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Page 2
Actually, saying a cloud is floating on air is a bit of a misnomer. When we think of floating in a technical sense, we think of an entity kept up within the medium by buoyancy forces due to density differences such as how a piece of wood floats on a lake, or how a hot-air balloon floats in the air. The cloud does not "float" in the air, because its water droplets are heavier than the surrounding air. The second question in the preceding paragraph is in fact where the error lies. Gravity does pull the cloud down to earth. The reason the cloud we are watching does not seem to fall to the ground is the result of two other mechanisms acting on those cloud droplets.
The most important of these is that the cloud, particularly our afternoon cumulus, has been formed within a rising current of air. There are many causes for this rising air, I wrote about several elsewhere (making clouds and updrafts). A rising air current is one of the important steps in the formation of a cloud. Rising air in a cloud core continues until the cloud reaches old age and begins to dissipate. Cloud droplets within this rising air are thus continually pushed upward at a greater rate than the rate at which gravity pulls them downward. The net result is that the cloud droplets are rising within the updraft, which you can see by watching the tops of that cumulus. If the cloud is not dissipating, you will see growing and changing bumps of white at the top of the cloud, an indication of updrafts and continued growth. Now put aside thoughts of the updrafts for a moment. We next look at what happens to those cloud droplets under the influence of gravity. Now we know that gravity wants to bring everything down to earth, and as Galileo showed us, the pull of gravity is independent of the object's mass. That is mostly true, because the objects he dropped off the Tower of Pisa were of approximately equal size and shape though not of equal mass. As a result, the forces of air resistance on the two objects were about the same, and the distance of fall not that great. Galileo's experiment works to perfection when objects fall through a vacuum, but we do not live in one nor do clouds. Our cloud droplets are falling through air which puts up some resistence to any object falling through it. This is technically known as aerodynamic drag, and its value depends on the object's mass, shape and size. Cloud droplets are more or less spherical (ice crystals have a wide variety of shapes that may increase their drag effect) and thus have a smaller drag effect than a less symmetric shape — say a feather. They also have a very small size, 12 micrometres or less in diameter.
The copyright of the article How Do Clouds Float? - Page 2 in Meteorology is owned by . Permission to republish How Do Clouds Float? - Page 2 in print or online must be granted by the author in writing.
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