All-sky TV images of wave structure in the near-infrared hydroxyl (OH) nightglow emission were recorded over a 3-month period during May, June, and July 1988 from a high-altitude site at the Mountain Research Station (40.0¿ N, 105.6¿ W, 3050 m), near Nederland, Colorado Well-defined, coherent wave patterns associated with the passage of short period (<1 hour) gravity waves were observed on a total of 22 occassions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ¿40.0¿N), with some eastward motion in May and June, was observed throughout the campaign. was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid-latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These ''blocking diagrams'' which vary with height and time were constructed for the OH altitude (~87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short-period waves reaching the upper atmosphere. ¿American Geophysical Union 1993