In the paper experimental measurements of the time-dependent velocity and density perturbations upstream of obstacles in linearly stratified flow are presented. Attention is concentrated on obstacles which generate turbulent separated wakes at Froude numbers, based on velocity and body height (U/Nh), of less than 0.5. The form of the upstream columnar modes is shown to be largely that of first-order unattenuating disturbances, which have little resemblance to the perturbations described by small obstacle height theories (e.g., McIntyre, 1972). For two-dimensional obstacles, the disturbances are similar to those found by Wei et al (1975) and it is shown that provided a suitable obstacle drag coefficient is specified, the lowest order modes (at least) are quantitatively consistent with the results of linear theory based on the Oseen inviscid model (e.g., Janowitz, 1981). The experiments complement earlier work and a study of the corresponding columnar disturbances upstream of three-dimensional obstacles is included. A discussion of the importance of towing tank end-walls and the relevance of the Foster & Saffman (1970) theory for the limit of zero Froude number is also included.