#condensation cloud

An F/A-18C Hornet from USN VFA 113 breaks the sound barrier during an air power demonstration over the aircraft carrier USS Carl Vinson - 6 June 2011

A nuclear explosion test, codenamed “Baker”, part of Operation Crossroads, at Bikini Atoll in the Marshall Islands (July 25th, 1946).

The second photo shows the white surface "crack" under the ships, and the top of the hollow spray column protruding through the condensation cloud.  In the background is Bikini Island beach.

The 40-kiloton atomic bomb was detonated 3.5 miles from Bikini Atoll, at a depth of 27m below the ocean surface.  The purpose of Crossroads tests was to study the effects of nuclear explosions on ships.  They would be the first of many nuclear tests held in the Marshall Islands, and the first to be publicly announced beforehand and observed by an invited audience, which included a press corps.

95 target vessels were assembled in Bikini Lagoon, with amphibious target ships berthed on Bikini Island.  At the centre of this target cluster, the density was 20 ships per square mile, or 7.7 ships per square kilometre, which was 3-5 times greater than military doctrine allowed.  The goal was to measure damage at various distances from the blast centre (as many different distances as possible).

The Army and the Navy had disagreed about how many ships should be allowed to sink.  The target fleet included four obsolete US battleships, two aircraft carriers, two cruisers, 11 destroyers, 8 submarines, many auxiliary & amphibious vessels, and three surrendered German & Japanese ships, including the Japanese battleship Nagato.

The ships carried sample quantities of fuel & ammunition, and scientific instruments to measure air pressure, ship movement and radiation. The support ship USS Burleson supplied live animals for some of the target ships – there were 204 goats, 200 pigs, 200 mice, 60 guinea pigs, and grains containing insects.  These animals would be studied for genetic effects by the National Cancer Institute.

USS LSM-60, a landing ship from WW2, was placed in the centre of the target fleet with the bomb suspended beneath it.  Baker was detonated 27m underwater, halfway to the ocean floor, which was 55m deep.  The bomb had a yield of 23 kilotons.  No identifiable remains of LSM-60 have ever been found: it was probably vaporized by the nuclear fireball.

So many unusual phenomena were produced by the Baker explosion that two months later, a conference was held in order to standardize nomenclature and define new terms to use in description and analysis.

The underwater fireball was in the form of a rapidly-expanding hot gas bubble.  It pushed against the water, creating a supersonic hydraulic shock wave that crushed the hulls of nearby ships as it spread outwards.  When the bubble's diameter had expanded to the water's depth (i.e. 55m), it hit the ocean surface and floor at the same time, 4 milliseconds after detonation.

At the ocean floor, it began digging a shallow crater, which was ultimately 9m deep and 610m wide.  At the ocean surface, it pushed the water above it into a “spray dome”, which burst through the surface like a geyser.

When the bubble reached the air, it set off a supersonic atmospheric shockwave, which was (like the “crack”) more visually dramatic than destructive. Behind the shockwave was a short-lived area of low pressure, which caused fog to instantly develop, shrouding the developing column in a condensation cloud, obscuring it from view for two seconds. The condensation cloud started out hemispherical and expanded into a disk, which then lifted up from the water, revealing the fully-developed spray column, then expanded into a doughnut and vanished.

(Not) A Jet Fighter Breaking The Sound Barrier

I seem to be making these into a bit of a series: ‘when social media gets it wrong’ (see: Not A Baby Polar Bear and Not A Baby Giraffe). More often than not it’s one of those scraper accounts that post images for cheap engagements. I assume they do it to create ad revenue from sponsored posts. It can’t be worth that much, right?

Anyway, here’s the next in the series, a fighter jet visually…