Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the mass of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. On average, a column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (2.28 lb_f) (A column one square inch in cross-section would have a mass of about 14.7 lbs and weight of about 65.4 N).

Standard atmospheric pressure[link]

The standard atmosphere(symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa.^[1] The following units are equivalent, but only to the number of decimal places displayed: 760 mmHg (torr), 29.92 inHg, 14.696 psi, 1013.25 millibars/hectopascal. One standard is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries. In 1971, the International Union of Pure and Applied Chemistry (IUPAC) said that for the purposes of specifying the properties of substances, "the standard pressure" should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.^[2] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).^[3] (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of air at standard temperature and pressure. For every 1,000 feet you ascend, the atmospheric pressure decreases by about 4%. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioner industry, the standard is often temperature = 0 °C (32 °F) instead. For natural gas, the Gas Processors Association (GPA) specifies a standard temperature of 60 °F (15.6 °C), but allows a variety of "base" pressures, including 14.65 psi (101.0 kPa), 14.656 psi (101.05 kPa), 14.73 psi (101.6 kPa) and 15.025 psi (103.59 kPa).^[4] For a given "base" pressure, the higher the air pressure, the colder it is; the lower the air pressure, the warmer it is.

Mean sea level pressure[link]

15 year average mean sea level pressure for June, July, and August (top) and December, January, and February (bottom).

Kollsman-type barometric aircraft altimeter as used in North America displaying an altitude of 80 ft (24 m).

Mean sea level pressure (MSLP) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature.

This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See Altimeter (barometer vs. absolute).

The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures that are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.

The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.

QNH

The barometric altimeter setting that will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield

QFE

The barometric altimeter setting that will cause an altimeter to read zero when at the reference datum of a particular airfield (in general, a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.

QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.

Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.92 inches of mercury (inHg) or 760 millimeters (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States, Canada, and Colombia where it is reported in inches (to two decimal places) of mercury. (The United States and Canada also report sea level pressure SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars.^[5] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals [1], while Environment Canada's standard unit of pressure is the same [2] [3].) In the weather code, three digits are all that is needed; decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050.0 mbar (105.00 kPa). The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes.

Altitude atmospheric pressure variation[link]

A very local storm above Snæfellsjökull, showing clouds formed on the mountain by Orographic Lift

Variation in atmospheric pressure with altitude, computed for 15 °C and 0% relative humidity.

This plastic bottle was sealed at approximately 14,000 feet altitude, and was crushed by the increase in atmospheric pressure (at 9,000 feet and 1,000 feet) as it was brought down towards sea level.

Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.^[6] Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.

Within the troposphere, the following equation relates atmospheric pressure p to altitude h

Failed to parse (Missing texvc executable; please see math/README to configure.): p = p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R \cdot L} \approx p_0 \cdot \exp \left(- \frac{g \cdot M \cdot h}{R \cdot T_0} \right),

where the constant parameters are as described below:

Local atmospheric pressure variation[link]

Hurricane Wilma on 19 October 2005–88.2 kPa (12.79 psi) in eye

Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.

Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.

Atmospheric pressure records[link]

The highest barometric pressure ever recorded on Earth was 1,085.7 hectopascals (32.06 inHg) measured in Tonsontsengel, Mongolia on 19 December 2001.^[7] The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (25.69 inches), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.^[8]

Atmospheric pressure based on height of water[link]

Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 millimetres (30 in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to visualize the pressure of one atmosphere.

One atmosphere (101 kPa or 14.7 psi) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.

Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c. (water column) or W.G. (inches water gauge). A typical gas-using residential appliance is rated for a maximum of 14 w.c., which is approximately 35 hPa.

In general, non-professional barometers are aneroid barometers or strain gauge based. See pressure measurement for a description of barometers.

Boiling point of water[link]

Boiling water

Water boils at about 100 °C (212 °F) at standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water.^[9] Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than 3,500 ft (1,100 m) above sea level requires adjustments to recipes.^[10] A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.^[11]

See also[link]

Underwater diving portal

• Atmosphere (unit)
• Barometric formula
• Barotrauma physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid.
• International Standard Atmosphere - a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature etc.) with altitude, at mid latitudes.
• NRLMSISE-00
• Plenum
• Subtropical high belts
• Cabin pressurization

References[link]

External links[link]

• How Atmospheric Pressure Affects Objects (Audio slideshow from the National High Magnetic Field Laboratory)
• NASA page on the 1976 Standard Atmosphere
• Source code and equations for the 1976 Standard Atmosphere
• A mathematical model of the 1976 U.S. Standard Atmosphere
• Calculator using multiple units and properties for the 1976 Standard Atmosphere
• Calculator giving standard air pressure at a specified altitude, or altitude at which a pressure would be standard
• Some of the effects of air pressure
• Atmospheric calculator and Geometric to Pressure altitude converter

Experiments[link]

• Movies on atmospheric pressure experiments from Georgia State University's HyperPhysics website - requires QuickTime
• Test showing a can being crushed after boiling water inside it, then moving it into a tub of ice cold water.

