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Air Pressure

Air Pressure

Atmospheric pressure is the pressure above any area in the Earth's atmosphere caused by the weight of air. Standard atmospheric pressure (atm) is discussed in the next section. Air masses are affected by the general atmospheric pressure within the mass, creating areas of high pressure (anti-cyclones) and low pressure (depressions). As elevation increases, fewer air molecules are above. Therefore, atmospheric pressure decreases with increasing altitude. The following relationship is a first-order approximation: :

\log_ P \approx

, where P is the pressure in pascals and h the height in metres. This shows that the pressure at an altitude of 31 km is about 10^(5-2) Pa = 1000 Pa, or 1% of that at sea level¹. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lbf. A 1 m² column of air would weigh about 100 kilonewtons. See density of air.

Standard atmospheric pressure

Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101.325 kilopascals (kPa). (see also Standard temperature and pressure) This can also be stated as:
- 29 117/127 inches of mercury ≈ 29.92 inHg
- 760 millimetres of mercury (mmHg) or torrs (Torr)
- 1013.25 millibars (mbar, also mb) or hectopascals (hPa)
- 14.6959 psia or 0 psig (pounds-force per square inch, absolute or gauge) (lbf/in²)
- 2116.2 pounds-force per square foot (lbf/ft²)
- 1 6517/196133 technical atmospheres (at) ≈ 1.03322745 at This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world. 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. However, this standard atmosphere is defined slightly differently: temperature = 68 °F (20 °C), air density = 0.075 lb/ft³ (1.20 kg/m³), altitude = sea level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32 °F (0 °C) instead. For natural gas, the petroleum industry uses a standard temperature of 60 °F (15.6 °C).

Mean sea level pressure (MSLP or SLP)

Mean sea level pressure (MSLP or SLP) is the pressure at sea level or (when measured at a given height on land) the station pressure reduced to sea level by an appropriate altitude dependant formula. This is the pressure normally given in weather reports on radio, television, and newspapers. 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. The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures which 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. See altimeter.
- QNH barometric altimeter setting which will cause the altimeter to read altitude above mean sea level in the vicinity of an airfield.
- QFE barometric altimeter setting which will cause an altimeter to read height above a particular runway threshold if set to the correct field elevation while on the ground. Average sea-level pressure is 1013.25 hPa (mbar) or 29.921 inches of mercury (inHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals, except in the United States and Canada where it is reported in inches of mercury. (The United States also reports 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. In Canada's public weather reports, sea level pressure is reported in kilopascals, while Environment Canada's standard unit of pressure is hectopascal.) 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 1032.0 mbar. The lowest measurable sea-level pressure is found at the centers of hurricanes (typhoons, baguios).

Atmospheric pressure variation

Atmospheric pressure varies widely on the Earth, and these variations are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.. The highest recorded atmospheric pressure, 108.6 kPa (1086 mbar or 32.06 inches of mercury), occurred at Tosontsengel, Mongolia, 19 December, 2001². The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip on 12 October, 1979². The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inHg) during Hurricane Wilma on 19 October 2005. Atmospheric pressure shows a diurnal (daily) rhythm. This effect is very strong in tropical zones, and almost zero in polar areas. A graph shows these rhymic variations in northern Europe on the top of this page. In tropical zones it may reach above 5 mbar variation. These variations follow a circadian (24 h) and at the same time semi-circadian (12 h) rhythm.

Intuitive feeling for atmospheric pressure based on height of water

Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 30 inches of mercury is often used to teach, make visible, and 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 conceptualize the amount of pressure in one atmosphere. 1 atmosphere (14.7 lbf/in²) is the amount of pressure that can lift water approximately 33.9 feet (10.3 m). Thus, a diver at a depth 10.3 meters under water in the ocean experiences a pressure of about 2 atmospheres (1 atm for the air and 1 atm for the water). In terms of city water pressure, one atmosphere is approximately one-half to one-fifth the pressure of typical city water mains (i.e., water pressure is around 2 to 5 atmospheres).

