Sorry if this seems like a dumb question, but...I don't get it!!
I was explaining the Water Cycle to my son the other day and got to wondering about evaporation. I understand that heat (energy) causes the molecules to move faster and break away from one another so that a liquid becomes a gas. Why, though, do those molecules not hang around closer to the earth's surface, but instead travel up into the troposphere (or higher) where they eventually can form into clouds? Even low mass objects are pulled down by gravity. So, why do these molecules move as far away from the earth's surface as they do and tend to remain up there until they recombine, glom onto particles, etc.? I guess even asked more simply - why isn't gravity pulling them back to earth?
Sorry if this seems like a dumb question, but...I don't get it!!
Sorry if this seems like a dumb question, but...I don't get it!!
Molecules with higher motion speed moving in random direction and in botom levels molecules moving with lower speed and thus more them in those levels are and molecules with bigger speed can't go through many lower speed molecules with bigger density... But going into higher level there is more space, but fewer hight speed molecules...
In a word: Displacement. The same function that allows wooden sailing ships and pieces of styrofoam to float in water. Heavier molecules displace lighter molecules as the earth's gravity pulls more on them than on the lighter molecules, thus leaving heavier molecules on the bottom, with nowhere for the lighter ones to go, but up. 
(PS, it DID seem like a stupid question at first, but when I formulated an answer, I saw how it might not be obvious.)
(PS, it DID seem like a stupid question at first, but when I formulated an answer, I saw how it might not be obvious.)
QUOTE (BigDumbWeirdo+Mar 27 2008, 06:38 PM)
In a word: Displacement. The same function that allows wooden sailing ships and pieces of styrofoam to float in water. Heavier molecules displace lighter molecules as the earth's gravity pulls more on them than on the lighter molecules, thus leaving heavier molecules on the bottom, with nowhere for the lighter ones to go, but up.
This makes sense (I had considered it, thinking of a mixture of say oil and water). I imagine, then, that the process of going from that proverbial pot of water up into the upper atmosphere isn't necessarily a speedy process. The lighter molecules are bouncing around a lot at first as they're knocking about through the heavier molecules. Some might have a clearer path and travel up fairly quickly. Others muck around a bit and eventually lose that energy. However, they will eventually filter up as the heavier molecules crowd them out.
Does that sound about right as far as a simplistic view goes?
This makes sense (I had considered it, thinking of a mixture of say oil and water). I imagine, then, that the process of going from that proverbial pot of water up into the upper atmosphere isn't necessarily a speedy process. The lighter molecules are bouncing around a lot at first as they're knocking about through the heavier molecules. Some might have a clearer path and travel up fairly quickly. Others muck around a bit and eventually lose that energy. However, they will eventually filter up as the heavier molecules crowd them out.
Does that sound about right as far as a simplistic view goes?
QUOTE (MGraser+Mar 27 2008, 01:52 PM)
This makes sense (I had considered it, thinking of a mixture of say oil and water). I imagine, then, that the process of going from that proverbial pot of water up into the upper atmosphere isn't necessarily a speedy process. The lighter molecules are bouncing around a lot at first as they're knocking about through the heavier molecules. Some might have a clearer path and travel up fairly quickly. Others muck around a bit and eventually lose that energy. However, they will eventually filter up as the heavier molecules crowd them out.
Does that sound about right as far as a simplistic view goes?
Sure, but there's one more factor I left out, (I'm not sure if it's what DavidD was talking about or not, though... I think he uses a free online translator to translate his posts into english...)
Heat. Hotter pockets of gas take up more space than colder pockets, but weigh the same, meaning that they're less dense, and hence: rise. (Heat is an emergent property of motion: hotter objects are those whose molecules are vibrating faster.)
So hotter pockets of gas rise up (that's why all the water vapor in clouds. The sunlights heats water until it produces steam, which (being hot) rises into the atmosphere, where it cools and condenses into clouds, which condense even further until they produce rain).
Does that sound about right as far as a simplistic view goes?
Sure, but there's one more factor I left out, (I'm not sure if it's what DavidD was talking about or not, though... I think he uses a free online translator to translate his posts into english...)
