We present a simple, novel kinematic criterion to separate line plumes from local boundary layers in a plane close to the hot plate in turbulent Rayleigh-Benard convection (RBC). We show that the horizontal divergence of the horizontal velocity field (∇H.u) has negative and positive values in plumes and boundary layers, respectively. Following the deformation mechanism inside plumes and boundary layers, we show that the plume regions given by the divergence criterion are also regions of the negative dominant eigenvalue of 2D strain rate tensor λD. We also explore the connection of these plume structures to Lagrangian Coherent Structures (LCS) in the instantaneous limit. Further, to study the effect of shear on plume spacings, we extracted plumes using divergence criterion in turbulent RBC in water and that from planforms of plume structures obtained from particle scattering experiments in turbulent mixed convection in air. Then, we propose a common scaling law that describes complex dependencies of mean plume spacing λ on Raw, Re, and Pr in RBC and MC.