References

# US Department of Defense Military Standard 810E # Burt, Christopher C., (2004). Extreme Weather, A Guide & Record Book. W. W. Norton & Company # U.S. Standard Atmosphere, 1962, U.S. Government Printing Office, Washington, D.C., 1962. # U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.

External links

- [http://www.atmosculator.com/The%20Standard%20Atmosphere.html? A detailed mathematical model of the 1976 U.S. Standard Atmosphere]

Experiments

- [http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl? An exercise in air pressure]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/patm.html#atm Movies on atmospheric pressure experiments] from Georgia State University's HyperPhysics website. (requires Quicktime) Category:Atmosphere Category:Meteorology Category:Units of pressure Category:Diving ko:대기압 ja:気圧

Pressure

:For the psychological or political context, see Peer pressure. Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. Mathematically: :

p = F/A\,

where p is the pressure, F is the normal force, and A is the area. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume. A closely related quantity is the stress tensor σ which relates the vector force F to the vector area A via :

\mathbf=\mathbf

This tensor may be divided up into a scalar part (pressure) and a traceless tensor part shear. The shear tensor gives the force in directions parallel to the surface, usually due to viscous or frictional forces. The stress tensor is sometimes called the pressure tensor, but in the following, the term "pressure" will refer only to the scalar pressure. shear

Example

As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall. Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. Unlike stress, pressure is defined as a scalar quantity. The gradient of pressure is force density. In the human body, baroreceptors monitor blood pressure.

Relative or gauge pressure

For gases, pressure is sometimes measured, not as an absolute pressure, but relative to atmospheric pressure; such measurements are sometimes called gauge pressure. An example of this is the air pressure in a car tire, which might be said to be "220 kPa," but is actually 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 kPa." Where space is limited, such as on gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)," is permitted. In non-SI technical work, a gauge pressure is sometimes written as "32 psig," though the other methods explained above that avoid attaching characters to the unit of pressure are preferred [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 ¹].

Scalar nature of pressure

In static gas, the gas as a whole does not appear to move, the individual molecules of the gas, which we cannot see, are in constant random motion. Because we are dealing with an extremely large number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per unit area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has a magnitude but no direction associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular to the surface.

Hydrostatic pressure

Hydrostatic pressure is the pressure due to the weight of a fluid. :p = ρgh where ρ (rho) is density of the fluid, g is acceleration due to gravity, and h is height of the fluid above the point being measured. See also Pascal's law.

Stagnation pressure

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation. The pressure of a moving fluid can be measured using a Pitot probe, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.

Units

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m^-2 or kg·m^-1·s^-2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N/m². Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (psi) and the bar. The cgs unit of pressure is the barye (ba). It is equal to 1 dyn·cm^-2. Pressure is still sometimes expressed in kgf/cm² or grams-force/cm² (sometimes as kg/cm² and g/cm² without properly identifying the force units). But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI; the unit of force in SI is the newton (N). The technical atmosphere (symbol: at) is 1 kgf/cm². Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa. The obsolete unit inch of mercury (inHg) is still sometimes used in the United States. Blood pressure is still measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. These obsolete manometric units of pressure are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer. When millimetres or inches of mercury are used today, they have precise definitions that can be expressed in terms of SI units. The water-based units depend on the density of water, a measured, rather than defined, quantity. The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows. :standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa. A rule of thumb commonly used by scuba divers is that one atmosphere is approximately equal to the pressure exerted by ten metres of water. Non-SI units presently or formerly in use include the following.
- atmosphere.
- manometric units:
- centimetre, inch, and millimetre of mercury (Torr).
- millimetre, centimetre, metre, inch, and foot of water.
- imperial units:
- kip, ton-force (short), ton-force (long), pound-force, ounce-force, and poundal per square inch.
- pound-force, ton-force (short), and ton-force (long) per square foot.
- non-SI metric units:
- bar, millibar.
- kilogram-force, or kilopond, per square centimetre (technical atmosphere).
- gram-force and tonne-force (metric ton-force) per square centimetre.
- barye (dyne per square centimetre).
- kilogram-force and tonne-force per square metre.
- sthene per square metre (pieze).

