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For most people, clouds are these fluffy white things floating up in the air, which may give the impression that clouds are light, but nothing could be further from the truth. Clouds are created when water vapor condenses into minute water droplets due to changes in pressure and temperature, and water has weight. Those large cumulus clouds that you see up in the sky actually weigh hundreds of tons! The reason why clouds don’t fall is due to in part the same reason why ships built out of steel don’t sink. The density of the clouds is lower than the density of the underlying air. In other words, a given volume of air below the cloud is heavier than the same volume of air in the cloud, thus the cloud floats on this air. In cumulus clouds, this happens because as the warm air rises from the land it expands and becomes less dense. Additionally, the water droplets that make up most clouds are microscopic, and the effect of gravity on them is negligible and easily counteracted by the updraft currents within the clouds. 
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During a large explosion the air is pushed away from the center of the explosion and compressed into a supersonic shock wave, which creates the loud sound we hear from the blast. Immediately behind the shock wave an area forms of low pressure and temperature. If the explosion is sufficiently strong and occurs in a humid environment, the temperature of the air behind the shock wave can briefly drop below the dew point causing water molecules to rapidly condense into water droplets forming a cloud. In the first atomic bomb tests carried out in the Bikini Atoll in 1946 (Operation Crossroads), these clouds were called Wilson Clouds. They were named after the Scottish physicist Charles Thomson Rees Wilson who won a Nobel Prize (shared with the American physicist Arthur Compton) in 1927 for the development of the cloud chamber which allowed the visualization of radioactivity. In the video below, an atomic bomb named “Baker” is exploded underwater next to unmanned ships to test its power. You can see the large Wilson cloud begin to develop around the huge central column of water 9 seconds into the video. Today Wilson clouds are called “condensation clouds”. Condensation clouds can also form during far weaker explosions under the right conditions such as the one that rocked the port of Beirut in 2020. The white condensation cloud begins to develop 11 seconds into the video. It envelops the central red-colored explosion cloud and then quickly dissipates. However, man-made condensation clouds are not only formed during explosions. Anything capable of generating a supersonic shock wave such as a fighter jet can generate such a cloud, which is this case is called a vapor cone. Warm air contains water vapor which is invisible, and warm air is less dense so it tends to rise. As the air rises, the temperature and pressure decreases. This progressively lowers the capacity of the rising air to hold water vapor until a point is reached (called the dew point) at a certain altitude where the water vapor in the air transitions to the liquid state forming minuscule water droplets. If enough of these water droplets are present, the water in the air will not be invisible anymore and will acquire a white color forming what we call a cloud. If the water droplets become larger, the cloud acquires a grey color. The air can continue to rise pushing the top of clouds to great altitudes and making them look fluffy with a lot of dome-like bumps. If the layer of air below the clouds is free of turbulence, the point at which the transition from water vapor to water droplets occurs will be located at more or less the same altitude over a large area of land, Because of this, all the clouds over this area will seem to have a more or less uniform flat bottom located at the same altitude. In the video below, you can see many clouds with fluffy tops which have flat bottoms located at roughly the same altitude, as though all of them were resting on an even surface. |
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