The Earth, Our Atmosphere, And You On A Mountain

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The Earth, Our Atmosphere, And You On A Mountain

The Earth, Our Atmosphere, And You On A Mountain

Page Type: Article

Activities: Mountaineering

Page By: Florida Frank

Created/Edited: Aug 27, 2010 / Sep 1, 2010

Object ID: 654545

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Page Score: 88.99% - 25 Votes

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Table of Contents

Introduction

Anyone who has climbed a major mountain is very aware of two climatic phenomena. 1) It’s harder to breathe the higher you go. 2) It gets colder the higher you go. If you are camping on the mountain add 3) it takes a lot longer to cook most food the higher you go. Though readily accessible, scientific values regarding these phenomena are often better understood by scientists, engineers, and meteorologists and less well known in the mountaineering community. I thought it would be worthwhile to post a table and few graphs with some basic explanation for future reference. Data referenced for this brief article are taken from the “CRC Handbook for Chemistry and Physics”, 76th edition. With apologies to the rest of the world, I’ve presented most of the data in American engineering units, i.e. – psi, degrees Fahrenheit.

Atmospheric Pressure

Air pressure at sea level is 14.7 pounds of force per square inch. This is basically because you have very tall column of air above and around you that exerts pressure (force per unit area) in all directions. As you ascend higher in elevation, there is less mass of air between you and space. Pressure and mass concentration of gases correlate linearly. The pressure and concentration of oxygen, nitrogen, and everything else found in air decreases as you ascend. At 2500 m or 8200 ft, it’s about three-quarters that found at sea level. At 5500 m or 18040 ft, it’s about half. Combine this with the fact that air is only about 21% oxygen. There’s a good reason why it’s hard to breathe up there! Not wishing to ignite a debate on global warming, but we would have no oxygen to breathe if it weren’t for plants. So, if you love your oxygen, plant a tree! The curve shown in the figure below illustrates how pressure drops with altitude. Of course weather fronts, hurricanes, and other climatic disturbances can also influence atmospheric pressure. In the grand scheme of things the curve below is pretty accurate, and is exactly how your barometric altimeter provides you with an elevation value.

Thanks to "Coldfoot", who has provided the following information about latitude dependence on air pressure at altitude:

"Just to add a subtlety about the lapse rate - it applies in the troposphere where the atmosphere transports energy convectively, and more or less for that reason, the pressure variation with altitude depends on the overall temperature. The standard atmosphere applies around mid-latitudes. Closer to the poles, the atmosphere is colder and the pressure drops off more quickly with altitude. This underlies the idea that the air is thinner at some altitude on Denali than at the same altitude in the mid-latitudes. The sea level pressure is the same in Alaska, but the pressure falls off faster with height.

Some diagrams:

Diagram 1

Diagram 2"

Water Boiling Point

As air pressure decreases, the temperature required to boil water decreases. A liquid being heated cannot exceed its boiling or phase change temperature. Much of our cooking relies upon moisture and temperature to convert raw food into something we can both enjoy and digest. As the boiling temperature drops, it takes much longer to cook many items. This is the guiding principle behind the pressure cooker. Increase the pressure, increase the boiling point, and you reduce the cooking time. Unfortunately those of us without Sherpas or llamas can’t readily haul a heavy kitchen appliance up the mountain with us. This is the reason why it becomes practically impossible to cook meals like rice and pasta at very high altitude. The graph below illustrates how the boiling point of water drops with altitude.