External links

- [http://calc.skyrocket.de/en/ Online unit converter] - conversion of many different units.
- [http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Exercise_In_Air_Pressure An exercise in air pressure]
- [http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html Pressure being a scalar quantity] Category:Diving Category:Meteorology Category:Physical quantity Category:Thermodynamics ko:압력 ms:Tekanan ja:圧力

Earth's atmosphere

Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen and 21% oxygen, with trace amounts of other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night. The atmosphere has no abrupt cut-off. It slowly becomes thinner and fades away into space. There is no definite boundary between the atmosphere and outer space. Three-quarters of the atmosphere's mass is within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 50.0 miles (80.5 km) are designated as astronauts. An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman line, at 100 km (62 mi), is also frequently used as the boundary between atmosphere and space.

Temperature and the atmospheric layers

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between the different atmospheric layers:
- troposphere: From the Greek word tropos meaning to turn or mix. The troposphere is the lowest layer of the atmosphere starting at the surface going up to between 7 km at the poles and 17 km at the equator with some variation due to weather factors. The troposphere has a great deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which then rise to release latent heat as sensible heat that further buoys the air mass. This process continues until all water vapor is removed. In the troposphere, on average, temperature decreases with height due to expansive cooling.
- stratosphere: from that 7–17 km range to about 50 km, temperature increasing with height.
- mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.
- thermosphere: from 80–85 km to 640+ km, temperature increasing with height. The boundaries between these regions are named the tropopause, stratopause, and mesopause. The average temperature of the atmosphere at the surface of earth is 14 °C.

Various atmospheric regions

Atmospheric regions are also named in other ways:
- ionosphere — the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
- exosphere — above the ionosphere, where the atmosphere thins out into space.
- magnetosphere — the region where the Earth's magnetic field interacts with the solar wind from the Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
- ozone layer — or ozonosphere, approximately 10 - 50 km, where stratospheric ozone is found. Note that even within this region, ozone is a minor constituent by volume.
- upper atmosphere — the region of the atmosphere above the mesopause.
- Van Allen radiation belts — regions where particles from the Sun become concentrated.

Pressure

:Barometric Formula: (used for airplane flight) barometric formula :Main article: Atmospheric pressure :Nasa mathematical model: NRLMSISE-00 Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).

Thickness of the atmosphere

The atmosphere is present to heights of 1000 km. or more. But at this height it is so thin and at such low pressure that it's almost like it isn't there.
- 57.8% of the atmosphere is below the summit of Mount Everest.
- 72% of the atmosphere is below the common flight height of airplanes, (about 10000 m or 32800 ft).
- 99.99999% of the atmosphere is below the highest X-15 plane flight on August 22, 1963 which reached an altitude of 354,300 ft or 108 km. Therefore, most of the atmosphere is below 100 km (99.9999%) although in the rarified region above this there are auroras, and other atmospheric effects.

Composition

aurora
Source for figures above: [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA]
Carbon dioxide and methane updated (to 1998) by IPCC TAR table 6.1 [http://www.grida.no/climate/ipcc_tar/wg1/221.htm]

Minor components of air not listed above include:
- The mean molecular mass of air is 28.97 g/mol.

