Aerodynamic properties of a plant of complex structure

Coefficients of momentum and vapour transfer of spruce shoots in a wind‐tunnel are measured, and are shown to depend on shoot density: transfer per unit needle area becomes less effective as shoot density increases. Comparisons with theoretical values derived from established coefficients for 1 mm and 3 mm diameter cylinders (representing needles and twigs, respectively) indicate that the magnitude of the implied shelter effect is the same for water vapour as for momentum.