# Momentum flux

From AMS Glossary

## momentum flux

The Reynolds stress (τ where ρ is air density. A direct measurement is possible with eddy-correlation techniques, or an indirect determination can be made using Monin–Obukhov similarity flux-profile relationships (

Momentum flux can be associated with either mean velocity components, internal gravity waves, or with turbulent velocity fluctuations. For turbulence, the momentum flux is also called the Reynolds stress. For waves, it is related to mountain wave drag.

_{R}) can be determined from the covariance of the fluctuations of the horizontal (*u*′,*v*′) and vertical (*w*′) wind components, by*also called*universal functions). The relation between the velocity scale (friction velocity*u*_{*}) and the momentum flux is