Anatomy of a mushroom cloud

Think “nuclear weapon,” and you probably picture a mushroom cloud—a stem supporting a puffy head. But photos from U.S. atmospheric nuclear tests (1945–1963) show more than the basic stem-and-head structure. The detonations produced different, sometimes odd, effects.

The basic mushroom

The iconic mushroom cloud begins as a fireball, a luminous bubble of extremely hot air and vaporized weapon residues. The fireball rises like a hot-air balloon, pulling air, water vapor, and debris, into its base to form the mushroom stem. As the fireball rises, it cools, losing its glow, and the vaporized material and water vapor condense and spread, forming the mushroom head.

Fireball

The fireball was gone in seconds, so photographing it during the tests required a rapid electronic camera called a rapatronic camera. Rapatronic photos of a test detonated on a tower, showed a fireball with “legs”—the tower’s guy-wires becoming a glowing plasma in the fireball’s heat. The phenomenon was called a rope trick.

A fake mushroom

This famous photo of the Crossroads Baker test (July 25, 1946) looks like a mushroom cloud but is not. Crossroads Baker was detonated 90 feet under water, so the “stem” in the photo is a hollow pillar of water. The “head” is a short-lived (just seconds), cloud of vapor, caused by moisture in the air condensing in the low pressure behind the explosion’s shock wave.

Slick and crack

Crossroads Baker also produced a “slick” and a “crack,” both caused by the underwater test’s shock wave’s first contact with the surface. The slick was an expanding circle of dark water resembling an oil slick, the source of its name. The crack was the shock wave’s disturbance of the water—a circle of white, ruffled water. 

Rings

A test in a warm, wet environment like the South Pacific sometimes produced rings of vapor instead of the type of vapor cloud seen in photos of Crossroads Baker. The rings were caused by the layers of humidity in the air. 

Caps

A test with explosive power measured in megatons (the equivalent of millions of tons of TNT) produced a cloud that rose rapidly (about 300 miles an hour) to great heights, pushing warm water vapor ahead of it. In the high, cold altitudes, the water vapor froze, forming one or more caps—ice caps.

Skirts and bells

Some tests produced “skirts” or “bells,” cone-shaped phenomena descending along the mushroom’s stem. These occurred when the dropping pressure and temperature of the high altitude caused humid air around the stem to condense into water droplets heavy enough to fall.

Lines

The vertical lines in many nuclear test photos are smoke trails from rockets. The rockets were fired so the progress of the test’s shock wave could be recorded against the pattern of lines provided by the smoke trails. ★