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MHS
2001-Dec-06, 07:17 PM
I know this has very little to do with astronomy, but I need the answer for The National Science-quiz here in Holland. Here's the question:

The weather is clear and sunny with a few clouds in the sky. Do these clouds float?

a. No, they don't float, but they fall very slowly, because water is heavier than air.

b. Yes, they float, because the specific gravity of the clouds is just a little less than that of air.

c. No, they rise slowly, because they expand heavily.

Anyone?

> Michiel <

Mnemonia
2001-Dec-06, 07:37 PM
On 2001-12-06 14:17, MHS wrote:
I know this has very little to do with astronomy, but I need the answer for The National Science-quiz here in Holland. Here's the question:

The weather is clear and sunny with a few clouds in the sky. Do these clouds float?

a. No, they don't float, but they fall very slowly, because water is heavier than air.

b. Yes, they float, because the specific gravity of the clouds is just a little less than that of air.

c. No, they rise slowly, because they expand heavily.

Anyone?

> Michiel <



I'm not an expert on this but I seem to recall the same problem from my high school days.

Wator Vapor (H20) has a molecular weight of 18, Molecular nitrogen (N2), the most common atmospheric element, is 28. So clouds are quite a bit lighter than most "air". so strike A from the list.

B is a trick. Gravity never allows things to float so it can not be a reason for why clouds stay aloft.

The correct answer is C. A sunny day will allow endothermic reactions to break down heavy molecules (like NO and NO2 into N2 and O2) and thus reduce slowly the density of the cloud, and the cloud will then rise. This is why on sunny days clouds appear higher in the sky than on rainy days and why nighttime fog rises not long after the sun comes up.

GrapesOfWrath
2001-Dec-06, 07:56 PM
On 2001-12-06 14:37, Mnemonia wrote:
B is a trick. Gravity never allows things to float so it can not be a reason for why clouds stay aloft.

Well, it was specific gravity, right? Can't ice cubes float in my drink? Gravity allows that.

CJSF
2001-Dec-06, 07:57 PM
If translated correctly, none of those answers are really correct.

Clouds "float" because they are made up of pretty much the same stuff as the air around them.

When an parcel of air is warmed, it rises and expands. This expansion causes the parcel to cool. When the water vapor in the air cools enough to reach the dewpoint, it condenses out into water droplets or ice crystals. The water droplets form on small particles in the air called condensation nuclei - these can be dust or smoke
particles for example - and become visible.

That is what a cloud is. Its characteristics are still basically the same as the surrounding air (meaning the same proportions of gases and particles), so it remains suspended.

On sunny days with "fair weather" clouds, the areas of clouds represent ares of upwardly moving air while clear areas are when air is descending and warming (this is a convection cycle). Convection also keeps the clouds "floating".

The whole process that keeps clouds up and determines their base level and top heights is rather complex. As clouds move through an air mass their bases can rise and fall, depending on the relative density/temperature of the air they encounter. Clouds are also contantly changing as water droplets evaporate and condense throughout the cloud's "life".

I suppose the words for C, "expand heavily" might refer to what I just wrote, but seem awkward to explain the process. Is that just a bad translation?

CJSF

Anyone with more of a meteorological background than myself care to help me here?
_________________
"Be very, very careful what you put into that head, because you will never,
ever get it out."
--Thomas Cardinal Wolsey (1471-1530)

<font size=-1>[ This Message was edited by: Christopher Ferro on 2001-12-06 15:06 ]</font>

MHS
2001-Dec-06, 08:21 PM
With 'expand heavily' I just mean that clouds 'grow quite big'; not that they gain weight or anything like that. Besides this I think I translaten the question correctly.

And to Grapes Of Wrath: you're right about 'specific gravity'; that is indeed what I meant.

> Michiel <

Tim Thompson
2001-Dec-06, 11:17 PM
Ferro is correct in that all 3 of the proposed answers are wrong. Clouds float because the cloud water droplets are held up by turbulent motion of the air. When the droplets get too heavy, the become "rain" and fall to the ground.

Water vapor is lighter than air, but liquid water is not. The lightness of water vaopr allows for the phenomenon that air that is actually cooler than the air around it can convect upwards anyway, if there is enough water vapor to privide a buoyant force.

The Rat
2001-Dec-07, 03:23 AM
On 2001-12-06 18:17, Tim Thompson wrote: Water vapor is lighter than air, but liquid water is not.

That's the key. Clouds are not vapour, but liquid in little tiny drops. I think the scientific term is 'teensy'.

