# Air getting thinner and thicker - what's the Straight Dope?

Answers for an 8-year old are welcomed.

I put a half-full bottle of liquid in the freezer, and it constricts in volume after a while. I guess it’s the air molecules being less active and coming closer together. You go up high and it gets colder, for reasons I don’t know, and the air gets thinner. Gravity? But it’s colder up there so the air should be less active and thicker, how does the air at the top of Everest become colder and thinner? Shouldn’t colder air become more dense, because of my bottle in a freezer constricting until I open it?

The bottle in my freezer contains no gasses, just a pretty inert liquid which is white wine. If I leave it too long, usually after passing out, it’ll then expand and possibly break the bottle and leak frozen wine all over my freezer. What is this bigger/smaller liquid and air phenomenon doing above and below freezing that seems to not apply to most other substances?

Air gets thinner on mountaintops because it’s under less pressure, and it’s under less pressure because there’s less air on top of it pushing it down.

Your wine breaks the bottle in the freezer because wine is mostly water, and water has the unusual (though not quite unique) property that it expands when it freezes (most substances contract when they freeze). This is because of the particular shape of the crystals of ice, which has the molecules held at a very low packing efficiency.

ETA partly ninja’d by excellent Chronos.

There are lots of interesting physics in the stuff you describe. And several disconnected ideas that you’ve mistakenly glued all together.

Let’s start with a couple of basics and see what you think. Everything I’m saying is simplified a bunch and ignores a lot of details.

Idea 1:

There’s a basic relationship with gases that PV=nRT or with some algebra PV/T=nR. Where P means pressure, V means volume and T means temperature. You can think of nR as how much gas is actually there.

If we put a bunch of gas in a sealed container then nR is a constant. So whatever we do next, PV/T is also constant.

If we heat it, T gets bigger. If we double T then to keep PV/T the same, that means PV has to double as well. Which can either be the pressure increasing; pushing harder on the sealed container walls, or volume increasing, like a sealed elastic balloon expanding, or some combo of both.

Likewise if we start from a steady state and squish the volume, pressure goes up and or temperature goes up to keep PV/T constant.
Next idea.

The atmosphere is not in a sealed container. The pressure we feel at sea level (14.7 psi officially) is the result of all the air stacked up to the “top” of the atmosphere 100ish miles up. Because air is compressible there’s more air molecules in a cubic foot of air at sea level than on Everest. If you make a tall stack of books, the bottom book is more squeezed than the top one. Pretty obvious. If you made a tall stack of real soft squooshy foam pillows, the bottom one would be squished much flatter than the second from the top one. Gas is the squooshiest of squooshy foam pillows. So the bottom layers of a gas are lots compressed versus layers near the top.

If you climb Everest the air is less dense; there’s less air per cubic foot. The air is also under less pressure since there’s so much less above you pushing down on the air where you are. For real round numbers on Everest you’re above 90% of the air, even though you’re only 6 miles above sea level and the atmosphere extends up to around 100 miles before it gets really, really negligibly thin. You’ve covered 6/100ths of the distance to the “top” and you’re above 90/100ths of the gas already.
Next idea.

The atmosphere is heated by the Sun. But what really happens is the light and heat flow down through the atmosphere without heating it hardly at all. The light and heat from the sun then heat the land and the sea. Those things in turn heat the air. So like a pot on a stove, the atmosphere is heated from below, not from above. Which is why it’s hotter at sea level and colder on a mountain top.

Air gets cooler the higher you go up, too. Why is that when the same sun still shines on higher grounds? They’re even nearer to the sun!

If you notice it’s nice and warm in Albuquerque or Denver in the summer. ABQ regularly gets above 100F and DEN is often in the upper 90F range. Despite both being about 5000 feet above sea level, 1/6th of the way to the height of Everest. So you’re right there’s more going on than in my first post.

The key thing about a mountain is it’s a small bump sticking up into what’s otherwise empty air. Most of the air around a mountaintop is way above the level of the valley below.

Imagine a 5000’ peak on a small volcanic island. Same height as Albuquerque. As the wind blows more or less horizontally, the air that was 5000 feet above the sea or the beach blows by the peak. It’s cold air because it’s 5000 above the heat source below: the ground or ocean. Yes, it’s being heated by the land right here on the mountaintop. But that’s a tiny little bump of land in what’s otherwise a vast blob of air a mile above the heat source.

For round numbers the temperature of Earth’s atmosphere declines at a rate of 3F (or almost 2C) per 1000 feet. So we’d expect that whatever the temp at the beach is, it’d be 15F cooler on our peak. Plus a small fraction of a degree for the heating taking place right there due to the warm rocks of the peak itself.

There’s more going on, but this is an easy idea to grasp most of it.

So how come when drillers go really deep down, it gets hotter and hotter? Surely all of the earth that stops the Sun’s heat would have been absorbed or reflected when you’re a mile under the ground?

The earth’s crust generates quite a lot of heat from radioactive decay of stuff in the crust, so it’s hot regardless of the sun. The bad news is that all of that radioactive stuff will run out in a few million years.

