Microclimate in an apple orchard.
During 1971 micro-meteorological measurements were made in March, May, July, August and October in a 7-year-old hedgerow-type apple orchard. The albedo ( alpha ) of the orchard varied considerably through the season, being highest ( alpha = 0.22) at full bloom and lowest ( alpha = 0.13) during a period of overcast weather in August. The long-wave balance (RL) derived by difference from measurements of solar (RS) and net radiation (RN) varied diurnally and seasonally in relation to RN. No explanation could be offered for the seasonal variation. Regressions of RN on RS(1- alpha ) and on RS alone were all highly significant, and a general equation is presented, derived from pooled data. Temperature and vapour pressure profiles in the orchard appeared to be less influenced by the trees than is the case in denser plant communities. In-crop wind profiles could, when the fetch was adequate, be almost exactly fitted by a theoretically derived profile. Useful regressions of the differences between bud and flower temperatures, and air temperature, on RN were obtained. The equations include a convective transfer parameter. Leaf temperatures could not be predicted by simple regressions, but variations in leaf-air temperature differences were satisfactorily explained in terms of the leaf Bowen ratio.