Heat. Hotter pockets of gas take up more space than colder pockets, but weigh the same, meaning that they're less dense, and hence: rise. (Heat is an emergent property of motion: hotter objects are those whose molecules are vibrating faster.)
So hotter pockets of gas rise up (that's why all the water vapor in clouds. The sunlights heats water until it produces steam, which (being hot) rises into the atmosphere, where it cools and condenses into clouds, which condense even further until they produce rain).
QUOTE (BigDumbWeirdo+Mar 27 2008, 07:02 PM)
Heat. Hotter pockets of gas take up more space than colder pockets, but weigh the same, meaning that they're less dense, and hence: rise.
OK! I think I almost have the complete picture now. I should have picked all this up in school years ago, but I wasn't interested back then...
So, the reason that the hotter pockets take up more space is again due to their motion. When they vibrate faster, they break away from the other molecules which means they tend to head away from each other. This causes the pocket of heat to expand - thus taking up more space with the same number of molecules (resulting in less density). Even an individual molecule that disperses from the main pocket is vibrating faster and will take up more space than a molecule that is vibrating more slowly, meaning that even that space within which it is vibrating is less dense also.
The difference in density is where the rules of displacement kick in and those molecules rise, eventually condensing.
Yeah?
OK! I think I almost have the complete picture now. I should have picked all this up in school years ago, but I wasn't interested back then...
So, the reason that the hotter pockets take up more space is again due to their motion. When they vibrate faster, they break away from the other molecules which means they tend to head away from each other. This causes the pocket of heat to expand - thus taking up more space with the same number of molecules (resulting in less density). Even an individual molecule that disperses from the main pocket is vibrating faster and will take up more space than a molecule that is vibrating more slowly, meaning that even that space within which it is vibrating is less dense also.
The difference in density is where the rules of displacement kick in and those molecules rise, eventually condensing.
Yeah?
Air containing water vapour is lighter than dry air, because water molecules weigh 18g instead of 29g for air as a mean. So vapour-charged air climbs.
As the air expands with altitude and lower pressure, it cools down up to a point where the vapour begins to condense. Then, the air+liquid becomes denser. Usually, it makes a cloud called a cumulus and stops climbing due to higher density. This explains why all cumulus have a flat base, all at the same altitude (500-3000m): it depends only on the vapour content and the air temperature.
Under special conditions, the condensation of vapour releases enough heat to keep the air warm and thus light, and the air+vapour+liquid can go on climbing to 10km in the surrounding cold air. This is a cumulo-nimbus, the cloud of electrical storms.
Both conditions can also be man-made.
The cooling tower of a power station evaporates some water at the basis of the tower to cool its own circuits. The huge air movement needed would cost enormous amounts of power to blow, but is obtained for free because air+vapour naturally climbs in the tower.
Some years ago, I read the description of a balloon which used damp air to climb in the atmosphere, allowing the condensed water to rain down as the balloon climbed.
When the condensing vapour is replaced by hot dust that heats the air, you get a dust devil, able to suck more dust as it moves across the soil.
Over a warm ocean, the hot air may suck more vapour, and you get a hurricane.
As the air expands with altitude and lower pressure, it cools down up to a point where the vapour begins to condense. Then, the air+liquid becomes denser. Usually, it makes a cloud called a cumulus and stops climbing due to higher density. This explains why all cumulus have a flat base, all at the same altitude (500-3000m): it depends only on the vapour content and the air temperature.
Under special conditions, the condensation of vapour releases enough heat to keep the air warm and thus light, and the air+vapour+liquid can go on climbing to 10km in the surrounding cold air. This is a cumulo-nimbus, the cloud of electrical storms.
Both conditions can also be man-made.
The cooling tower of a power station evaporates some water at the basis of the tower to cool its own circuits. The huge air movement needed would cost enormous amounts of power to blow, but is obtained for free because air+vapour naturally climbs in the tower.
Some years ago, I read the description of a balloon which used damp air to climb in the atmosphere, allowing the condensed water to rain down as the balloon climbed.
When the condensing vapour is replaced by hot dust that heats the air, you get a dust devil, able to suck more dust as it moves across the soil.
Over a warm ocean, the hot air may suck more vapour, and you get a hurricane.
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