“Shot DIXIE of Operation Upshot-Knothole was set off on April 6, 1953.”
It wasn’t a terribly large explosion by the standards of the day — “only” 11 kilotons, so smaller even than the Hiroshima bomb (though still large enough to, say, take out downtown Boston). It was dropped from a plane and detonated some 6,000 feet above the ground, which is some three to four times higher in the air than the Hiroshima and Nagasaki bombs had been set off. As a result, its cloud was a mushroom “cap” without a stem — it was too high off the ground to suck up the debris and dirt necessary to form a vertical column.
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I occasionally go through the photo library of the DOE’s Nevada Site Office, which has a rather large collection of scanned photos of atmospheric tests online. The Upshot-Knothole series is quite a large one, and there is a numbing effect looking at so many of them. So when I saw this one of the DIXIE event, I was initially just totally bamboozled…