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I see clouds traveling at speed where is no updraft, keeping the same altitude, in windy conditions...why they don't fall? Also I experienced clouds hovering in valleys and stationary clouds above areas with no updrafts in windy conditions. Why are those not moving? Thank you kindly

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  • $\begingroup$ I was thinking now that clouds may hover on a layer of moving air (wind), as we have different wind speeds at different altitudes, an air cushion effect $\endgroup$ – Ross T Alba Feb 19 '17 at 5:07
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First, a cloud is not an object like a snowball or a raindrop. It is a grouping of microdroplets which have formed in a region of air where the temperature is at the dew point (the temperature at which the air cannot hold any more water vapor), or below the freezing point and ice microcrystals have formed. The droplets and crystals are of such a size that the mass-to-area ratio is so small that very small air currents will keep them suspended, much like fine dust. Where you believe that there are no updrafts, there may actually be moderate updrafts of moist air, continually refreshing the cloud.

You should investigate the ideas of aerosols and colloidal suspensions if you want to know more about the behavior of very small particles.

Large cumulus clouds that seem to be floating and stable are actually losing and gaining droplets continually. The regions around clouds which are "empty" actually may have the same water vapor presence (or not), but the local temperature is above the dew point, so none of it condenses. If you look carefully at cumulus clouds you will notice that the lower parts of them are flat. That's the boundary line when the temperature drops to the local dew point.

When winds are blowing but clouds seem stationary, again it's because the cloud forms where the local temperature is low enough. You don't see the droplets forming or evaporating because the boundaries of the temperature region aren't moving.

Moving clouds are simply the transport of droplets or ice crystals along with the cold air and surrounding water vapor that resulted in their formation in the first place, but again, the cloud is continually changing. It is not a static entity.

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  • $\begingroup$ Clouds often form where there is an updraft. The rising air cools. This causes the water vapor to condense, forming the cloud. $\endgroup$ – mmesser314 Feb 19 '17 at 5:23
  • $\begingroup$ @mmesser314 That's true. That's not the only mechanism. $\endgroup$ – Bill N Feb 19 '17 at 15:20
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Because of gravity, air is pushed together more at lower heights (since the air above it is sitting on top of it). This makes the air near the earth have move friction than air above. This effect is called the wind gradient, and it what you are seeing is likely seeing. Since friction is different air higher up will be slowed down less than air below. So if you are observing almost no wind near you, that does not guarantee that the air above you is moving at the same speed.

If you've ever tried flying a kite, you'd know this effect. It's tricky to get a kite up in the air, but once you can get it high enough it'll be up there for good!

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