;^)

_________________
Free speech; exercise it or shut up!

<font size=-1>[ This Message was edited by: The Rat on 2001-12-06 22:26 ]</font>

DStahl
2001-Dec-07, 05:48 AM
You guys zeroed in nicely, in my humble opinion. In the problem--sunny day, a few clouds--it's probably that just as was suggested the clouds are at the top of columns of warm rising air; the air cools at altitude and the invisible water vapor condenses into droplets at an altitude precisely defined by the base of the cloud. In that case the cloud can't fall, because if a lump of its misty self does fall off the column of rising air and descend, it will warm up and the liquid water droplets will turn back to invisible water vapor--just as someone suggested. You guys are just so smart! I'm impressed.

Other cloudulous situations might involve a sheet of warm air moving over a pool of cold air, with clouds forming where the warm air is cooled at the interface, or warm air blowing up over a mountain range and once again making clouds as it ascends and cools.

--Don

Glutomoto
2001-Dec-07, 05:56 AM
I guess most of the time i see a horse or maybe a face but this morning the very cloudy skies over st. louis, mo. reminded me of those close up images of jupiter, no giant red spot, but plenty of banded and swirling clouds.

Once i did the mind flip it was really amazing, i felt like a mountian the size of the whole sky was going to fall on me, it was great.


i guess this isn't astronomy, but hey the clouds got in the way.

MHS
2001-Dec-07, 11:26 AM
Thanx for all these answers! But there's one problem; this is an official, national thing, so I guess that one answer must be right... I think you're saying that answer 'c' comes close, am I right? Because I ('we' actually) have to choose.

> Michiel <

GrapesOfWrath
2001-Dec-07, 11:45 AM
Official national quiz? This is a take-home quiz? What happens when you're done, officially?

MHS
2001-Dec-07, 01:49 PM
On 2001-12-07 06:45, GrapesOfWrath wrote:
Official national quiz? This is a take-home quiz? What happens when you're done, officially?


This quiz is printed in a national newspaper and everyone can send in their answers. There will be a show on TV where they demonstrate/explain most of the answers. The winner will also be anounced on this program. Is this official enough for you /phpBB/images/smiles/icon_wink.gif?

> Michiel <

ToSeek
2001-Dec-07, 02:04 PM
On 2001-12-06 14:57, Christopher Ferro wrote:

When an parcel of air is warmed, it rises and expands. This expansion causes the parcel to cool. When the water vapor in the air cools enough to reach the dewpoint, it condenses out into water droplets or ice crystals. The water droplets form on small particles in the air called condensation nuclei - these can be dust or smoke
particles for example - and become visible.

That is what a cloud is. Its characteristics are still basically the same as the surrounding air (meaning the same proportions of gases and particles), so it remains suspended.


After thinking about this for a while, I decided I don't like the question much at all. As you indicate, clouds aren't a "thing" so much as a "phenomenon" - a region of the atmosphere where certain things happen rather than a chunk of water vapor. I don't think any of the answers given are good ones.

David Simmons
2001-Dec-07, 03:46 PM
On 2001-12-06 14:17, MHS wrote:
I know this has very little to do with astronomy, but I need the answer for The National Science-quiz here in Holland. Here's the question:

The weather is clear and sunny with a few clouds in the sky. Do these clouds float?

a. No, they don't float, but they fall very slowly, because water is heavier than air.

b. Yes, they float, because the specific gravity of the clouds is just a little less than that of air.

c. No, they rise slowly, because they expand heavily.

Anyone?

> Michiel <



According to the Encycloaedia Britannica clouds are created and maintained in the air by "upward moving air currents."

The water droplets are on the order of 0.01 mm in diameter so that even almost insignificant updrafts will keep them aloft.

Mnemonia
2001-Dec-10, 07:35 PM
On 2001-12-07 08:49, MHS wrote:


On 2001-12-07 06:45, GrapesOfWrath wrote:
Official national quiz? This is a take-home quiz? What happens when you're done, officially?


This quiz is printed in a national newspaper and everyone can send in their answers. There will be a show on TV where they demonstrate/explain most of the answers. The winner will also be anounced on this program. Is this official enough for you /phpBB/images/smiles/icon_wink.gif?

> Michiel <


Not to toot my own horn but if I was taking the quiz I'd still go with C. Why would the question say sunny day if a thermal reaction was not taking place? Keep in mind it doesn't take much sunlight to vaporize water droplets, making them warmer and thus more expansive than the surrounding air.

