A periodic wave structure with a vertical wave length typically 2 km between 14 and 23 km altitudes has been identified in lidar measurements of relative density profiles at 589 nm that were made by the University of Illinois group during the March/April AIDA 1989 campaign effort. The equivalent temperature fluctuations are several degrees in magnitude. Also seen are stratified aerosol layers at altitudes near 25 km with a vertical extent of ~1 km. Both of these phenomena show a downward phase speed between 1 and 3 cm s-1. The lower periodic structure is believed to be the same phenomenon seen previously in 430-MHz measurements of winds obtained at Arecibo Observatory. The source of this structure remains uncertain. Cornish and Larsen (1989) and Sato and Woodman (1982a) considered this structure to be a quasi-inertial long period wave. Hines (1989) proposed instead the identification of this structure with a stationary wave system generated by orographic waves created through the interaction of the subtropical trade winds with the mountainous terrain of Puerto Rico. The higher altitude feature seen in the AIDA lidar observations is believed to be an aerosol layer with a scattering ratio of about 1.15. An alternative explanation is clear air turbulence, but the signature of the 10 K fluctuation deduced from the lidar observations was not found in the San Juan balloon measurement of the temperature profile for the lower stratosphere. ¿ American Geophysical Union 1993