The expression of grain sizes on a logarithmic scale. The phi value (ϕ) is related to grain diameter (d) by the expression ϕ = -log2d. Increasing positive phi values are for grain diameters progressively finer than 1000 μm, and increasing negative values for grain diameters progressively coarser than 1000 μm (e.g. 2ϕ = 250 μm; 1ϕ = 500 μm; 0ϕ = 1000 μm; -1ϕ = 2 mm; -2ϕ = 4 mm).
Subjects: Environmental Science — Earth Sciences and Geography.