If you look on some photos of nuclear explosion experiments you can see the smoke stripes near the mushroom. It seems that many people are asking everywhere: “What are those funny lines in those pictures of nuclear tests”? It’s a shame but even though I’m experimentalist I’ve never asked this questions. Well, maybe I haven’t seen much of the pictures with nuclear explosions, since I’ve never done research specifically related to that process. 🙂
This is actually a visualization technique which is used for investigation of shock wave propagation. Let’s look on the picture below:

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Operation Upshot-Knothole “Grable” – Nevada, 1953 A shell armed with a 15 kiloton nuclear warhead has just been fired from the cannon in the foreground. That idea of a nuclear cannon is just crazy.  And overall I don’t want even think how many soldiers died after all those tests from radiation consequences. Safety measures were really poor, especially in US, where even civilians  were coming to take a look on nuclear mushroom. Look at this picture:

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Exercise Desert Rock – 1950’s. 18-25 kilotons. Soldiers are sitting just half a mile away without any protection from the epicenter of the explosion.

An estimated 18,000 DOD personnel participated in observer programs, tactical maneuvers, scientific studies, and support activities. Members of all four armed services participated in Exercise Desert Rock V.

This operation exposed exercise personnel to nuclear tests, and thus radiation, more aggressively than previous ones. Observation by troop formations was conducted at what was (at the time) calculated to be the minimum safe separation distance, with many personnel being exposed to multiple tests. Under current occupational radiation exposure limits (0.3 rem/week and 5 rem/year) this would would limit maximum exposures to 3.3 rems over the 11 week operation. Approximately three thousand soldiers reached or exceeded this limit, with 84 exceeding the annual limit (the highest recorded exposure was 26.6 rem). These exposures do not produce observable symptoms, they simply increase the lifetime risk of cancer a small amount.

The effect on the downwind civilian population was much worse. Uphot-Knothole released some 35,000 kilocuries of radioiodine into the atmosphere (for comparison, Trinity released about 3200 kilocuries of radioiodine). The radioactive cloud rose to an altitude of 11,500 meters and passed directly over St. George, Utah, 160 km to the east. The following summer film ‘The Conqueror’ starring John Wayne, was shot in a canyon near St. George. Legend has it that he developed cancer, and later died, due to this test. 91 members of the 220 member cast developed cancer.

You can see smoke grid on the sides by the way on both of the pictures.

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The cosily named Operation Teapot was a series of 14 desert based tests. The explosion above was the MET test which was detonated on a tower and yielded 22 kilotons. You can see smoke tracers all around.

So, what actually is going on. 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.

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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 an vertical array of reference lines against which the progress of the shock front could be photographed.

That’s it. I decided to compile information from several sources for myself since I used smoke visualisation in wind tunnels for subsonic flows and it looked like quite interesting detail for me.

Sources:
http://nuclearweaponarchive.org/Usa/Tests/SmokeTrails.html

https://www.youtube.com/watch?v=GhJpTmPfF6E

http://www.planetdeadly.com/human/incredible-nuclear-explosion-photos

http://www.sonicbomb.com/modules.php?name=Content&pa=showpage&pid=21

 

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