*Measured* air pressure at a given time and place has a well defined meaning. However, "average air pressure" is poorly defined. The value "760 mm Hg" is little more than a conventional common reference value, which is seldom realized naturally with great precision anywhere at any time.
Calculation of "average air pressure" depends strongly on the *definition* used for the calculation and is of questionable value. ---- Original message ---- >Date: Fri, 12 Feb 2010 04:32:26 -0800 (PST) >From: "John M. Steele" <[email protected]> >Subject: [USMA:46601] Re: Average air pressure >To: "U.S. Metric Association" <[email protected]> >Cc: "U.S. Metric Association" <[email protected]> > > I am amazed it agrees as well as it does; I > CERTAINLY would not expect five digits of precision. > > While there is nothing inherently wrong with naive > models, (hell the standard atmosphere is a naive > model), you have repeatedly replaced detail with > averages and assumed it all averages out. Gravity > varies around ±½% from pole to equator. The earth > is an ellipsoid, the average radius you used is > correct for volume, not necessarily for surface > area. As the atmosphere is a gas, and can move, > variations in the vertical temperature profiles are > critical to the local pressure (and cause variation > day to day). Finally the assumption that averaging > works has a flaw that there is (mostly) sea below > sea level, while mountains intrude into the air > space and displace air all over the world; the > assumption that a uniform mass of air is uniformly > resting on a sphere has a BUNCH of problems. (I > have NO idea how to solve them all.) > > We know air pressure varies day to day with weather, > however, the total atmosphere is relatively > constant. I believe a better way to proceed is to > average a large number of simultaneously > measurements at (or corrected to) sea level, all > over the earth. The stations should be chosen to > represent a good sampling plan. It is certainly > true that the overly precise 101.325 kPa is nothing > but 760 mm Hg expressed in SI. Wikipedia's article > on the atmosphere has a nice map of 15 year average > air pressure in December and June, > http://en.wikipedia.org/wiki/Atmospheric_pressure > > I think it shows two things: > *The average is NOT 101.325 kPa everywhere > *The standard value of 101.325 kPa is much more in > the middle of data than your value. > > ------------------------------------------------ > > From: Pat Naughtin > <[email protected]> > To: U.S. Metric Association <[email protected]> > Cc: U.S. Metric Association <[email protected]> > Sent: Fri, February 12, 2010 12:17:05 AM > Subject: [USMA:46597] Average air pressure > Dear Bill and All, > I have had another thought on finding the average > pressure at (or near) the surface of the Earth. As > you will see, I am trying to derive the average by > calculation. Our previous correspondence is below. I > would appreciate any thoughts you might have on this > way of proceeding and particularly on my > calculations. > > National Center for Atmospheric Research > > According to the National Center for Atmospheric > Research, "The total mean mass of the atmosphere is > 5.1480 x 1018 kg with an annual range due to water > vapor of 1.2 or 1.5 x 10^15 kg depending on whether > surface pressure or water vapor data are used; > somewhat smaller than the previous estimate. The > mean mass of water vapor is estimated as 1.27 x > 10^16 kg and the dry air mass as 5.1352 ±0.0003 x > 10^18 kg." > > Calculation > > Mass of Earth's atmosphere > = 5.1480 x 10^18 > kilograms > > Weight of atmosphere = mg = m x 9.80665 m/s2 > = 50.48463 x 10^18 > newtons > > Surface area of Earth = 4 π r2 = 4 x π x (6.378 > 137 x10^6)^2 m = 511.207 8 x 10^12 square > metres > > Average pressure at sea level (Force ÷ area) > = 98.755 58 x 10^3 > pascals = 98.756 kPa > > Irrelevant information about the atmosphere and the > Earth > > The average mass of the atmosphere is about 5 > teratonnes (= 5.1480 x 10^18 kilograms). > > The average mass of the Earth is about 6 zettatonnes > (= 5.9742 × 10^24 kilograms). > > The proportion of the whole Earth to its atmosphere > is roughly 1 000 000 to 1, that is if the atmosphere > was valued at 1 cent then the Earth would be valued > at $10 000.00. > > Cheers, > > Pat Naughtin > Author of the ebook, Metrication Leaders Guide, that > you can obtain > from http://metricationmatters.com/MetricationLeadersGuideInfo.html > PO Box 305 Belmont 3216, > Geelong, Australia > Phone: 61 3 5241 2008 > Metric system consultant, writer, and speaker, Pat > Naughtin, has helped thousands of people and > hundreds of companies upgrade to the modern metric > system smoothly, quickly, and so economically that > they now save thousands each year when buying, > processing, or selling for their businesses. Pat > provides services and resources for many different > trades, crafts, and professions for commercial, > industrial and government metrication leaders in > Asia, Europe, and in the USA. Pat's clients include > the Australian Government, Google, NASA, NIST, and > the metric associations of Canada, the UK, and the > USA. See http://www.metricationmatters.com/ to > subscribe. > On 2009/12/05, at 08:51 , Bill Hooper wrote: > > On Dec 4 , at 11:38 AM, Pierre Abbat wrote (in > response to my earlier note): > > (I wrote) Normal or > > average atmospheric pressure is about 101.3 > kPa. (There may be different > > standards for identifying "normal" or > "average" air pressure.) > > (Pierre replied) > 101.325 kPa, to be exact, is the standard. > > The value of 101.325 kPa is the exact value (the > "standard") specified by CGPM for the standard > atmospheric pressure, as Pierre correctly points > out. That value is as close to a universal value > as is possible. > In addition, however, there are other > circumstances where different standards are used. > Here are two I've encountered (from Wikipedia): > > ... International Union of Pure and Applied > Chemistry (IUPAC) recommended that for the > purposes of specifying the properties of > substances, “the standard pressure” should > be defined as precisely 100 kPa ... rather than > the 101.325 kPa value of “one standard > atmosphere”. ... For natural gas, the > petroleum industry uses a standard temperature > of 15.6 °C (60.1 °F), pressure 101.56 kPa > (14.730 psi). (air pressure) > > The CGPM standard is probably more precise than is > reasonable for many uses, such as reporting air > pressure in weather reports. For such > measurements, a rounder value of 101.3 kPa or even > 101 kPa might be suitable as a state "normal" or > "average". > Certainly, atmospheric pressure is affected by > altitude and other factors. For specific locations > or specific special conditions, the CGPM standard > atmosphere may not be too close to the actual > average of the ambient pressure. > This is not meant to be argumentative. I recognize > the value of 101.325 kPa as the best value to use > for most technical purposes. I just felt it was > necessary to indicate in my note that there are > different possible averages or normal values that > might be used. > Regards, > Bill Hooper > > ============================== > If you have not already done so, > please note my new email address: > > [email protected] > > (Old address will still work OK temporarily.)
