Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 08/07/2007 14:48:57
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If clouds are composed of water droplets or ice crystals, what keeps these particles aloft? Why don't they rain down straight away?
Chris
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This question is similar to something I was puzzling about the other day. When it rains, why doesn't all the rain fall down straight away , like bursting a balloon full of water, rather than taking sometimes hours to fall?
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let me have a go at this.
Clouds are formed at the hight they appear at because of the low temperature, water vapour condenses into droplets. The droplets of course are water so they tend downward. But when water vapour condenses, it gives off heat and this heat keeps the droplet and air mixture lighter or equal to around air. That is why clounds are floating.
Also, the water molecule is lighter than air. air being oxygen and nitrogen.
........or something like that.....possibly!
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I believe clouds are made out of water vapour, a gas which is lighter than air and rises due to warm air currents
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I believe clouds are made out of water vapour, a gas which is lighter than air and rises due to warm air currents
Clouds are droplets, not vapour. Vapour is simply invisible, clouds are visible and appear white because the light is diffracted and spread in all directions.
If the droplets remain suspended up in the air, it is because the air around them moves up upwards as fast as the droplets would fall. That is why glider pilots search for clouds, or rather for the upward moving air in and beneath them.
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yea basically the gasous h20 is lighter than the air around it so it stays up there really high
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ok to end the confusion instead of .
Clouds are made out of water vapour.
I shall put.
Clouds are made out of water vapour which has condensed. :)
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If clouds are composed of water droplets or ice crystals, what keeps these particles aloft? Why don't they rain down straight away?
Chris
Good question Chris, never thought about it.
Instead of searching for the answer in books and internet (maybe eric has already given the right answer) I want to try an intuitive answer: maybe it also depends on adhesion of air molecules to the droplet's surface, keeping it floating.
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The only thing which can keep the individual droplets up there is rising air and for the air to do so the droplets must be very very extremely small, how small i dont know but they must be tiny. :)
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If clouds are composed of water droplets or ice crystals, what keeps these particles aloft? Why don't they rain down straight away?
Chris
Maybe they are - raining down straight away.
The only difference being that the rain is falling in air that is rising, so to someone outside of the rising air, it looks stationary.
As Michael suggests, the size of the droplets will matter, in that it effects the ease with which it will be caught in the updraught, and when the droplets get too big, the mass of the droplets increases faster than its surface area, so it falls faster than the updraught can keep it aloft.
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fine dust and droplets take longer to settle out than coarser drops and particles and it is possible to have drops so small that they never settle out because of the buffeting they get from the gas molecules in the air
The fine droplets that make up the "steamy bits of clouds just fall so very slowly that they mostly stay where they are but can merge to form larger droplets.
Now one of the main features of most shower clouds is an updraft of air and this updraft can keep larger drops in suspension. These updrafts can be very strong indeed in large thunderclouds. Beyond a certain size raindrops become unstable and can fall apart to become mall again so the size of the drops depends a bit on the nature and height of the clouds but when the rain freezes to become hail (its cold up there)they can get fery big ans hailstones as big as eggs that have been kept up by the strong updrafts in powerful storms sometins happen.
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So what triggers a cloud to decide that now's the time to unload its rain (or snow or hail)?
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I thought that this was simply the point at which the coalesced droplets became too big for the internal forces of the cloud to keep up, and hence precipitated.
Are there not also static charges at work here too?
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As a child, I was always told that when the temperature drops below the dew point it begins to rain. This has to do with the saturation of air with water vapor, which is related to the relative humidity.
Here's a link to an informative site about dew point, relative humidity and precipitation
http://weathersavvy.com/Q-dew_point1.html
As to why clouds appear in the sky...
Clouds, as stated earlier, are composed of tiny water droplets or ice crystals, which diffract the light. White clouds have fewer droplets/ice crystals than dark clouds (obviously.)
The warm air currents below the clouds keep the ice crystals aloft. Also, the conditions in a cloud are not static. As some of the droplets/crystals fall they have enough time to re-evaporate into the air below.
When the air temerature reaches the dew point, the relative humidity becomes 100%. Then, the falling droplets/crystals can no longer evaporate, because the air below the cloud is already saturated with water vapor. At that point the cloud "rains" or some sort of precipitation occurs.
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As mentioned, when the clouds are floating, they're in equilibrium: the updrafts and air currents cancel out the falling of the small ice crystals.
When the air around a cloud starts to cool, it will pass it's dew point as Cut Chemist said, and the water molecules will have to leave the air and condense somewhere. The cloud is made of lots of tiny ice crystals which should act as nucleation sites for the condensing water, and it will start to form droplets in the cloud. The droplets are too big to be held aloft by the air currents, and so they fall down as rain/snow/hail.
I don't know what various causes could start the cooling/condensing process, but certainly a cold front can do it, which explains why rain often accompanies a cold front passing through.
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Thanks everyone for a very thorough and interesting analysis!
Chris