In the first milliseconds after a nuclear explosion in the atmosphere, the rapidly growing fireball and shock wave of the explosion are one and the same. The surface of the expanding fireball is in fact the front of the shock wave. Once the fireball cools to 300,000 degrees C (which is 15 milliseconds after detonation for a 20 kt explosion), the shock front and the fireball separate - a phenomenon called "breakaway". After that moment the shock front quickly becomes invisible as it loses strength and can no longer make air incandescent through compression heating. This makes it difficult to record the progress of the shock front. Shock pressure gauges can be used, but they are difficult to deploy anywhere but near the ground where interactions between the shock wave and the surface complicate their interpretation. A solution to this problem was suggested by a serendipitous observation in the very first nuclear test, the Trinity shot on 16 July 1945 (see at left). Berlyn Brixner photographed the cable of barrage balloon (the vertical white line at the left edge of the picture) behind the fireball which was visible due to the smoke from the vaporizing cable. As the shock front passed in front of the cable, which was in the background, an apparent break appeared in the cable - an optical illusion caused by refraction of light by the compressed air behind the shock front. The arrow in the second and third pictures shows the movement of this break, which coincides with the location of the shock front. Several years later this phenomenon was put to use with the aid of smoke rockets launched from the ground seconds before the detonation. This created a vertical array of reference lines against which the progress of the shock front could be photographed.
Source:
http://nuclearweaponarchive.org/Usa/Tests/SmokeTrails.html