The sky way grey and overcast, but there was a distinct, wavy pattern in the cloud cover, like ripples on water, or on sand:
The similarity to waves on a water surface is not by chance - in fact, the physical phenomenon, called gravity waves  or buoyancy waves, is the same: If two layers of different fluids meet at an interface, disturbances of the interface can spread as waves. For waves on a water surface, the fluids are the water, and the air above it.
But the same can also happen at surfaces between layers of water with different density, for example with different salinity and temperature, or at the interface between distinct layers of air. If the upper layer of air is below the dew point and carries a close cover of cloud, disturbances of the interface layer can show up as a wavy pattern in the ceiling - it's just the same as waves on water. However, since the differences in density between the two fluids are much smaller in the atmosphere than at the interface water-air, the wavelength of gravity waves in the atmosphere is much longer, and the frequency is lower.
But they can look quite impressive, if they come as Giant Atmospheric Waves Over Iowa.
 Be careful not to mix up gravity waves - waves at the interface of fluids in a gravitational field - and gravitational waves - undulating disturbances of the space-time metric.