Climate in a Glasshouse (2)

Abstract

Data of seasonal and hourly changes of radiation balance components in and out a glasshouse were used for study of radiation balance conditions inside the glasshouse to those outside the glasshouse. Humidity conditions inside the glasshouse were also investigated as a preliminary to the study of evapotranspiration in the glasshouse.Net radiation inside the glasshouse (iS) is nearly the same as the net short-wave radiation iR(1-α) and iS is determined mainly by transmission coefficient (tm) or daylight coefficient (f), which varies with solar altitude, light components and structure factors of the glasshouse such as the area ratio of iron framework to roof area, thickness of roof glass, etc. This result implies that in the determination of net radiation during daytime (iSd) the net long-wave radiation inside the glasshouse can be neglected.Mean transmission coefficient of total short-wave radiation during the observation period was 0.57. The ratio of net radiation during daytime (iSd) to shortwave radiation outside the glasshouse (0R) varied from 0.4 to 0.5 with the change of transmission coefficient.Protection index (P.I.) defined by HANSON (1963) was about 60%, which is very small as compared with 93% obtained by Hanson. The reason for this discrepancy may be due to the effect of dew at the roof glass and to underestimation of the nocturnal net long-wave radiation in the open (0Sn) on account of dew drops formed on the polyethylene cover of a Funk-type net radiometer.Heating coefficients (γ) (MONTEITH, 1961) at the floor surface of the glasshouse were 8-14% of those reported by MONTEITH for fields. Such low values of γ-value seem to be due to the relatively large long-wave back radiation from the inside glass roof.Variation of air humidity in the glasshouse depends mainly on the evapotranspiration rate determined by the (iS-iB)/(1+β) and on the ventilation rate. On the assumption of a constant rate of ventilation through a whole day the amplitude of the variation of water vapour pressure inside the glasshouse was calculated from Eq. (10). The amplitude (A) calculated decreases with Bowen's ratio as shown in Fig. 7.