Julius Sumner Miller (May 17, 1909 – April 14, 1987) was an American physicist and television personality. He is best known for his work on children's television programs in North America and Australia.

Off-screen[link]

Julius Sumner Miller was born in Billerica, Massachusetts the youngest of nine children. His father was Latvian, his Lithuanian mother spoke 12 languages.^[1][2]

Miller graduated with a Philosophy degree and a Master's in Physics from Boston University in 1933 but due to the Depression worked as a butler for a wealthy Boston doctor for the next two years during which time he married the doctor's maid, Alice Brown; they were to have no children, but he was to go on to reach millions of children through his popular science programs.^[2]

After making over 700 applications for employment, in 1937 he was offered a place in the Physics Department of Dillard University, a private, black liberal arts college in New Orleans. During World War II he worked as a civilian physicist for the U.S. Army Signal Corps while holding fellowships in physics at the universities of Idaho and Oklahoma and was a Ford Foundation Fellow at UCLA. In 1950 he enrolled in the Institute for Advanced Studies in Princeton, N.J. where he was a student of Albert Einstein. He became a lifelong friend of Einstein and went on to amass a collection of Einstein memorabilia that included Einstein's birth certificate. Miller also taught at Princeton but disliked large institutions, leaving in 1952 to join the Physics Department at the then small El Camino College in Torrance, California (1952–1974), to maximum student enrollments due to his great popularity^[2] and where he was instantly recognizable by his casual hair and horn-rimmed spectacles.

Miller was intolerant of misspelled words and misplaced punctuation and often angered his colleagues because he charged that the students of most faculty were not learning enough. During an interview in the 1940s, he stated that intellectual life in America was in trouble, a belief he held for the rest of his life.

"We are approaching a darkness in the land. Boys and girls are emerging from every level of school with certificates and degrees, but they can't read, write or calculate. We don't have academic honesty or intellectual rigor. Schools have abandoned integrity and rigor."^[2]

From 1963 to 1986 he was the visiting lecturer for the Physics Department of the University of Sydney ^[3] and from 1965 to 1985, the US Air Force Academy.

Television[link]

Miller began hosting his educational program, "Why Is It So?" in 1959 on KNXT Channel 2 in Los Angeles. From 1962 to 1964, Miller was Disney's "Professor Wonderful" on new introductions, filmed at Disneyland, to the syndicated reruns of The Mickey Mouse Club. During the same period, he appeared on a semi-regular basis, performing physics experiments, on Steve Allen's late night TV show in Hollywood, syndicated by Group W.

Miller's first television appearance in Australia was on Bob Sanders People in 1963. In an improvised physics demonstration he attempted to drive a straw through a raw potato. A paper straw does not have the strength but if you pinch the end, the trapped air acts as a piston, easily piercing the potato. For the first time in his career he could not get it to work and loudly exclaimed "Australian straws ain't worth a damn". The next morning, Miller arrived at his Sydney University laboratory to find one million drinking straws on the floor with a telegram reading "You might find one of these fitting your requirements". He later stated "I sat amongst the straws with straws stuck in my hair and ears. But clearly I had made a mistake. I should have said: "Australian potatoes ain’t worth a damn," and I’d have cornered the potato market!"

Shortly after, he was offered a job presenting science for ABC Television. When asked how much money he wanted he replied he never asked, he listened to an offer then "multiplied it by a factor between two and ten". Due to budget constraints the offer was withdrawn but an agreement was reached for Miller to host his own science based TV series which was filmed at the University of Sydney where he taught. Why Is It So? (his eventual stock phrase), was broadcast from 1963 to 1986 and became an instant hit known for its "cool experiments, interesting science, and fantastic hair." The 1960s program became Demonstrations in Physics (also called Science Demonstrations when it was aired on American Public television). He introduced each episode with the line: "How do you do, ladies and gentlemen, and boys and girls [sometimes adding "and teachers"]. I am Julius Sumner Miller, and physics is my business."

"My first TV series on demonstrations in physics - titled Why Is It So were now seen and heard over the land. The mail was massive. The academics were a special triumph for me. They charged me with being superficial and trivial. If I had done what they wanted my programs would be as dull as their classes! I knew my purpose well and clear: to show how Nature behaves without cluttering its beauty with abtruse mathematics. Why cloud the charm of a Chladni plate with a Bessel function?"^[4]

Miller's on-air popularity was due to an enthusiasm not normally associated with serious science. Shows would be liberally sprinkled with phrases such as "He who is not stirred by the beauty of it is already dead!" and he also liked to trick the audience. A common ploy would be to hold up an empty glass and ask guests to confirm it was empty....then chide them for not noticing it was full of air. Before each demonstration he would usually ask for a show of hands to indicate which of several results they expected. Often he would then add "hands up those who don’t care".

