TRANSMISSION OF SOLAR RADIATION IN GLASSHOUSES

Measurements of daily totals of solar radiation were made inside and outside glasshouses in the north and the south of England over a period of 5 years. Kipp solarimeters were used and connected to magnetic amplifiers and electrolytic current integrating meter. Propagation houses orientated E-W had a winter transmission which was high compared with that of single-span vinery glasshouses orientated N-S. In multi-span N-S vinery houses there was also mutual shading which further reduced the transmission. On cloudy days the transmission in all the houses was constant with time of year, but on sunny days this was not so.

In order to obtain more detailed information, especially about new types of glasshouse, the outputs of the inside and outside solarimeters were compared continuously on chosen days, using a potentiometric ratio recorder. The measurements were made on overcast days and on cloudless days at the four seasons of the year and included an outside instrument fitted with a shade ring to determine the proportion of diffuse radiation on the sunny days.

On overcast days the transmission was measured at various stages of construction of new houses. This enabled the amounts of obstruction due to the structure, cladding and glazing to be estimated separately.

The transmission on sunny days was always higher in the south than in the north half of a wide clear span house orientated E-W and was also higher in winter than in summer. To assess the overall performance of a glasshouse at a given site, the proportion of diffuse (sky radiation) and direct (sunshine) received throughout the year had to be taken into account.