Why don't clouds fall?

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Why don't clouds fall?
« on: 11/04/2011 10:30:02 »
mike.elliott asked the Naked Scientists:
Clouds: water is heavier than air, so why do clouds of water hover high up in the sky rather than falling straight down? is it water vapour buoyed up by up currents (like you see above a cup of tea)?

And why do they have such flat bases and tops to them, the cloud layers? My 7 year old son, John, asked me this one and despite having a PhD in physics I couldn't answer)
Many thanks

Ps - I love your podcast!

What do you think?
« Last Edit: 11/04/2011 10:30:02 by _system »


Offline JMLCarter

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Why don't clouds fall?
« Reply #1 on: 11/04/2011 18:10:57 »
Fine water droplets are small and light enough to float on air currents. When the air is warming, e.g. in sunlight, the rising and circulating air is enough to keep the droplets airborne.

The atmosphere tends to have a layered structure due to winds in different direction or different temperature and humidity not being able to occupy the same layer without colliding. Also the atmosphere is itself a very thin layer around the earth.
When the conditions for cloud formation occur they tend to occur in one atmospheric layer at a time. Intermediate size droplets will fall to the bottom of the cold layer, but not over a boundary into the warm layer.

Another couple of questions;

how come all the raindrops fall at the same time?
when the density of droplets in the whole cloud reaches a critical level the droplets start to combine, as the fall through the cloud they grow larger. This process is triggered by macroscopic conditions on a homogeneous cloud, so impacts the whole cloud at the same time. I wonder if there might be some kind of other feedback process (anyone know)?

what shape are raindrops?
They are not teardrops. More like pancakes - they get blown inside out by the apparent wind when they fall.

« Last Edit: 11/04/2011 18:16:48 by JMLCarter »


Offline Soul Surfer

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Why don't clouds fall?
« Reply #2 on: 11/04/2011 23:31:01 »
Standard flat based fluffy cauliflower shaped cumulus clouds are caused when rising air cools and condenses as this cooling and condensation depends on the dew point of the air this always happens at about the same point assuming that all the air has a similar water content at that time.  In meteorological terms this level is called cloud base.

Fine droplets of water in air can be kept up by brownian motion but the rising air currents in major clouds can suspend large raindrops and even very large hailsones if they are very strong.
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