Investigation of a proposed relationship between the skin friction part of the drag on a natural surface and a simultaneous transfer of mass or (sensible) heat led to measurements of the dimensionless transfer coefficients Cd, Cv and Ch for the exchange of (stream-wise) momentum, mass, and heat, between a single artificial leaf and the airflow in a wind-tunnel. It was shown that Cv, h = C0(D/v, k/v)2/3 where D, k and v are the molecular diffusivities in air of gas or vapour, heat, and momentum, and where C0 is a generalized mass or heat transfer coefficient almost independent of ϕ, the angle of incidence between the leaf and the airflow, Cd, however, made up of a bluff-body or pressure part Cb, in addition to a molecular skin friction part Cf, depended strongly on ϕ. C0 was close to the theoretical skin friction drag coefficient at ϕ = 0 of a thin flat plate with dimensions similar to the leaf. As a general relation between the corresponding coefficients of a natural rough surface CF = βC0, where β lies between 0·1 and unity, depending on the form and inclination (ϕ) of the roughness elements, and on wind speed.

C0 was shown experimentally to be proportional to (wind speed)−1/2 in a regime of fully-forced convection. When the evaporation from one side of the leaf was stopped, the transfer coefficient for the removal of vapour from the other side increased by about 30 per cent. An accurate form of the psychrometric constant of the leaf includes the factor (k/D)2/3, which has the value 0·90 for water vapour.