Heterosphere

Below an altitude of about 100 km, the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above. However, above about 100 km, the Earth's atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude, but at a rate which depends on the molecular mass. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.[http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html]

Density and mass

The density of air at sea level is about 1.2 kg/m³. Natural variations of the barometric pressure occur at any one altitude as a consequence of weather. This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space due to variable solar radiation The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites. The total mass of the atmosphere is about 5.1 × 10¹⁸ kg, or about 0.9 ppm of the Earth's total mass. The above composition percentages are done by volume. Assuming that the gases act like ideal gases, we can add the percentages p multiplied by their molar masses m, to get a total t = sum (p·m). Any element's percent by mass is then p·m/t. When we do this to the above percentages, we get that, by mass, the composition of the atmosphere is 75.523% N₂, 23.133% O₂, 1.288% Ar, 0.053% CO₂, 0.001267% Ne, 0.00029% CH₄, 0.00033% Kr, 0.000724% He, and 0.0000038 % H₂. ppm This graph is from the NRLMSISE-00 atmosphere model, which has as inputs: latitude, longitude, date, time of day, altitude, solar flux, and the earth's magnetic field daily index.

The evolution of the Earth's atmosphere

model The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research. The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat (from the still-molten crust, and the sun) dissipated this atmosphere. About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen (though very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen [http://newsrelease.uwaterloo.ca/news.php?id=4348]). This second atmosphere had approximately 100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing. During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the cyanobacteria. Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth’s atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen). Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere. Photosynthesizing plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen. As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, resulting in mass extinctions and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

References

- [http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html The thermosphere: a part of the heterosphere], by J. Vercheval (viewed 1 Apr 2005)

External links

- [http://nssdc.gsfc.nasa.gov/space/model/models_home.html#atmo NASA atmosphere models]
- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth Fact Sheet]
- [http://atmospheres.agu.org/ American Geophysical Union: Atmospheric Sciences]
- [http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm Layers of the Atmosphere] Category:Atmospheric sciences Category:Atmosphere Category:Environments ko:대기권 ms:Atmosfera ja:大気

Weight

:See also weight function. For the 1994 album by the group Rollins Band, see Weight (album). In the physical sciences, weight is the interaction of matter with a gravitational field. It is equal to the mass of the object multiplied by the magnitude of the gravitational field. The word weight entered Old English sometime around the 9th century, and meant the quantity measured with a balance -- the same as mass in both common and scientific usage. In common usage, weight still means the same as mass.

Weight and mass

"Weight" is often used as a synonym for mass. For instance, when we buy or sell goods "by weight", we are interested in the amount of goods exchanged, not how hard it presses down on the table. Similarly, in measurements of body weight we are primarily interested in the amount of tissue (fat, muscle, etc.) present. Correspondingly, weight is often given in kilograms and other units of mass. In the physical sciences, people usually distinguish between weight and mass. Under most circumstances, this ambiguity is not a problem, because the weight of an object is directly proportional to its mass, and the constant of proportionality -- the strength of the gravitational field -- is approximately constant everywhere on the surface of the Earth (around 9.8 m/s²). For instance, a body will exert less force if it is located on the Moon than if it is on the Earth, since the gravitational field of the Moon is weaker; its mass, on the other hand, does not depend on position. Although terms such as "atomic weight", "molecular weight", and "formula weight" may still be encountered, such usage is often discouraged; terms like atomic mass are used instead. Mass is measured using a balance which compares an item in question to matter of known mass; this method is independent of gravity. Alternately, a spring scale or Hydraulic or pneumatic scale is used to measure force (which physicists call weight). Most scales measure weight using a spring. Related to the historical identification of mass and weight, the pound has been used both as a unit of mass and as a unit of force. In the United States, United Kingdom, and elsewhere, the pound is and always has been officially defined as a unit of mass. The corresponding force is called a pound-force, and similarly the weight of a kilogram of material on Earth is called a kilogram-force. However, the use of pounds to measure forces is still common in engineering, and it occurs in derived units like p.s.i. (pounds per square inch). In most countries, scientists have adopted SI units, which use kilogram for mass and newton for force non-interchangeably.

