The linear and weakly nonlinear stability characteristics of spatially growing, long, low frequency baroclinic waves are examined in a continuous atmosphere on a midlatitude β-plane channel in the presence of Ekman friction and Newtonian cooling (NC). Under the assumption that the damping mechanisms are sufficiently weak, the linear spatial instability problem is solved analytically and explicit expressions for the zonal wavenumber and spatial growth rate are obtained. The expressions show that Ekman damping is stabilizing whereas NC is destabilizing. The tendency of NC to dominate over Ekman damping increases linearly with frequency and β, and quadratically with reduction in zonal wind at the surface. The zonal wavelength of the disturbance is affected only by the flow supercriticality and NC, which always oppose each other; increasing the NC (supercriticality) increases (decreases) the zonal wavelength.