Temperature

Temperature drop with altitude is much less precise than either air pressure or boiling point, but some rules of thumb can still be applied. There is a thermodynamic term called the adiabatic lapse rate. The basic concept is that if a gas loses pressure or expands without heat transfer, it will cool. This strictly doesn’t apply to climbing a mountain, as the same air doesn’t accompany you up the hill. The more accurate factor for an increase in altitude is the environmental lapse rate. This predicts a 3.6-degree F drop in temperature for every 1000 feet of elevation gain for dry air. There are a several caveats to be applied to the temperature drop rule of thumb: (1) It’s for dry air. Humid air behaves differently. (2) It often works reasonably well if you are in an airplane. If you are standing on a large land mass, i.e – a mountain, the thermal mass of the mountain can greatly affect the local temperature. (3) Air often does not behave ideally. The best example that many of us have observed in the colder months is a temperature inversion. Cold, still air can settle into valleys overnight. In these conditions, the mountain top may be warmer than the valley floor. All things considered, the following graph can still be a useful rule of thumb when planning gear for your next outing.

Images

Disappointment Cleaver - Mt. Rainier - Mar., 1982

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nader

Great Article

Voted 10/10

I was still in high school when I got my first (old style) altimeter. I became very interested in finding a formula that would calculate the barometric pressure as a function of elevation. I came up with various formulas that gave an approximate answer. This is a formula that I came up with myself:

Pressure=Square root of (0.01664 hXh+1067089)-0.109h
Where h is in meters and Pressure in milibars
For example for 1000 m, P=932 mb

It wasn't until I went to college when I came across this formula that gives the exact number:

P=(e) to the power of (-0.000125h)

where e is the natural log function
h in meters and P in atm

Posted Aug 28, 2010 11:58 am


	nader	Re: Great Article
	Voted 10/10	Looking at the numbers in your chart, they seem to be a little off. To begin with, at sea level, air pressure is 76 cm mercury. With a density of 13.6 grams/cm3 for mercury, the pressure becomes 13.6X76=1033 mb. I am not sure why your chart says 1013 mb. Likewise, for 1000 m elevation, both the second formula that I mentioned and my old altimeter show a pressure of 912 mb, your chart says 899.
		Posted Aug 28, 2010 3:48 pm


	Florida Frank	Re: Great Article
	Hasn't voted	Thanks for your comments. As I referenced, the values for the chart are taken from the CRC Handbook of Chemistry and Physics. It's a standard reference text. I appreciate your interest! I re-checked the reference. The base document the data are taken from is "U.S. Standard Atmosphere, 1976", published by US NOAA, NASA, and the Air Force. It defines the representation as "idealized, steady state, and during a period of moderate solar activity". Also, in the truth of full disclosure of references, the boiling point data aren't directly from the CRC handbook. I took the air pressures and interpolated saturated steam temperature from a steam table. OK, enough techno-speak for me :-)
		Posted Aug 29, 2010 2:44 pm


	nader	Re: Great Article
	Voted 10/10	Thanks for the reply. I did not mean to be nit picky. I especially enjoyed the info about the boiling point of water at various elevations.
		Posted Aug 29, 2010 6:47 pm

Sierra Ledge Rat

Thanks

Voted 10/10

It would be a lot easier if the rest of the world switched to Imperial units instead of metric. (:

Posted Aug 28, 2010 10:29 pm

coldfoot	Latitude dependence
Hasn't voted	Just to add a subtlety about the lapse rate - it applies in the troposphere where the atmosphere transports energy convectively, and more or less for that reason, the pressure variation with altitude depends on the overall temperature. The standard atmosphere applies around mid-latitudes. Closer to the poles, the atmosphere is colder and the pressure drops off more quickly with altitude. This underlies the idea that the air is thinner at some altitude on Denali than at the same altitude in the mid-latitudes. The sea level pressure is the same in Alaska, but the pressure falls off faster with height. Some diagrams: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter8/graphics/500mb_np_eq.free.gif http://apollo.lsc.vsc.edu/classes/met130/notes/chapter8/isobar_chart.html
	Posted Sep 1, 2010 1:25 am


	Florida Frank	Re: Latitude dependence
	Hasn't voted	Coldfoot, this is a great addition, of which I was unaware. I hope you don't mind that I added your note to the article text. Thanks much!
		Posted Sep 1, 2010 12:45 pm


