He weaseled out. Clouds do have weight. Saying that they float and therefore have no weight is poppycock. Next I suppose he’ll say fish have no wieght either because they float (in water).
rhinobird is referring to this column :
Are we talking about the column? I think the relevant quote is “Because of course weight isn’t the same as mass–a cloud put on a scale wouldn’t weigh anything.” That seems OK to me. I’m still trying to figure out Dave Morgan’s comment that “the water vapor is less dense than the surrounding air.”
Besides, check out the first sentence: “You’re going to have to weasel on this one.”
Well, the cloud must be less dense than the air underneath it, right? Otherwise, it’d come crashing to the ground. Since it appears that both Cecil and Dave agree on that statement, I assume that there are no other applicable factors that keep the cloud way up in the sky. Or are you tryin’ t’ figger out why water vapor is less dense? In which case, I’m curious, too.
When we refer to clouds we are referring not to air laden with water vapor, but to air containing both water vapor and liquid droplets. The visible part of the cloud is not a vapor but a liquid. These droplets have a density that approaches 1000x that of the air around them, thus they are constantly falling.
Owing to their tiny size, thus their large surface area/volume, terminal velocity is low and they easily evaporate as they fall below the cloud deck (where relative humidity <100%).
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Yes, let’s really pile on here. Unca Cece, who as we all know is never wrong, allowed the good Ph.D. to distract him from a fundamental point when thinking about clouds: they aren’t solids; they are an aggregation of solids occupying a specific space at the specific moment observed. What the individual solid items (the individual droplets) are doing at any given moment is not the same was what we observe the cloud doing.
In short, clouds don’t float. A cloud is a volume of air in which water droplets have condensed. Therefore, the ‘cloud’ is not floating.
As an example of the difference, I direct everyone’s attention to the famous Sierra wave formation, a wonderful type of cloud that forms above the crest of the Sierra Nevada. For an example, see here, from which I quote the following text:
Floating clouds indeed (snort).
OK, color me confused. What jrepka and DSYoungEsq seem to be saying is that the condensed droplets in a cloud are continually falling, but evaporate upon reaching a lower altitude. Meanwhile, I assume there must be some bulk upward movement of the surrounding air, in order to replace the condensed (and falling) water droplets. Correct?
All right, if so, then 1) why are cumulus clouds fluffy? Seems like the process described would lead only to horizontal sheets of clouds at the “evaporation threshold” (or whatever the correct technical term is). And 2) (possibly related): why do clouds billow? From occasional, unofficial, passenger aircraft observation, it appears that the bulk vapor movement in a cloud is dominated by roiling turbulence, not a contistent up/down motion.
Or, if I’m misinterpreting what you’re saying, just what are individual droplets doing in an ordinary cumulus cloud?
I’m going to wade in here with a slightly different than usual perspective - that of a pilot (strictly small aircraft, no 747s)
Correct. Under each of the puffy cottonball cumulus you see on a summer day there is an updraft. Next to the puffball is usually a downdraft of equal magnitude. In small aircraft, such as I fly, these up-and-downdrafts not only give an interesting roller-coaster effect but also help maintain the financial health of barf-bag producers. Once you get above these clouds, though, the turbulence is usually much less.
In larger clouds, such as thunderstorms, there are even bigger up-and-downdrafts. A big thunderhead sucks air into itself and upward - the updraft is what piles them up so high. You also get some other interesting air currents going sometimes, like tornados.
Stratus clouds - the flat, sheet-like ones - don’t have such updrafts. Where they sit in the sky has to do with dry air vs. moist air density and such factors as inversion layers.
There is an “evaporation threshold” as you put it - that’s where you tend to find the flat clouds. And there is turbulence in puffy clouds due to convection. Convention occurs with unequal heating. The top of the cloud is heated more by sunlight than the bottom of a cloud, the terrain underneath the cloud may radiate heat (such as sand, pavement, farmfields with relatively light colored vegetation) or absorb it (tress, lakes) so as the cloud passes over these features it is pushed and pulled from beneath, stirring the pot so to speak. The air mass of the cloud rolls around under the influence of all these changing forces.
You can also have a mix of stratus and cumulus (flat and puffy).
Weather is a natural phenomena and it’s messy.
They could be doing a lot of different things. They could be hanging out as microscopic droplets well above freezing. They could be microscopic and frozen (on the ground this is called “ice fog”). They can be snow, hail, rain, or sleet, suspended in an updraft powerful enough to keep these small collections of water aloft. They can be “supercooled” - liquid water that is actually colder than the temperatue water normally freezes. Running into these in an airplane is definately serious, since they’ll freeze the instant they strike an object. A heavy infestation can encase a plane in ice thick enough to be measured in inches in a matter of minutes. All that ice definitely has weight and can bring a plane - even a big plane - down out of the sky well before the scheduled landing. You can have all of the above in the same cloud at the same time, due to the interactions of air masses of different temperatures and conditions.
