How do clouds stay up?

[abyrne]

How do clouds stay up?- surely if they're made of water vapour then they'll be heavier than air and fall down?

[OBonio]

Steam rises, so water vapour is not necessarily heavier than air.  When the density of a cloud does exceed that of the air around it, we get rain.

[bernardo]

They are very light and fluffy, so it takes ages for them to settle (i.e. fall).  They keep being blown about by the wind, so it takes ages.  By the way, clouds are no longer water vapour (gas) - they have already condensed into zillions of tiny droplets (liquid).  But it is as if you had a load of dust on the floor, and you kept on blowing it around.

[rod]

They go to bed early!

[Davidgrey]

 

Clouds occur as a function of rising air. As the air rises, it also cools (about 2 degrees per thousand feet). The water condenses out and hey presto there are your clouds. If the air was to drop then it would warm up and the water vapour would disappear again.

 

[brachiopod]

d g is qite right, if you have ever seen time-lapse photography of clouds, especially,say, over a mountain range, you will see the cloud accreting as the moist air rises above it's condensation level, and disappearing again ('ablating') as the airflow once more causes the condensed droplets to evaporate.

 

Then again, the water droplets that make up clouds would not form at all if it weren't for all the condensation nucleii that are present in the atmosphere. These are particles of dust, wind-blown soil, salt (from the seas and oceans), pollen, insects, volcanic particles, etc etc that are ever-present in our atmosphere - all of which are 'heavier than air' but are kept aloft by the winds and updraughts.

Think of it as 'muddy water' - given still conditions (a jar or bucket) it soon settles out - but in an ever moving river, the water stays 'muddy'.

[newtron]

Many times, as an air mass travels over a mountain range, water condensates, and rains out  as the temperature drops. As condensation is a exothermic reaction, the temperature of the air mass increases so that the air mass is relatively warm and dry as it decends the other side of the mountain range.  This describes the formation of a rain shadow. 

As far as the muddy water analogy goes, moving water or turbulence is not always necessary to keep sediments suspended.    Small clay particles can be held in suspension due to electrostatic interactions as a result of  eletrical charges associated with the surfaces of the clay particles.  I don't know if this mechanism is important in the atmosphere.  Many airborn particles are electrically charged, but the air is not as viscous as water.