Two different approaches of the aerological method were used for estimating evaporation in northwestern Europe in the period 1969-1970.

Daily operational aerological observations (»the synoptic approach«) gave better results for homogeneous areas (e.g. the Baltic Sea) than for a nonhomogeneous surface. One drawback in this approach was the non-uniform station network due to a lack of observations and resulting in scattered values even for monthly means.

Based on annual mean moisture fluxes at the aerological stations (»the statistical approach«) the evaporation from the Bothnian Sea was estimated to be about 300 mm/year. The main convergence of the water vapor flux occurred below the 900 mb level and was largely due to the mean part of the flux. The results appeared to be quite dependent on the homogeneity of the data sample.

In both approaches the main source of error is the lack and low quality of data, because divergence calculations are sensitive to errors in wind data. The linearity approximation used may also fail in low levels. Careful data checking and the use of complete data samples are needed when the aerological method is applied.

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