Observations of the “quasi-two-day” wave are reviewed and it is concluded that the weight of evidence indicates that the wave is a solstical phenomenon, with maximum amplitudes in low latitudes of the summer mesosphere. It is suggested that the properties of this wave and of a similar wave found in a numerical model of the middle atmosphere may be consistent with an origin via baroclinic instability of the easterly jet in the summer mesosphere. This suggestion is supported by the results of a stability analysis of a one-dimensional model of the summer mesospheric flow. Abstract Observations of the “quasi-two-day” wave are reviewed and it is concluded that the weight of evidence indicates that the wave is a solstical phenomenon, with maximum amplitudes in low latitudes of the summer mesosphere. It is suggested that the properties of this wave and of a similar wave found in a numerical model of the middle atmosphere may be consistent with an origin via baroclinic instability of the easterly jet in the summer mesosphere. This suggestion is supported by the results of a stability analysis of a one-dimensional model of the summer mesospheric flow.