Weight as a force

The SI unit for weight is the newton (N), or kilogram metres per second squared (kg m s⁻²). The weight force that we sense is actually the normal force exerted by the surface we stand on, which prevents us from being pulled to the center of the Earth, and not the weight itself. This normal force, that we can call the apparent weight is the one that is measured by a weighing scale, not the weight itself. A good evidence of this is given by the fact that a person moving up and down on his toes does see the indicator moving, telling that the measured force is changing while his weight, that depends only on his mass, the Earth mass and the distance between his center of mass and the center of Earth obviously do not change. In contrast, in free-fall, there is no apparent weight because we are not in contact with any surface to provide such a normal force. The experience of having no apparent weight is known as weightlessness or microgravity.

Comparative weights on bodies of the solar system

The following is a list of the weights of a mass on some of the bodies in the solar system, relative to its weight on Earth: For weight variations on Earth, see gee, physical geodesy and gravity anomaly.

Human weight in the medical sciences and ordinary language

Although many people prefer the less-ambiguous term body mass to body weight, the term weight is overwhelmingly used in daily English speech and in biological and medical science contexts. Body weight is measured in kilograms throughout the world. Most hospitals in the United States use kilograms for calculations, but use kilograms and pounds simultaneously for other purposes (a pound is 0.45 kg). Many people in the United Kingdom still measure their weight using the stone equal to 14 lb (6.35 kg).

Sports usage

Participants in sports such as boxing, wrestling, judo, and weight-lifting are classified according to their body weight, measured in units of mass such as pounds or kilograms. See, e.g., wrestling weight classes, boxing weight classes, judo at the 2004 Summer Olympics, boxing at the 2004 Summer Olympics. In horse racing, weight is used to handicap horses. A weight also refers to the physical objects used in weight-lifting and other sports such as the hammer throw.

Air mass

In meteorology, an airmass or air mass is a large volume of air having fairly uniform characteristics of temperature, atmospheric pressure, and water vapor content. Air masses cover many hundreds or thousands of square miles, and slowly change in accordance with the terrain they are over. Air masses are classified according to their temperature and moisture content. The terms Arctic, Polar, and Tropical define the temperature of an air mass with arctic being the coldest and tropical being the warmest. Maritime air is a moist air mass, whereas continental air is relatively dry. These terms are combined; i.e. a maritime tropical air mass would be warm and moist. The terms refer to the fact that air masses acquire the properties of the terrain over which they move. Thus, cold arctic air masses are most common in the arctic regions and maritime air masses generally form over water. Air masses do move however, and a maritime air mass that moves over land will slowly lose its moisture and eventually become continental, just as a tropical air mass that moves north will cool and become polar, or even arctic. Air masses can not be defined by perfect lines or borders, however there is a very small region of interaction of two or more air masses where mixing occurs. This region is called a weather front, and visible weather and significant weather changes will occur there. As air masses move and displace each other, the associated fronts also move, thus causing weather changes for the terrain below. Fronts are always named for the air mass that is advancing. Thus a cold front would occur where a cooler air mass is displacing a warmer one. Air masses are not to be confused with small scale events like microbursts. Though these smaller events do involve masses of air, the term air mass is reserved for weather systems that span large areas. See also: weather front. category:Meteorology category:weather ko:기단 ja:気団

