A transient condensation cloud, also called a Wilson cloud, is observable surrounding large explosions in humid air. When a

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

or a large amount of a conventional explosive is detonated in sufficiently humid air, the "negative phase" of the

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

causes a rarefaction of the air surrounding the explosion, but not contained within it. This rarefaction results in a temporary cooling of that air, which causes a condensation of some of the water vapor contained in it. When the pressure and the temperature return to normal, the Wilson cloud dissipates.

Mechanism

Since heat does not leave the affected air mass, this change of pressure is adiabatic, with an associated change of temperature. In humid air, the drop in temperature in the most rarefied portion of the shock wave can bring the air temperature below its

dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...

, at which moisture condenses to form a visible

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

of microscopic water droplets. Since the pressure effect of the wave is reduced by its expansion (the same pressure effect is spread over a larger radius), the vapor effect also has a limited radius. Such vapor can also be seen in low pressure regions during high–g subsonic maneuvers of aircraft in humid conditions.

Occurrence

Nuclear weapons testing

Scientists observing the

Operation Crossroads Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the ...

nuclear tests in 1946 at

Bikini Atoll Bikini Atoll ( or ; Marshallese: , , meaning "coconut place"), sometimes known as Eschscholtz Atoll between the 1800s and 1946 is a coral reef in the Marshall Islands consisting of 23 islands surrounding a central lagoon. After the Seco ...

named that transitory cloud a "Wilson cloud" because the same pressure effect is employed in a

Wilson cloud chamber A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation. A cloud chamber consists of a sealed environment containing a supersaturated vapour of water or alcohol. A ...

to let condensation mark the tracks of electrically-charged sub-atomic particles. Analysts of later nuclear bomb tests used the more general term ''condensation cloud''. The shape of the shock wave, influenced by different speed in different altitudes, and the temperature and humidity of different atmospheric layers determines the appearance of the Wilson clouds. During

nuclear test Nuclear weapons tests are experiments carried out to determine nuclear weapons' effectiveness, Nuclear weapon yield, yield, and explosive capability. Testing nuclear weapons offers practical information about how the weapons function, how detona ...

s, condensation rings around or above the fireball are commonly observed. Rings around the fireball may become stable and form rings around the rising stem of the

mushroom cloud A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke and usually condensed water vapor resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently ener ...

. The lifetime of the Wilson cloud during nuclear air bursts can be shortened by the

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...

from the fireball, which heats the cloud above to the dew point and evaporates the droplets.

Non-nuclear explosions

TNT detonation on Kaho'olawe Island during Operation Sailor Hat, shot Bravo, 1965

Any sufficiently large explosion, such as one caused by a large quantity of conventional explosives or a volcanic eruption, can create a condensation cloud, as seen in

Operation Sailor Hat Operation Sailor Hat was a series of explosives effects tests, conducted by the United States Navy Bureau of Ships under the sponsorship of the Defense Atomic Support Agency. The tests consisted of two underwater explosions at San Clemente Island ...

{{cite web , title=KN-11352 Operation "Sailor Hat", 1965 , url=https://www.history.navy.mil/our-collections/photography/numerical-list-of-images/nara-series/kn-series/KN-10000/KN-11352.html , publisher=Naval History and Heritage Command , accessdate=7 August 2020 or in the 2020 Beirut explosion, where a very large Wilson cloud expanded outwards from the blast.

Aircraft and Rockets

The same kind of condensation cloud is sometimes seen above the wings of aircraft in a moist atmosphere. The top of a wing has a reduction of air pressure as part of the process of generating lift. This reduction in air pressure causes a cooling, just as above, and the condensation of water vapor. Hence, the small, transient clouds that appear. The

vapor cone A vapor cone (also known as a Mach diamond, shock collar, or shock egg) is a visible cloud of condensed water that can sometimes form around an object moving at high speed through moist air, for example, an aircraft flying at transonic speeds. W ...

of a

transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...

aircraft or rocket on ascent is another example of a condensation cloud.

References