http://blogs.agu.org/wildwildscience/2012/02/06/condo-wave-clouds/

Cool air offshore was very nearly at the saturation point, with a
temperature near 20ºC and a dew point of about 19.5ºC. The air at this
temperature can only hold a certain amount of water vapour, and how much it can
hold depends heavily on the temperature. If you add more water into the air, a
cloud will form, but you can also get a cloud to form by cooling the air. Drop
the temperature, and it can no long hold as much water vapour, so some of it
will condense out and a cloud will form.

In this case, the air was cooled by lifting it about 50 meters over the top of
the condos. A parcel of unsaturated air will cool when lifted at a rate of 1ºC
per 100 meters. In this case, it probably cooled about 0.5 degrees C, but that
was all it took! On the back side of the condos, the air slowly sinks back down
and warms at the same rate. As it warms the air can hold more water vapour and
the cloud evaporates and disappears!