Anti-cyclone

In meteorology, an anticyclone (i.e. opposite to a cyclone) is a weather phenomenon in which there is a descending movement of the air and a relative increase in barometric pressure over the part of the earth's surface affected by it. At the surface the air tends to flow outwards in all directions from the central area of high pressure, and is deflected on account of the earth's rotation (see Ferrel's law) so as to give a spiral movement. In the northern hemisphere an anticyclone rotates in the clockwise direction, while it rotates counterclockwise in the southern hemisphere. The rotation is caused by the movement of colder higher pressure air that is moving away from the poles towards the equator being affected by the rotation of the earth. Since the air in an anticyclone is descending, it becomes warmed and dried, and therefore transmits radiation freely whether from the sun to the earth or from the earth into space. Hence in winter anticyclonic weather is characterized by clear air with periods of frost, causing fogs in towns and low-lying damp areas, and in summer by still cloudless days with gentle variable airs and fine weather. Anticyclones generally bring fair weather and clear skies as the dynamics of an anticyclone lead to downward vertical movement which suppresses convective activity and generally lowers the mean relative humidity, in contrast to the upward vertical movement in a cyclone. However as the anticyclone moves over the earth surface it may heat up locally, acquire water from the land or oceans or encounter warmer wet air. Local geography may cause a range of localised weather phenomena specific to anticyclones, while the interaction of the different air masses, which occurs at weather fronts, may cause a range of weather events.

Origin

Sir Francis Galton proposed the existence of the anticyclone. An ingenious man who traveled widely and left his marks upon the world, he wrote Meteorographica, or Methods of Mapping the Weather (1863). The discovery of the anticyclone enabled meteorologists to draw the modern weather map.

Dry Air

All anticyclones are produced by dry air that settles to the surface of the earth and accumulates, forming air masses. The absence of aqueous vapor (water vapor) increases the density of air which means that each volumetric unit of dry air weighs more than the same volumetric unit of humid air at the same temperature, and vapor pressure. The two most common parts of the air are nitrogen (roughly 78% of the total) and oxygen (roughly 21% of the total). Together, the two components weigh more than 99% of the total weight of the atmosphere. When air takes on aqueous vapor (water vapor), vapor pressure displaces some of the heavier Nitrogen and Oxygen, thus, a mixture that is lighter in weight overall is created. Displacement by vapor pressure produces intense tropical storms called hurricanes, typhoons, or baguios. The weight of air is called its air pressure.

Cool or cold dry air type

Cool or cold dry air settles onto land and forms shallow anticyclones or high-pressure cells which often move across the terrain and create fair weather with little cloudiness or precipitation, then dissipate and vanish after reaching the open sea. The types of anticyclones display different patterns of movement.

High-latitudes maritime type

In the months of winter, many strong cyclones appear at high latitudes. Rising air in them eventually descends to form anticyclones. Tall anticyclones appear at some places each year during the coldest months. They may exceed 35,000 feet or 10,200 meters in height. The position of each anticyclone is at about the same place on the surface as it is far above the surface. The sea-level pressure may exceed 1040 millibars (hectopascals) (hPa) (SI). They tend to linger close to the place at which they had appeared. The Denmark Strait along the east coast of Greenland is a place where they often appear, particularly during the winter. They form part of the North Atlantic Oscillation that significantly influences the weather in that region of the Northern Hemisphere. The Beaufort Sea is an arm of the Arctic Ocean that exists north of northwestern Canada. An anticyclone called the Arctic High or the Beaufort High forms there. [http://nsidc.org/arcticmet/patterns/anticyclones.html NSIDC]

Warm, dry air type

An anticyclone composed of warm dry air may be situated over much of the North Atlantic ocean during most of the year. The warm dry air type of anticyclone is tall and may be observed on weather charts above three miles (5km) in height. The warm dry air type of anticyclone is usually described as being semipermanent. Frontal activity is not associated with it. Transoceanic in extent, in Europe it is called the Azores High, and in the United States it is known by the name Bermuda High. Since it has a tropical origin, its most proper name is extratropical anticyclone (but see the last paragraph, below). It has a characteristic "vertical displacement" that shifts its center away from its surface position towards the equator and westwards, too. Far above the surface of the North Atlantic ocean at a height of 3-4 miles (5-7km), the center of the high-pressure cell may be seen about 3,000 miles (5,000km) southwestwards of its surface position (which is in the general vicinity of the Azores Islands). The maximum sea-level pressure in this type of anticyclone is not very high. It may reach, perhaps, 1025 millibars (hectopascals) or thereabouts during the summertime, which is a mere twelve millibars above the average sea-level pressure of 1013 millibars. Similar anticyclones that are built of warm dry air exist over other oceanic areas of the world, such as the South Atlantic ocean. The anticyclone that is located there is practically a mirror-image of the anticyclone that is located over the North Atlantic ocean. Its "vertical displacement" is also towards the equator and westwards, too. The warm dry air is continually being produced in the Intertropical convergence zone (ITCZ) by thunderstorms. Confusion may be created by the fact that the term "subtropical anticyclone" is used by meteorologists in Australia in place of "extratropical anticyclone", which is the term that used in the United states. Except for the wordage, there is no difference; there are no separate or different types of warm dry air anticyclones being generated by the ITCZ. Both extratropical cyclones and extratropical anticyclones are seen in the Northern Hemisphere, year-round. For an example of the Australian term, see: [http://www.bom.gov.au/lam/glossary/epagegl.shtml Equatorial trough]

