As a jet accelerates it pushes air in front of it and compresses that air. At a point just before breaking the sound barrier, the compressed air instantly decompresses, which causes it to dramatically cool, causing the moisture in the compressed air to condense and creating a vapor cloud that exists for a fraction of a second.

This image was photographed from a rooftop overlooking San Francisco Bay during an air show. It captured the vapor cloud created when the jet reached a speed where the air could no longer be compressed.

Photographed by Dan Friedman
Camera: Canon 5D Mk2
Lens: Canon EF100-400mm f/4.5-5.6L IS at 400mm
Exposure: ISO 400, f/11, 1/1250sec