The Soviet Union had abrogated the 34 month old de facto nuclear testing moratorium on 1 September 1961 by initiating an unprecedented series of atmospheric nuclear tests. The initial U.S. response was to commence its own test series (Operation Nougat) two weeks later. Nougat was an underground series however, restricted to very low yield devices.
Numerous atmospheric tests, many at high yield, were of course on the drawing board at the weapons labs, some carried over from planning for previous test series. The Dominic Sunset test successfully reached the full design yield with a yield-to-weight ratio of 4.06 kt/kg. The vapor cloud rose to over 60,000 feet.
Nuclear detonations produced high above the ground do not create mushroom clouds. The heads of the clouds themselves consist of highly radioactive particles, primarily the fission products, and are usually dispersed by the wind, though weather patterns (especially rain) can produce problematic nuclear fallout.
Nuclear explosions are often accompanied by short-lived vapor clouds known variously as “Wilson clouds”, condensation clouds, or vapor rings. The low pressure region causes a sharp drop in temperature, causing moisture in the air to condense in a shell surrounding the explosion. When the pressure and temperature return to normal, the Wilson cloud dissipates.