How Do You Explain...
This is a competition. How do you explain to an 11 year old why it's colder on top of a mountain than it is at sea level, despite being nearer the sun? Submissions will be judged on lucidity and brevity (rules out Mat then).
Competition closes 31st January 2008. Emailing your answer to me, rather than posting it as a comment will preserve the tension and stop folk plagiarising and subsequently improving on your submission. Only one entry will be judged for each contestant, but you may ask for an earlier submission to be replaced by a later one.
A prize (which will be contained in a bottle ~ but it won't be a ship or a message) will only be awarded if there are entries from more than one person. EtU's decision is final.
Answers which start off "Stands to reason dunnit..." will probably not succeed on the grounds of (inter alia) lack of brevity, as they will have wasted four words already.
EtU
With regard to Ian's explanations (see comments) I'm not entirely convinced by either, particularly the (er) scientific one. I have no problem with compressing a gas (or a liquid for that matter) generating heat, and vice versa for releasing pressure, but this is not a lasting effect. Once compressed or released the heat (or lack of it) dissipates and the gas assumes the temperature of its surroundings. If Ian's explanation held good, pressurised containers would remain permanently hot. Also, as gasses and liquids tend to behave similarly, wouldn't it be permanently warm at the bottom of the ocean (or the bottom of a mine) where there's lots of pressure?
Going back to his first explanation, I don't see why rarified should necessarily mean colder. A bottle of hydrogen at normal atmospheric pressure would be much rarer and would hold less heat than a bottle of nitrogen, but would be at the same temperature and not therefore feel any colder.
But this is helping; a germ of understanding is growing and I am now much nearer understanding and being able to articulate the reason why it's colder up there. Thank you.
The competition remains open.
EtU.
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This is the on-line journal of a small ill-defined group of mostly geriatric runners who share a love of the fells. It also covers a sub-group, best described as drinkers with a running problem, who struggle to combine dwindling athletic ability with an increasing thirst for cheap alcohol. A sort of "Last of the Summer Whine" (sic) in Walshes. Posting limited to registered contributors, but comments welcome.
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4 comments:
The warmth you feel is that of the air around you rather then the sun and as there is less air at the top of mountains than at the bottom and less air means less warmth and therefore it is colder. The irony is that at the top of mountains the sun may be strong enough to give you sunburn or snow blindness but because of the lack of air you would still be cold.
More scientifically -
When you pressurise air (or any gas), it gets hotter, and when you release the pressure on air it gets colder. So a bicycle pump gets hot when you pump up a tyre, and a spray paint can or a C02 cartridge gets cold as you release the pressurised gas. A refrigerator puts both of these processes together, pressurising gas on the outside of the refrigerator to release heat and decompressing it inside the refrigerator to absorb heat
Let me try again but brevity and simplicity are casualties
The top of the mountain is cold because it is immersed in a bath of cold air. How the air gets cold is the rest of the story.
Air is a gas mixture, mostly nitrogen some oxygen and tiny amounts of others. When a gas, including air, is compressed its temperature goes up.
The opposite effect happens when a gas is allowed to expand. A CO2 fire extinguisher can demonstrate this. Stop by a fire station (you should have asked when you did) and see if you can get a fire safety demonstration, including the discharge of a CO2 fire extinguisher. The compressed gas when it expands through the nozzle can actually freeze moisture out of the air creating a small snow shower.
The next thing we need to know is that hot air is lighter than cold air. OK, I know it sounds like I am going backward since I am trying to make it seem reasonable that mountain-top air is colder than valley air, but bear with me a while. The hot air balloon gives evidence that air which is hotter than the air surrounding it, tends to rise. When the sun shines on the surface of the Earth the solar energy raises the temperature of the ground but the temperature increase depends on how much energy is absorbed and this differs from place to place. The air over each little hot spot begins to rise and the cooler surrounding air sinks to take its place. The result of all this is cells of rising and sinking air establishing a vertical circulation throughout the lower atmosphere up to 18,000 feet or so.
The last piece of this puzzle is the effect of altitude on atmospheric pressure. At sea level a column of air one square foot in area and extending up to the edge of space weighs a little over a ton. This works out to a sea level atmospheric pressure of about 15 pounds per square inch. At 18,000 feet the pressure is about half this value so rising air is allowed to expand and falling air is compressed.
Remember that for air to keep rising it needs only be warmer than the air at its level, not warmer than air at the surface. So the air cools due to expansion as it rises and warms by compression as it falls. The net result is a decrease in temperature with increasing altitude.
I agree wiyh FS shuffler.
As someone who is ruled out of the competition,not unjustifiably, on account of my inability to adhere to the strictures of lucidity and brevity(you could have cited senility as well)from observation could I ask EtU(nsteady) the following; isn't the answer to be prepared for a 11 year-old rather than a 'O' level student? Arn't answers to be emailed rather than posted? Have you had any answers by email? I ask all of this because as you know I am well into my second childhood. As a result I am in a good position to know the type of answer that us 11 year-olds would appreciate. I could post something on the blog so that competitors would have an idea of the level at which their answers should be pitched. I can also offer an alternative solution. I await your comments with shield at the ready.
Matt
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