Space vehicle operations are sensitive to density disturbances in the thermosphere. The quantification of such disturbances is therefore a matter of importance. In this paper we discuss the contribution from mesoscale waves and conclude that ''external'' waves cause much larger disturbances than the internal gravity waves. The internal waves are restricted to relatively small amplitudes by convective overturning, while the external waves, typified by the lamb wave, do not suffer from this limitation. The energy in a Lamb wave is largely confined to the lower atmosphere which is also its expected source region. But we demonstrate that the exponentially small penetration of this wave to high altitudes produced an exponentially increasing relative disturbance in the more tenuous upper atmosphere. We conclude that the Lamb wave is a significant disturbance in the upper atmosphere and that it may provide density variations that are problems for space vehicle reentry and performance.