The dependence of prevailing wind drift in the Celtic sea upon prevailing winds is discussed and a proposal made to use wind records for the Scilly Meteorological Observatory for assessing drift currents for use in biological investigations.

If there is no seasonal change in the direction of the prevailing winds, nevertheless, the wind drift in summer will be more northerly than in winter.

The terms ‘current’, ‘residual water movement’ and ‘drift’, as used in this series of papers, are denned. The Ekman–Thorade equation for computing wind drift from wind is discussed and applied to the drift of the biological indicator Muggiaea atlantica in 1938. The two approaches agree to suggest that either method may be applied in biological distribution studies.

Some effects of drift, due to variable winds, on plankton distribution are outlined.