Here’s a stab at an anlogy. Take two ounces of sugar. Make half into cotton candy. Make the other half into a solid sugar cube. In both instances you have an ounce of sugar, but the cotton candy occupies a much bigger space. Put the sugar cube in your coffee it will sink immediately because it is denser than the coffee. Put the cotton candy on your coffee it will float (until it dissolves). When water sits in your glass, in puddles on the ground, and so forth it’s in a dense form. When it’s in a cloud it’s more like cotton candy, so un-dense it floats.
A way of saying this to minimize hair splitting is that
the WATER in a cloud weighs more than a 747.
OK, Broomstick, that was a pretty informative post. Thanks, and welcome aboard.
Now, I want to clarify some things. I’m counting three or four possible reasons that clouds “float”:
- Water condenses into droplets, falls, and evaporates at a lower elevation. Meanwhile, updrafts bring moist air upward, and water condenses out (and falls, etc.)
- Updrafts prevent condensed water droplets from falling.
- Clouds are buoyant (Cecil’s explanation)
- (what I’m interpreting from the above analogy) Condensed water droplets have an incredibly low terminal velocity, and are, in effect, suspended.
I sense that the real answer is a combination of (1) and (2), and Cecil is wrong. Err, right?
For shame - you know Cecil is never wrong, although his staff may occassionally not be fully informed.
Actually, all of the above are correct. Who said there had to be just one reason clouds float?
As an additional fact - water vapor is, in fact less dense than a dry mixture of nitrogen/oxygen/other trace gasses. Why I don’t know. I don’t know why solid water (ice) is less dense than liquid water but the ice cubes in my soda always float. It just is. I’m sure someone has a complicated scientific explanation but for now I’ll take it as a given.
Occassionally we pilot types have to take that into account on a very humid day, because the additional water vapor in the air makes the air on that particular day less dense than it would be on a different, less humid day. It’s usually not a huge difference, but if you’re planning to take off or land from a runway that’s already a little short for your plane it could make a significant (even if small) difference because it takes longer for a plane to get off the ground in thinner air, and the plane doesn’t climb as fast as in denser air. Real bummer if you misjudge by a couple feet and hit trees while trying to take off - snagging the landing gear in the top branches can really mess up your day, your bank account, and possibly your body.
The reason true water vapor (as opposed to suspended droplets) is lighter than dry air is easy. Water is H[sub]2[/sub]O, with a molecular weight of approximately (21)+16=18. Nitrogen is N[sub]2[/sub], with a molecular weight of approximately 214=28. Oxygen is O[sub]2[/sub], with a molecular weight of approximately 2*16=32. Air, then, has an approximate molecular weight of 29. Since all gasses, at a given temperature and pressure, have the same number of molecules per unit volume, their density is in proportion to their molecular weights. And 29 is a lot more than 18.
Just to clarify my post, I’d like to note that I never said the water droplets in clouds are always falling. Indeed, in the lenticular wave clouds I referenced, the water droplets form as the moist air is moving horizontally. While I am sure there is SOME z component to the movement, at the speed the air is travelling, by the time the droplet moves to the other side of the cloud and evaporates again, it ain’t moved too far down. 
Anybody notice that Dave the Ph.D overreached a bit when he mentioned the concept of a skydiver crashing into “the AstroTurf” at 120 mph? As Cecil posited the scenario, the skydiver was falling through a VACUUM. The commonly-accepted “terminal velocity” of a falling human is dependent on the retarding effect of the earth’s atmosphere, something you don’t find much of in a vacuum.
Just an observation…
so what r you guys up too
going to put clouds in glas and see if you can weigh them. next
and then hole thing startede with that dave disagress with cecil in that you can’t weigh clouds.
and you can’t .o(not on a weight anyway)
but cecil still said they did weigh more that a 747 so therefore are not weightless.
now what is the problem (besides dave )
Well, I think everyone agrees that clouds have mass. What intrigues me is the question of whether or not clouds have buoyancy. Do clouds float because they’re less dense than the surounding air? Or are individual cloud particles continuously falling? If the former, then one can argue that clouds have “negative weight”, in the same sense that a helium baloon does, and thus “weigh” less than a 747. This, of course, depends on the precise definition of “weight”, which, in general usage, is imprecisely defined.
And Al, please don’t take this the wrong way, but your post is a little hard to follow what with the misspellings and lack of capitalization. Since you’re posting from Denmark, I’m assuming English your second language; however, I think people would be more apt to respond to what you say if they didn’t have to puzzle out what you meant.
well.
i’ll try to write alittle slower.
and btw i think that its the idea that counts not the spelling, or do you look down at Albert.E cause he could’t drive?
I’ll say that the individual cloud particles are continuously falling slowly, and their floating is caused by the change of temprature and warmer upgoing winds. Cause if we think of a cloud in a closed box it would fall to the ground, So therefore must be affected alot by their surroundings .oO(and ohh) dot.
King_Al, since the only way you can communicated with us on the SDMB is by your writing, yes, spelling and grammar are very relevant. Otherwise how are we supposed to understand what you are saying? I personally also found your post hard to decipher.
So i’m hard to decipher, But if its any help to you,
I better on english than my primary language .oO(heheh)
btw.
“I personally also found your post hard to decipher.”
Do you mean that its okay, The way i made my 2nd post?
(If yes don’t answer this.)
So now lets go back too the clouds.
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