1

Clouds are made of extremely small droplets of water ranging from 1 to 100 microns. Their high surface area to volume ratio means that air resistance would greatly slow their fall.

However, regardless of a water droplets total size, water's density is still much greater than air, and over time the earth's gravitational pull should cause the water droplet to fall with respect to its surrounding gas.

Why then, do water droplets in clouds appear to float at great altitudes rather than fall to the earth? If drawing a Newton force diagram of a water droplet in a cloud, what forces pulling upwards equalize the force of gravity pulling it down?

Qmechanic
  • 201,751
Cory Klein
  • 147
  • @Ziggurat The referenced question approaches the question from a macro-cloud scale. I'm curious from a newtonian mechanics view at the scale of a single cloud droplet, what prevents it from falling? Neither answers nor their provided links explain this. – Cory Klein Jun 16 '17 at 19:31
  • Would those downvoting care to share ideas on how to improve the question? I have read the on topic page for this site and the question appears to be appropriate and follow the guidelines for asking a good question. If this is off topic I'm happy to close, and if it's a bad question for some reason, I would like to improve it. – Cory Klein Jun 16 '17 at 19:38
  • Even disregarding the droplets' slow rates of descent, I think the hail/updraft analogy justifies it fairly well. Even if there aren't constant upward forces, updrafts hit the droplets with great enough force frequently enough to keep them up. –  Jun 16 '17 at 19:44
  • But updrafts are a zero-sum game, for every updraft there is a down-draft as well, so this does not explain the kind of cloudy skies as shown in the link I gave above where the clouds just float at the same altitude, neither moving up nor down. – Cory Klein Jun 16 '17 at 19:47
  • @CoryKlein Well, that "macro" point is answered at the other question: clouds form when humid air rises, often across a very large front. – rob Jun 16 '17 at 19:54
  • @rob I must be misunderstanding what you're trying to say. Can it be possible that all the air visible in the video is rising? Wouldn't that create an enormous vacuum effect in order to pull air in to move it upwards? In the video the trees aren't moving and the winds seem very calm, yet it seems to cover miles and miles of present clouds. And wouldn't that explanation only account for clouds not falling at weather fronts? Why don't they fall elsewhere? – Cory Klein Jun 16 '17 at 20:00
  • @CoryKlein The entire atmosphere doesn't need to rise in order for clouds to exist. They only take up a portion of the atmosphere. It's been established that it doesn't take an immense upward force to keep clouds aloft, so the occasional updraft, which could be counterbalanced by an opposite draft in a part of the sky without clouds, is sufficient. –  Jun 16 '17 at 20:11
  • @Ziggurat Are you saying that statistically speaking clouds exist in locations where the net-wind over a sustained period of time is upwards in a rough equivalent to the speed at which the cloud is falling? That would explain why we never see clouds blown downwards I guess. – Cory Klein Jun 16 '17 at 20:13
  • @CoryKlein Yes; it should also be mentioned that if part of a cloud began moving down, it would likely evaporate in the warmer air, and that as air rises, the water vapor in it will tend to condense. –  Jun 16 '17 at 20:22
  • @CoryKlein Note that your video doesn't show statically-shaped clouds being transported, like cardboard cutouts in a school play. There's lots of new condensation forming --- mostly as new clouds billowing noticeably upwards --- and many examples of cloudlets disappearing as the droplets that comprise them evaporate. It's clear from the way the clouds evolve that the wind speed increases at higher altitudes, and the planetary boundary layer is visible. Weather is complicated. – rob Jun 16 '17 at 22:13
  • Also if you ever have a chance to fly in an unpowered glider on a nice day like that, your pilot will take you over those nice puffy clouds to gain altitude, because that's where the thermal updrafts are. – rob Jun 16 '17 at 22:14

1 Answers1

2

This link does the calculations, including viscosity. They do fall:

A water droplet with a 10 nm radius falls at 12 nm/s in air. It would take 2.6 years for this droplet to fall one meter. It is only when the small droplets begin to coalesce into larger droplets that they fall with significant speed.

Italics mine.

From the link provided by Rob:

Cloud dropplets are about a mm apart and on average of micron size, and most water is mainly as water vapor in all but thunder clouds. They fall so slowly that even small updrafts keep them aloft.

anna v
  • 233,453
  • Ten nanometers seems unrealistically small for a cloud droplet; a quick search brings up one of my favorite web treasures as a source that 10 μm is more typical. But since $v_\text{terminal} \propto R^2$ we have straightaway a terminal velocity 12 mm/s --- much faster, but still slower than typical speeds of air movement. – rob Jun 16 '17 at 19:43
  • Assuming the original calculation didn't have other errors, that is. – rob Jun 16 '17 at 19:53