The meteorological record from the manual Moel Cynnedd climate station at Plynlimon in the Welsh Uplands has been supplemented with solar radiation data, initially from the Institute of Hydrology's Dolydd Office, and later from an adjacent automatic weather station, in order to calculate Penman potential evapotranspiration for the entire 27 year data set, 1969-1995. The methods of data capture are consistent with Meteorological Office criteria throughout the entire record, establishing an unbiased and probably unique indicator of climatic variability and change for this type of environment. Values of Penman Et calculated from these data provides an independent index of atmospheric demand for moisture as an adjunct to the hydrological studies being carried out in the Plynlimon and neighbouring catchments. Analysis of the long term data indicates considerable year-to year variability in the component variables, including some cyclical changes and possible long term trends in measured temperature. Annual variability in Et is less than in the component variables, and there is an indication of a possible long term cycle, but no evidence of an overall trend in Et during this particular study period. The results indicate that some of the observed variability can be explained by inevitable changes in exposure within this forest clearing site rather than changes in regional or global climatic patterns. A single meteorological station sited in a forest clearing at a relatively low altitude may underestimate potential evaporation across the catchment, as this will also include areas of exposed hillside and forest canopy.\u
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