In 1964 Miller suffered a near fatal heart attack. Scheduled to give a lecture in Australia, he sent Sydney University a telegram saying; "I've dropped dead here." He would suffer a second heart attack in 1986.^[2]

In 1966 questions from his show with an answer to the previous day’s question were published as "Millergrams" for The Australian newspaper. 112 of these questions were published as a book "Millergrams; Some Enchanting Questions for Enquiring Minds". Further books were published in 1967: "The Second Book of Millergrams: Some More Enchanting Questions for Enquiring Minds" and 1988: "Why is it So?: The Very Best Millergrams of Professor Julius Sumner Miller".

Sample Millergram:
Q32: A juggler comes to a foot-bridge of rather flimsy design. He has in hand four balls. The maximum load is no more than the juggler himself and one ball. Can he get across the bridge by juggling the balls, always having at most one ball in the hand (and three in the air)?
A: No. A falling ball exerts a force on the hand greater than its own weight.
Rather, a 'thrown' ball exerts greater force than a 'held' one. That is, the additional force equal and opposite to that imparted to a flung ball, in addition to the juggler's mass, would exceed the bridge's tolerance (the bridge can tolerate a juggler and held ball, but not the additional downward force associated with forcing a ball 'up').

Miller appeared as "The Professor" in the Canadian series The Hilarious House of Frightenstein (1971), in a 2- to 3-minute segment each episode where he demonstrated physics experiments and explained the principles involved.

Miller was also an occasional guest in the 1970s on The Tonight Show Starring Johnny Carson in the United States.

Commercials[link]

During the 1980s Miller appeared in a famous series of Australian television commercials for Cadbury chocolate, using his stock phrase "Why is it so?", demonstrating a simple scientific principle, and describing how each block of chocolate "embraces substantial nourishment and enjoyment," and contained "a glass and a half of full-cream dairy milk."

While in Australia, Miller also appeared in ads for non-stick saucepans and Ampol Petroleum which included demonstrations of real principles of physics, albeit briefly. The ads were sufficiently popular to be played for some years after his death.

Foundations[link]

In February 1987 Miller became ill while visiting Australia and returned to the United States where he was diagnosed with leukemia. Miller died six weeks later on April 14, 1987, in San Jose, California. Miller willed his body to the University of Southern California's School of Dentistry, no services were held at his request.^[2] Professor Miller's wife, Alice Brown Miller, wanted to perpetuate the memory and achievements of her husband, and so conceived the idea of the Julius Sumner Miller Foundation, which was established in 1998.^[1]

Through an offer by Cadbury-Schweppes Pty Ltd, the Cadbury-Julius Sumner Miller Scholarship for Academic Excellence was set up to provide undergraduate scholarships in the School of Physics at the University of Sydney.

In 1993 the Australian Science Foundation for Physics established the Julius Sumner Miller Fellowship in his memory. The fellowship is currently held by Karl Kruszelnicki, best known as "Doctor Karl" for his appearances on Australian radio and television as a science commentator and author.

Popular culture[link]

In 1966 Miller devised a word game "Milleranagrams" which was published in Australia by John Sands Limited. The game, whose only materials were 200 Scrabble-like letter tiles, required players in their turn to draw an unseen tile from the "pool" then either make a word from their stock of tiles or add a tile to a word already on the table (rearrangement of the letters being permitted, hence the name) to form another word.

Miller remains popular in Australia, where he is still quoted. As Professor Julius Sumner Miller often asked, "Why is it so?" and its variations are still often used in newspaper articles that pose questions unrelated to science.^[5]

Bibliography[link]

• Millergrams; Some Enchanting Questions for Enquiring Minds, Ure Smith, 1966
• The Second Book of Millergrams: Some More Enchanting Questions for Enquiring Minds, Ure Smith, 1967
• Quiz Questions in Physics, Horwitz-Martin, Australia 1967
• Why It Is So, ABC books, 1971 ISBN 0-642-97296-6
• The Kitchen Professor, ABC books, 1972 ISBN 0-642-97352-0
• Why It Is So: Heat and Temperature, ABC books, 1973 ISBN 0-642-97496-9
• Why It Is So: Sound and Electricity & Magnetism, ABC books, 1973 ISBN 0-642-97584-1
• Why It Is So: Mechanics, Heat & Temperature, Sound and Electricity, ABC books, 1978 ISBN 0-642-97523-X
• Enchanting Questions for Enquiring Minds, Currey/O'Neil, 1982 ISBN 0-85902-280-3
• Why is it so?: the very best Millergrams of Professor Julius Sumner Miller, Australian Red Cross Society, Ringwood, Vic; Penguin Books, 1988
• The Days of My Life: an autobiography, Macmillan, 1989. ISBN 0-333-50337-6

References[link]

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2008)

External links[link]

Wikiquote has a collection of quotations related to: Julius Sumner Miller

• Julius Sumner Miller Foundation
• Australian Julius Sumner Miller tribute page
• Julius Sumner Miller page at a Hilarious House of Frightenstein tribute site
• Julius Sumner Miller at TV.com
• Julius Sumner Miller at the Internet Movie Database
• Why is it so? site at Australia's ABC.net.au featuring original episodes
• http://www.physics.usyd.edu.au/foundation/Outreach/DRK/jsm.pdf Julius Sumner Miller 1909 - 1987, Physics Department, University of Sydney.