Specifically speaking though nothing floats, some things just fall faster than other. The question should have said "Do these clouds appear to float."

By the way, was the question printed in English, or did you have to translate it from Dutch? Something may have been lost in a translation to really make a difference in what answer is correct.

MHS
2001-Dec-11, 02:04 PM
I translated the question to Dutch, but I'm pretty sure it's correct. We went with C by the way. Thanx again for the answers.

> Michiel <

ToSeek
2001-Dec-11, 07:39 PM
On 2001-12-11 09:04, MHS wrote:
I translated the question to Dutch, but I'm pretty sure it's correct. We went with C by the way. Thanx again for the answers.


You will of course let us know what the "official" answer turns out to be.

MHS
2001-Dec-11, 08:21 PM
Ofcourse I will, but please stop 'teasing' me about the use of the word "official" a little too much :-s!

> Michiel <



On 2001-12-11 14:39, ToSeek wrote:


On 2001-12-11 09:04, MHS wrote:
I translated the question to Dutch, but I'm pretty sure it's correct. We went with C by the way. Thanx again for the answers.


You will of course let us know what the "official" answer turns out to be.

2001-Dec-12, 02:02 AM
a. No, they don't float, but they fall very slowly, because water is heavier than air.

> Michiel <



I think that the answer can't be boauncy. The cloud is made of little liquid droplets that are denser than air. There is water vapor in a cloud, but it doesn't refract light very much so you can't see it. By "cloud," one means liquid water droplets not water vapor. Example: If you are in a region of high relative humidity, and the visibility is fairly good, you don't say "I am inside a cloud because I am in water vapor." It is understood that "cloud=liquid water droplets." So the answer that the clouds "float" is wrong.

It is true that there are cnovection currents in the air. Parcels of air do rise up carrying droplets with them. However, convection could not keep the droplets up forever because the parcels eventually come down. A convection cell has both rising an falling air. In fact, most cumulous clouds are near the top of the convection cell where the vertical component of air velocity is slowest. So the droplets can't be said to be held aloft by air currents.

Turbulence is by definition completely random motions of air. Turbulence would carry droplets down an equal number of times than up.

The real answer is more like this. The droplets are falling. The speed of fall is limited to the terminal velocity by air friction. This is on the order of an inch per minute. It would get to the ground in a couple of hours (maybe days) if it would last that long. However, it generally doesn't.

The troposphere, which is the lowest layer of our atmosphere, almost always has a temperature that decreases with height. Indeed, that is the definition of troposphere. This is caused by the fact that sunlight is absorbed by the ground, not by the air. Therefore, heat flows upward from the ground by convection. However, the speed of an air parcel is not large near the ground so the air near the ground is warmer. This is true even at night because the ground retains heat from the sun. A higher temperature means a smaller relative humidity.

Droplets form very high in the air, in regions where the relative humidity is 100% or greater. There is more water vapor in the air than the air can carry, so condensation forms a liquid droplet.

After a couple of hours, the droplet will travel a good distance. It may be carried down by part of the convection cell, or by turbulence, or even by gravity if the air is really still. I would prefer the "clouds are falling" answer because that at least gravity has a fixed direction down.

As the droplet falls, the relative humidity goes down. Temperature up, relative humidity down. The air can contain more water vapor. Therefore, the droplet evaporates.

Water vapor is less dense than air. Therefore, the water vapor is carried up by bouancy. The water vapor can thus be said to "float," although it is not the part of the cloud one can see.

As the water vapor floats up, the relative humidity increases. At the lower temperatures, the air can no longer support the water vapor. Therefore, a liquid droplet forms at the top of the convection cell (where the humidity is equal to or greater than 100%).

So the cloud as a whole doesn't fall. The water droplet are destroyed at lower levels and form at higher levels. The convection cell only serves to speed up the back and forth motion of the water (vapor/liquid).

Thus, my answer is (a), the cloud is falling.

BTW, Velikovsky got this answer wrong./phpBB/images/smiles/icon_rolleyes.gif Read his essay, "A Universe without Gravity."


<font size=-1>[ This Message was edited by: Rosen1 on 2001-12-11 21:04 ]</font>

ToSeek
2001-Dec-12, 02:22 PM
On 2001-12-11 15:21, MHS wrote:
Ofcourse I will, but please stop 'teasing' me about the use of the word "official" a little too much :-s!



Not my intention. I only wanted to imply that just because it's official doesn't mean it's right.