As an aircraft flies it sends forward a pressure wave at the speed of sound.
If the plane flies at the same speed a shock wave of sound is produced called the ‘sonic boom’.
U.S. Navy Ensign John Gay captured one of the best images ever taken of a sonic boom (the breaking of the sound barrier) in 1999. He snapped a photo of an F/A-18 Hornet on a humid day from the weather deck of the USS Constellation in the Pacific Ocean. Because aircraft wings generate both low-pressure regions (because of lift) and amplified low-pressure disturbances, large low-pressure regions exist near the aircraft, especially under sonic flight conditions. The lowered pressure condenses the water in the air, creating a vapor cloud. As the jet produces these pressure waves and propagates ahead of them, the regions of lower pressure are usually strongest behind the nose of the jet, on the wings and body. As the aircraft continues to speed up, the vapor cloud will appear farther toward the rear of the aircraft. Then, just as the aircraft bursts through the sound barrier, the air is locally disturbed by the resulting shock wave and the condensation/vapor cloud disappears. Ensign Gay snapped his photo at the moment he heard the boom, just before the cloud vanished. Thus, it literally appears as if the F-18 is pushing through the sound barrier at the instant the photo was taken.
Credit : Scientific American
Picture Credit : Google