# Drag law

From AMS Glossary

## drag law

The relationship between wind speed and force caused by the wind against objects or along surfaces.

Force per unit area of the wind parallel to the ground is called surface stress, τ, resulting in a drag law of the form where

*C*_{D}is a drag coefficient,*M*is wind speed, ρ is air density, and*u*_{*}is friction velocity, which when squared equals the kinematic surface stress τ/ρ. Most atmospheric flows near the surface are turbulent, resulting in drag coefficients that must be determined empirically rather than by using molecular properties such as viscosity or Reynolds number.*See*drag.