in hydrology and fluid mechanics, dimensionless quantity used to indicate the influence of gravity on fluid motion. It is generally expressed as F = v/(gd)^-1/2, in which d is depth of flow, g is the gravitational acceleration (equal to the specific weight of the water divided by its density, in fluid mechanics), v is the celerity of a small surface (or gravity) wave, and F is the Froude number. When F is less than 1, small surface waves can move upstream; when F is greater than 1, they will be carried downstream; and when F = 1 (said to be the critical Froude number), the velocity of flow is just equal to the velocity of surface waves. The Froude number enters into formulations of the hydraulic jump (rise in water surface elevation) that occurs under certain conditions, and, together with the Reynolds number (q.v.), it serves to delineate the boundary between laminar and turbulent flow conditions in open channels.