F-18 at transonic speed.
The cloud is due to the Prandtl-Glauert Singularity.
In aerodynamics, the sound barrier is the apparent physical boundary stopping large objects from becoming supersonic. The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a grab-bag of unrelated aerodynamic effects, and fell out of use in the 1950s when aircraft started to routinely "break" the sound barrier.
As a plane approaches the speed of sound, the way air flows around its surfaces changes and it becomes a compressible fluid. Along with a number of changes in the way that lift is generated, this change also gives rise to a rapid increase in drag, known as the wave drag.
At first the exact nature of the wave drag was not well understood. It appeared that it increased exponentially, as it does for a limited range of speeds. With only the limited power of piston engines to drive them, planes could not overcome this rapid increase in drag, and even large increases in power would result in only tiny increases in performance. It appeared that an infinite amount of power would be needed to reach supersonic speeds, and thus everyone started talking about the sound barrier.
Artillerymen knew better. Starting with Ernst Mach in the 19th century, they were aware that after a point the drag no longer increased, and in fact dropped again. The challenge then became how to provide this amount of power. With the introduction of the swept wing to lower drag, and the jet engine to provide the power, by the 1950s a number of aircraft were able to fly supersonically with relative ease.