Atmosphere thinner at the poles?

[dtilque]

I’m reading something about a bicycle ride to the South Pole. It says:

The plateau is at an elevation of roughly 8,500 feet but it feels more like 12,000 feet to your body because the Earth’s atmosphere is thinner at the North and South Poles.

I’ve never heard this before and I’m fairly certain this is total nonsense. Am I right or wrong?

The source, for those interested, is an article in a Bontrager catalog.

[Broomstick]

Nope, not nonsense.

The atmosphere really is slightly thinner at the poles vs. the equator.

The Earth is also of a slightly greater diameter when measured at the equator as opposed from pole-to-pole circumference.

[dtilque]

Yes, I know the polar diameter is less, but the atmosphere should be the same everywhere on the surface, modulo altitude above sea level. Sea level is an equipotential surface, so the atmosphere should be the same everywhere on it.

[sbunny8]

It makes sense to me that relevant question is whether the forces which make the solid Earth bulge at the equator would have a lesser or greater effect on the atmosphere than they do on the rock. I find it easy to believe that the effect would be greater on the atmosphere and it would bulge even more than the rock does.

[Kimstu]

@dtilque

Yes, I know the polar diameter is less, but the atmosphere should be the same everywhere on the surface, modulo altitude above sea level.

Nope. Temperature differences, primarily, result in huge variation in atmospheric thickness (nearly three times as thick at the equator as at the poles).

@sbunny8

It makes sense to me that relevant question is whether the forces which make the solid Earth bulge at the equator would have a lesser or greater effect on the atmosphere than they do on the rock. I find it easy to believe that the effect would be greater on the atmosphere and it would bulge even more than the rock does.

Mostly, AFAICT, it’s about temperature differences rather than rotational forces:

The gravitational pull of the Earth holds a veil of gases in close proximity; these gases are collectively known as the atmosphere. The primary gases are nitrogen and oxygen; other gases, including argon and carbon dioxide, occur in much smaller amounts. […] Although it extends many kilometers (miles) above the Earth’s surface, there exists no definitive upper limit of the atmosphere; rather, it gradually merges with outer space. […] In order of closest to the surface to farthest away, these layers are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. […]

Generally, depth and thickness of the atmosphere are measured in terms of its lowest layer, the troposphere. The troposphere extends from the Earth’s surface upward to between about 6 and 20 kilometers (4 and 12 miles). The exact extent depends on location. The troposphere is shallowest – or narrowest – at the poles, and deepest – or thickest – at the equator. At the geographic North and South Poles, the troposphere reaches only 6 kilometers (4 miles) high, while at the equator, it extends nearly 20 kilometers (12 miles) high. Depths and degree of thickness gradually shift between these two extremes in the middle latitudes.

[dtilque]

Wouldn’t the cold temperatures make the atmosphere denser at the poles? If the atmosphere is ballooning out in the tropics, that should make it less dense, just like heating in a desert makes a thermal low pressure area.

[Kimstu]

Oh, and getting back to the OP’s specific question:

@dtilque

I’m reading something about a bicycle ride to the South Pole. It says:

The plateau is at an elevation of roughly 8,500 feet but it feels more like 12,000 feet to your body because the Earth’s atmosphere is thinner at the North and South Poles.

I’ve never heard this before and I’m fairly certain this is total nonsense.

Well, not to make you feel bad that a Bontrager catalog is smarter (or at least better-informed) than you, but no, it’s not nonsense. It’s to do with the difference between physical, geographic altitude and “pressure altitude”:

[…] just like wind and humidity affect the temperature that you feel; latitude, temperature, and weather affect the altitude that you really feel […]

Standard altitude-pressure tables allow mountaineers and aviators to determine their approximate height by measuring atmospheric pressure. Inversely, the height of a mountain approximately determines the pressure of the air on its summit. This general relationship usually works quite well; however, factors other than physical altitude, such as latitude, temperature, and weather also effect the air pressure and contribute to how you truly feel at a particular elevation. […]

Latitude:
Just like there is no uniform sea level, there is no uniform atmospheric pressure throughout the world. The lowest portion of Earth’s atmosphere, the troposphere, is wider at the equator (10 mi) than at the poles (5 mi). So the further north or south you go from the equator, the lower the air pressure will be at a given altitude. Contrary to popular belief, this variation in the troposphere is only slightly attributable to the gravitational shifts at higher latitudes due to the Earth’s rotation and shape. The lower air pressure is caused almost exclusively by the lower temperatures encountered at more northern and southern latitudes. […]

Temperature:
Since cold air is more dense than warm air, at low temperatures the entire atmosphere is compressed downwards. Therefore, a specified air pressure will lie at a lower altitude in a cold environment compared to a warmer one. […]

Weather:
Anyone who has ever owned an altimeter knows that you can gain or loose a few hundred feet just by sleeping in your tent overnight. This is due to high or low pressure weather systems. Because of thermal activity, air rises and falls within the atmosphere which changes the ambient pressure.

[Kimstu]

@dtilque

Wouldn’t the cold temperatures make the atmosphere denser at the poles? If the atmosphere is ballooning out in the tropics, that should make it less dense, just like heating in a desert makes a thermal low pressure area.

