Before revision: where: is the local shear stress, which is by definition the stress component in the direction of the local flow velocity;See lift force and vortex induced vibration for a possible force components transverse to the flow direction is the local mass density of the fluid;Note that for the Earth's atmosphere, the air density can be found using the barometric formula.
After revision: Cauchy momentum equationIn the non dimensional form of the Cauchy momentum equation, the drag coefficient or skin friction coefficient is referred to the transversal area (tha area normal to the drag force, so the coefficient is locally defined as:where: is the local shear stress, which is by definition the stress component in the direction of the local flow velocity;See lift force and vortex induced vibration for a possible force components transverse to the flow direction is the local mass density of the fluid;Note that for the Earth's atmosphere, the air density can be found using the barometric formula.