The cooling of two naturally ventilated (nv) and two fan ventilated (fv) greenhouses were compared over two summers. Air temperatures and relative humidities in and above the canopy and leaf temperatures were measured. The nv houses used fogging nozzles while the fv houses had evaporative pads. In 2003, one nv house and one fv house were planted with tomato while the other house in each pair was empty. Combinations of fog, vent openings, ventilation rates and evaporative pads were applied. No difference in fruit yield was observed; however, the fruits in the nv house were slightly smaller with more defects. With evaporative pads, the temperatures (air and canopy) were lower in the fv houses than in the nv houses. Without evaporative pads, air temperatures were always higher in the fv houses, but canopy temperatures were sometimes lower in the fv house and sometimes not, depending upon the treatment in the nv house. In 2004, all four houses were planted. The nv houses used all three vents plus fog and the fv houses used an air flow of 0.087 m~3 m~(-2) s~(-1) plus evaporative pads. Both leaf and air temperatures in the fv houses were lower than in the nv houses, with the largest advantage occurring at the beginning of the season, declining as the season progressed. Although the temperature differences did not affect total yield over the life of the study, there was an early yield advantage to the fv houses that lasted for about a week. In addition, the nv houses produced a larger number of defectsuntil the temperature differences between house types reached essentially zero.
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