See previous post. Colder temperatures lower the altitude where a given atmospheric pressure occurs. That’s why your source says that a polar plateau that’s physically only 8000 feet feels like a 12000-foot elevation would in a warmer climate.

[Kimstu]

And furthermore: Remember that atmospheric pressure isn’t identical to atmospheric density, especially locally. Atmospheric pressure, which is what determines how readily you can take in oxygen, is determined by the total amount of the air above you, not how dense the air is right around you.

So in warmer, less dense air, you can go up quite a ways and still have a lot of the air mass above you: namely, higher atmospheric pressure. If you go up the same amount of elevation in colder denser air, you’ve got a larger amount of the air mass below you: namely, lower atmospheric pressure. Consequently, your body’s more stressed at the same elevation in the colder region than in the warmer region.

[watchwolf49]

@dtilque

[snip] … Sea level is an equipotential surface, so the atmosphere should be the same everywhere on it.

This is true … but the OP speaks of 8500 feet in elevation … that’s not equipotential … the atmosphere is more oblate a spheroid than the water is …

[dtilque]

OK, I get that the atmospheric pressure at the poles decreases faster with altitude. And that’s what they really meant.

So the text I was objecting to is guilty of being misleading by using the wrong term. “Thinner”, when applied to air, means lower pressure. When you talk about thin air up in the mountains, you mean the pressure is lower. The North Pole is at sea level, so its pressure is going to be the same as at sea level in the tropics (modulo local variation). If the South Pole were at sea level, ditto.

[Richard_Pearse]

The atmosphere is thinner in both ways. The pressure drops off quicker AND the layer of air is not as deep physically as at the equator. The troposphere (layer of atmosphere where temperature drops with height and all the weather happens), is about 7km at the poles in winter and 20 km at the equator.

Troposphere

[Kimstu]

@dtilque

So the text I was objecting to is guilty of being misleading by using the wrong term.

Don’t think so; everybody else here seems to have understood it correctly. I even went and found what appears to be the article itself just to double-check, and the author seems to have expressed it accurately and clearly.

[QUOTE=dtilque]

“Thinner”, when applied to air, means lower pressure.

[/quote]

Which is exactly what the article is talking about. Namely, a certain elevation (say, 8000 feet) at the pole has lower atmospheric pressure than the same elevation in warmer regions, so it feels like a higher elevation.
You know, dtilque, it’s no shame to initially misunderstand something because you’re ignorant about it, and it’s not much shame to ignorantly proclaim your certainty that the thing you misunderstood is “total nonsense”. But trying to backpedal from your gratuitously arrogant error by proclaiming your source “guilty of being misleading” is taking you farther into shame territory.

[Broomstick]

@dtilque

Wouldn’t the cold temperatures make the atmosphere denser at the poles? If the atmosphere is ballooning out in the tropics, that should make it less dense, just like heating in a desert makes a thermal low pressure area.

Yes, that is indeed the case. Cold air is denser air.

The effect is large enough that pilots make adjustments in flight plans in regards to take offs and landings with even relatively small temperature differences. Back when I was flying small planes it might take twice the time and distance* for me to take off in the same airplane on the same runway in summer as opposed to winter.

• Roughly. Don’t want to bog down the thread in minutiae.

[watchwolf49]

@Broomstick

[snip] … The effect is large enough that pilots make adjustments in flight plans in regards to take offs and landings with even relatively small temperature differences. Back when I was flying small planes it might take twice the time and distance* for me to take off in the same airplane on the same runway in summer as opposed to winter … [snip]

It’s common in Southern California that when there’s a forest fire the weather is very hot in the lowlands where the airports are … these borate bombers are loaded to the max and seem to use every last inch of the runout strip at the end of the runaway … have to quickly retract their landing gear to keep from tangling with the cyclone fence … and then just roaring a hundred feet over the houses … obviously the pilots include a margin of error but it sure doesn’t look like it …

[dtilque]

@Kimstu

Don’t think so; everybody else here seems to have understood it correctly. I even went and found what appears to be the article itself just to double-check, and the author seems to have expressed it accurately and clearly.

So I’m wrong when I contend that the air pressure at the North Pole is the same (modulo local variation) as in the tropics at sea level? If so, can you give a cite?

[Richard_Pearse]

@dtilque

So I’m wrong when I contend that the air pressure at the North Pole is the same (modulo local variation) as in the tropics at sea level? If so, can you give a cite?

No, that’s not wrong. But the moment you get away from sea level the pressure will be lower (this is an immediate effect and is not just noticeable at high altitudes). The difference is significant and causes a pressure altimeter to over-read by about 8% for every 10 degrees C colder than ISA. In winter at the South Pole the ISA deviation is approximately -70ºC (ISA is 15ºC, South Pole winter temp is about -55ºC). This means a pressure altimeter situated at head height (say 6’) will tell you it’s at 9’ and at 100’ it will say it’s at 150’. So aside from the specific case of sea level, the atmosphere is thinner both in terms of it’s physical thickness and the pressure.