From: Paul Doherty (pauld@exploratorium.edu)
Date: Fri Aug 20 2004 - 18:53:21 PDT
From: "Paul Doherty" <pauld@exploratorium.edu> Subject: Re: pinhole atmospheric pressure and latitude Date: Fri, 20 Aug 2004 18:53:21 -0700 Message-ID: <web-4102156@exploratorium.edu>
Hi Geoff
Good news Coriolis has little to do with it.
Here's an article from the Antarctic sun that says that
cold air decreases more rapidly in pressure as altitude
increAases than warm air. So polar atmospheric pressure
decreases more rapidly in pressure with altitude because
polar air is colder.
Paul D
Higher and higher at the South Pole
By Melanie Conner
Sun staff
When Holly Carlson landed at the South Pole in early
November and stepped off the plane, she felt a shortness
of breath and light-headedness, as though she had just
landed on top of a mountain peak.
Coming from Park City, Utah, Carlson was accustomed to
living at 7,000 feet above sea level. Landing on the
polar plateau, located at 9,300 feet should have been
easy. But Carlson and others at the South Pole are
breathing air that is like an 11,000-foot peak in Utah or
Colorado.
At the South Pole the barometric air pressure is on
average about 20 percent lower than expected for an
elevation of 9,300 feet (2,800 m). This is the result of
cold weather patterns in Antarctica that create the
effect of ?thinner? air at an equivalent elevation.
To understand weather and altitude changes related to
pressure, first it is important to understand atmospheric
pressure.
?Think of pressure as simply the weight of the overlying
column of air,? said Thomas Parish, professor at the
University of Wyoming, Dept. of Atmospheric Science in
Laramie. ?It is no more than the weight of fluid above
you.?
Atmospheric pressure changes with altitude. As altitude
increases, pressure decreases. Under less pressure,
oxygen molecules are more widely dispersed throughout the
atmosphere, making the air feel thinner.
Similar to the North Pole, the Antarctic region already
has relatively low surface pressure. McMurdo Station,
located at sea level has an average surface pressure of
approximately 990 millibars (a measurement for barometric
pressure), compared to the standard atmospheric pressure
at sea level of 1013.2 mb.
?That?s a bit less than average sea-level pressure in the
mid-latitudes and is a consequence of the circulation of
our atmosphere,? said Parish.
Pressure altitude, while it exists near polar coastal
areas, is even more pronounced on the polar plateau,
where the physical altitude is well above sea level.
?The second part to understanding pressure altitude is
related to how fast pressure changes with height. It
always decreases with height, but how fast it decreases
depends on temperature,? said Parish. ?In the winter when
it?s cold, the pressure decreases faster with height.
Because it?s cold in Antarctica all the time, the
pressure is lower at an elevation such as the South Pole
than at a similar elevation in the middle latitudes.?
An area located in the mid-latitudes at 9,300 feet (2,800
m) in elevation with an average temperature of about 42 F
(6C), would have an atmospheric pressure of approximately
716 mb. However, at the same elevation at the South Pole,
located at 90 degrees south in latitude, an area that is
much colder, an average temperature of minus -4F (-20C)
produces a pressure of 691 mb.
A commonly mistaken explanation of pressure altitude is
that it is the result of the centrifugal forces of the
Earth?s spin that draws the atmosphere toward the equator
to form an ?equatorial bulge.?
The equatorial bulge is a result of centrifugal forces and
represents the Earth?s physical shape, not that of the
atmosphere, Parish said.
?Our atmosphere is thermally driven, not mechanically
driven,? said Parish.
Atmospheric pressure creates weather patterns in order to
maintain thermal equilibrium over the globe. High
atmospheric pressure is associated with fair weather
while low pressure is generally associated with cold
weather. Pressure fluctuates throughout the globe in
relation to atmospheric weather conditions.
?The poles are cold and the tropics are warm. The reason
we have weather is because the atmosphere is trying to
distribute heat. Otherwise the tropics would keep getting
hotter and hotter and the polar regions would get
colder,? said Parish. ?The weather, coming from pressure,
transports heat.?
On Fri, 20 Aug 2004 16:43:33 -0700
Geoff Ruth <gruth@leadershiphigh.org> wrote:
>Hello,
>
>This summer I went up to Denali Nat'l Park in Alaska, and
>read a perplexing fact about the O2% content at very high
>latitudes: at high latitudes, there is less oxygen
>present than at lower elevations at the same elevation. I
>am assuming that this is because the total atmospheric
>pressure is lower at high latitudes, and not because the
>% composition of the air is different at high latitudes.
>Is this assumption correct? If so, why does the
>atmospheric pressure decrease at high latitudes? (I dread
>that answer has to do with the the Coriolis Effect, my
>least favorite atmospheric phenomenon.)
>
>Thanks so much,
>Geoff
>
>---------------------------------------------------------------------------
>
>To unsubscribe from pinhole, send an email to
>requests@exploratorium.edu
>with the words 'unsubscribe pinhole' (without the quotes)
>in the SUBJECT of the email.
>
>To subscribe to the digest and only get 1 combined
>message a day, send an
>email to requests@exploratorium.edu with the words
>'subscribe digest pinhole' (without the quotes) in the
>SUBJECT of the email.
>
>Check out what your colleagues have written on Pinhole in
>the Pinhole archives at:
>http://saturn.exploratorium.edu/ti/alumni/pinhole.html
>---------------------------------------------------------------------------
>
>
This archive was generated by hypermail 2.1.3 : Mon Aug 01 2005 - 16:06:42 PDT