Air flows may be decoupled from the underlying surface either due to strong stratification of air or due to canopy drag suppressing cross-canopy mixing. During decoupling, turbulent fluxes vary with height and hence identification of decoupled periods is crucial for the estimation of surface fluxes with the eddy-covariance (EC) technique and computation of ecosystem-scale carbon, heat, and water budgets. A new indicator for identifying the decoupled periods is derived using forces (buoyancy and canopy drag) hindering movement of a downward propagating air parcel. This approach improves over the existing methods since 1) changes in forces hindering the coupling are accounted for and 2) it is based on first principles and not on ad-hoc empirical correlations. The applicability of the method is demonstrated at two contrasting EC sites (flat open terrain, boreal forest) and should be applicable also at other EC sites above diverse ecosystems (from grasslands to dense forests).