Actually, all this radioactive stuff all came from the supernova dust our solar system condensed out of … a fairly sizable chunk of it has already decayed … however it takes time for the energy to work it’s way to the surface and radiate away from the Earth … no need to worry about the Earth dynamo freezing up before our Sun expands and boils us off into interplanetary space … you’re still okay buying unripe bananas …

It’s the radiation? I always thought is was, I don’t know, closer to the boiling innards of the earth. I’m sure I embedded this thought when as a kid on the (active) volcanic island of Lanzarote they showed tourists how you can boil an egg in the ground.

What is at the center of the earth, then?

A sphere of superheated Iron-Nickle liquid (or semi-liquid).

thnx. from that cite:

Temperature and pressure
The temperature of the inner core can be estimated by considering both the theoretical and the experimentally demonstrated constraints on the melting temperature of impure iron at the pressure which iron is under at the boundary of the inner core (about 330 GPa). These considerations suggest that its temperature is about 5,700 K (5,400 °C; 9,800 °F).[3] The pressure in the Earth’s inner core is slightly higher than it is at the boundary between the outer and inner cores: it ranges from about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm).[12] Iron can be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (see the Clausius–Clapeyron relation).[13]

A report published in the journal Science [14] concludes that the melting temperature of iron at the inner core boundary is 6230 ± 500 K, roughly 1000 K higher than previous estimates.

So when do the two thermoclines (starting here and starting from surface as mass increases) intersect?

The reason it’s hotter at the bottom of a deep hole or mine IS that it’s closer to the very hot center of the Earth.

The next question is WHY it’s hotter at the center. The answer is compressional heating left over from the planet’s formation, plus radioactive decay heating left over from the early era.

That question does not follow (that I can see) from that cite. Which two thermoclines do you think you’re talking about?

If you mean residual heating conducting up from below versus solar heating conducting down from above the two lines cross a few feet below the surface. That’s where weather stops having an effect and you get below the frost line.

uh…I just read that because of all the OIL IN THE ATMOSPHERE, which we are all breathing, world intelligence is going DOWN.ALL over the world. yeah, we’re gassing ourselves into the film “Idiocracy.” they have it figured out, this will keep going, our IQs will be way down in 20 yrs. pretty far. guess what happens to a big planet of too many STUPID PEOPLE? GUESS!!!

ITS CALLED “SAVAGERY.”(SPELLING? I live next to a 5-lane highway, mine is going fast) we will also have MORE PEOPLE, less food and water. go ahead, guess!! RIGHT!! ENDLESS WAR!!! you’ll even get so dumb, frustrated, unable to work, do stuff we can do now, we’ll be close to dementia–or retardation. And with that many angry helpless people, i do not wanta be around. I am 70 now, next summer. i do not regret never having had kids now. I am (was) one of the very smart. i guess dumb people are happier, they can’t see all the s— going on.

Yes you understood the query–and the source of it–correctly. I just wanted to correct the initial over emphasis on radiation from the ground material by radioactive decay, independent of current solar irradiation, in this thread about temperatures/heat --although interesting in its own right.

Capital letters at the start of sentences will be the first to go …

What’s interesting is that the atmosphere is heated from the bottom … solar energy is absorbed by the Earth and re-radiated to the very lowest levels of the air. This is what causes the atmosphere to circulate vertically. The ocean is heated at the top surface, so the oceans are actually warmer above than below, which is called a “temperature inversion” and vertical circulation is inhibited.

Addendum to above: re-reading my post above, I see the " over-emphasis" was
mine only. I was reacting to a post that explicitly states (although rather obliquely for a too-fast skim-- not “to worry” about “unripe bananas.”

Meaning Grand Central Station and, more often cited in this context, Hoover Dam, are hotter than outside (except for everything else) because of the radiation from all that concrete.

It was the “unripe” that threw me. Are you saying the Banana -equivalent dosage standard must be amended?

Because most of the energy in sunlight is in the form of visible light. Most of it goes straight through the atmosphere without being absorbed (i.e. without warming up the air). Or it gets reflected by clouds and go back out into space - again, without warming up the atmosphere.

If/when sunlight reaches the ground, it does get absorbed there, warming up the ground. Which warms up the atmosphere around it through conduction, but also by re-radiating the heat in the form of infrared - which DOES get absorbed by the atmosphere.

Each layer of the atmosphere receives infrared from the ground, but also from the amosphere above and below. It also LOSES heat by emitting infrared. The layers closest to the ground is warmest because it receives infrared from the ground AND all the atmosphere above it. The layers higher up receive less from above, and is therefore colder. (The infrared received from below is pretty much the same, because the infrared from the ground is blocked by the atmosphere below, and replaced by the infrared radiation from that atmosphere.)

Once all the radionuclides have decayed into stable elements AND all the heat energy from the original gravitational collapse have Second Law of Thermodynamicked out into space … supposedly the dynamo that creates the Earth’s magnetic field will also decay and be lost. This may have the effect of making Earth inhospitable to life what with all the direct exposure to solar radiation and some of her more corrosive aspects …

… but that won’t happen for at least a few days … so you’ll be safe buying unripe bananas at market since we have the time for them to ripen up and eat them …

I using the time standard of “buying unripe bananas” as a facetious metaphor when anyone points out some dread catastrophe that will happen in several billion years from now … we should probably be more worried about people using thermodynamic as a verb than the Earth’s internal heat disappearing …

“Don’t buy unripe bananas” is a droll way of saying you’re going to die soon (i.e. before the bananas ripen).