	FortMental	Re: Latitude dependence
	Hasn't voted	These graphs are really worth putting on this page....only because they serve to clear up a common misconception: that higher latitudes see lower air pressures. This is only a half-truth because at sea-level, in the norther hemisphere, it is mostly untrue. However, as these graphs make clear, it is true at increasing altitudes. For sea-level air pressures as a function of latitude, see here: http://wapedia.mobi/en/Atmospheric_pressure http://www.climate4you.com/AirPressureVariations.htm For reasons: http://geography.about.com/od/climate/a/highlowpressure.htm Synopsis: Subtropical High-Pressure Cells: Located between 20° N/S and 35°N/S this is a zone of hot, dry air that forms as the warm air descending from the tropics becomes hotter. Because hot air can hold more water vapor, it is relatively dry. The heavy rain along the equator also removes most of the excess moisture. The dominant winds in the Subtropical high are called westerlies. Subpolar Low-Pressure Cells: This area is at 60° N/S latitude and features cool, wet weather. The Subpolar low is caused by the meeting of cold air masses from higher latitudes and warmer air masses from lower latitudes. In the northern hemisphere, their meeting forms the polar front which produces the low pressure cyclonic storms responsible for precipitation in the Pacific Northwest and Europe. In the southern hemisphere, severe storms develop along these fronts and cause high winds and snowfall in Antarctica. Polar High-Pressure Cells: These are located at 90° N/S and are extremely cold and dry. With these systems, winds move away from the poles in an anticyclone which descends and diverges to form the polar easterlies. They are weak however because there is little energy available in the poles to make the systems strong. The Antarctic high is stronger though because it is able to form over the cold landmass instead of the warmer sea.
		Posted Sep 3, 2010 11:48 am

bdynkin	Latitude dependence: Denali - Bolivia comparison
Hasn't voted	I climbed both Denali and a couple of 6K summits in Bolivia so your article prompted me to compare the actual atmospheric pressure in these locations. Luckily, there were semi-permanent weather stations on Denali (5700m), Illimani (6265m), and Sajama (6542m) on some years. It took me maybe 2 minutes to find the actual data for these places. Look here if curious: http://www.iarc.uaf.edu/mt_mckinley/mt_mckinley_weather.php http://www.geo.umass.edu/climate/bolivia/data.html My conclusion after a brief look at the data? Well, in May/June it typically feels 500 to 600 meters higher on Denali compared to the Bolivia mountains! Whoa! Also, there are pretty extreme pressure variations on Denali between high and low pressure weather events. So summiting Denali on a good day versus bad day could be a difference of up to 700 feet of altutude equivalent!
	Posted Sep 10, 2010 10:45 am

oldandslow

Orographic Precipitation

Voted 10/10

A third phenomenon is the increased liklihood that you will be rained or snowed upon. As the wind blows moist air toward the mountains, the air rises, expands and cools, thereby reducing its capacity to hold water. The result is orographic precipitation. How many time have you returned wet and cold from the mountains and found dry pavement on the flatlands? Probably too many if you live on the west side of the Cascades.

Posted Sep 30, 2010 5:24 pm


	Florida Frank	Re: Orthographic Precipitation
	Hasn't voted	Yes, orthographic precipitation is a fascinating phenomenon. I've always been struck by ranges like the Cascades and Sierras that are so wet on the side of the prevailing wind and on top and then have a "rain shadow" out of the mountains on the opposite side.
		Posted Oct 1, 2010 10:05 am

oldandslow

Rain Shadows

Voted 10/10

There are some remarkable rain shadows along the Pacific Coast. The most dramatic that I know of is on the Olympic Penninsula where the western slopes of the Olympic Mountains approach 200 inches of rainfall annually and the town of Sequim on the east side has less than 20 inches. (Do you really mean "orthographic?)

Posted Oct 1, 2010 2:35 pm


	Florida Frank	Re: Rain Shadows
	Hasn't voted	Yes, I've always seen it referred to as "orthographic". Thanks for commenting!
		Posted Oct 13, 2010 8:31 pm

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