drip) film 0.1 and 0.05mm thick, and polyethylene 0.1 and 0.05mm thick. Shape of tunnels was 6m long, 110cm in base width, and 50cm high. Tunnels were set in parallel in an east-west-direction. Temperatures at the height of 25cm above the ground surface and 5cm deep in the soil were automatically recorded.The record showed that day-time air temperature was higher in the drained PVC film than in ordinary PVC or polyethylene film. In cloudy or rainy nights, air temperatures in PVC and polyethylene films showed little difference and were higher than the outside temperature, while in clear nights, temperature in PVC films was higher than that in polyethylene film. An unexpected phenomenon that air temperature in tunnel fell lower than the outside air temperature late in clear nights was frequently observed.Thickness of film affected temperatures in the tunnels, though not so significant in most cases.Relative order of soil temperatures in the tunnels covered with different films was the same as that of the air temperatures.Small tunnels were set on dry sand in order to compare temperature conditions covered with the films, the inner surface of which was free from water droplets. Temperature difference between ordinary and drained films of PVC could not be observed in this case. This result showed that water droplets condensed on the inner surface of the film had a marked effect on the day-time temperatures in tunnels, probably due to their reflection of sun light.Transmission rates of the films to radiation were measured in the range from ultra-violet to infrared. It was revealed that there was little difference between PVC and polyethylene films in transmission rate in the range from visible to infra-red up to about 6μ, while polyethylene was more transmissible to the infra-red longer than 6μ than PVC. It is known that radiation from black body, temperaturue of which is as high as the soil surface under the tunnel, is mostly in the range longer than 6μ and has its maximum at about 10-11μ. Lower temperature in the tunnel covered with polyethylene film than in PVC film in clear nights seemd to be due to its higher transmission rate to the longer portion of infra-red radiation.A probable cause of the phenomenon that air temperature in the tunnels fell lower than the outside air temperatue during clear night was briefly discussed.