Transport of a quantity, expressed in units of that quantity crossing a unit area per unit time. Examples are heat flux in units of J m−2 s−1 (equivalent to W m−2), momentum flux in units of kg (m s−1)(m2 s)−1 (equivalent to kg m−1 s−2), or mass flux in units of kg m−2 s−1. When a flux in dynamic units is divided by air density (or for heat flux by air density times specific heat of air at constant pressure), the result is the flux in kinematic units, which is often easier to measure by conventional weather instruments such as anemometers and thermometers.