References

External links

- [http://stormwiki.unk.edu/index.php/Anticyclonic_rotation Glossary Definition: Anticyclonic rotation] - StormWiki
- [http://rsd.gsfc.nasa.gov/rsd/images/goes8_lg.jpg Intertropical Convergence Zone photo] - NASA Goddard Space Flight Center Category:Weather Category:Tropical cyclone meteorology

Depression (meteorology)

A depression (also called a 'low') is an area of low pressure caused by rising air. This is opposite to an anti-cyclone which is an area of high pressure. Water vapour in the rising air cools and condenses to form clouds and precipitation. For this reason depressions normally bring cloudy, wet and windy weather. In the northern hemisphere this rising air tends to circle in an anticlockwise direction due to the coriolis effect. For more information see Low-pressure cell

Links

- [http://www.metoffice.com/education/curriculum/lesson_plans/weathersystems/partd.html MET Office, UK] Category:Weather

Molecule

A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. According to the strict definition, molecules can consist of one atom (as in noble gases) or more atoms bonded together. The concept of monatomic (single-atom) molecule is used almost exclusively in the kinetic theory of gases. In molecular sciences, a molecule consists of a stable system (bound state) comprising two or more atoms. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates. A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond). Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positvists such as Mach. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy. microwave spectroscopy (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres; white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single bonds.]]

Chemical bond

:See main article chemical bond In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O₂), or of different elements, as with water (H₂O).

Size

Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes. The smallest molecule is the hydrogen molecule. The interatomic distance is 0.15 nanometres (1.5 Å). But the size of its electron cloud is difficult to define precisely. Under standard conditions molecules have a dimension of a few to a few dozen Å.

Empirical formula

:See main article empirical formula The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

Chemical formula

:See main article chemical formula The chemical formula reflects the exact number of atoms that compose a molecule. The molecular mass can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a ¹²C isotope atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

:See main article molecular geometry Molecules have fixed equilibrium geometries—bond lengths and angles—. A pure substance is composed of molecules with the same geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.

Molecular spectroscopy

:See main article spectroscopy Molecular spectroscopy is the study of the response (spectrum) of a molecule to a signal of known energy (or frequency, according to Planck's formula). This signal is usually an electromagnetic wave or a beam of electrons, but new molecular spectroscopies, such as the positron spectroscopy, are under development. The molecular response can be signal absorption (absorption spectroscopy), emission of another signal (emission spectroscopy), fragmentation, or a change in its chemical nature. Spectroscopy is recognized as the most powerful tool in the investigation of the microscopic properties of molecules, and, in particular, their energy levels. Nowadays, in order to extract the maximum microscopic information from the experimental results, spectroscopical studies are very often coupled with computational chemical investigations. The theoretical background of spectroscopy is the scattering theory.

Metre

:This article is about the unit of length. For other uses of metre or meter, see meter (disambiguation). The metre (Commonwealth English) or meter (American English) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second. Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

The word metre is from the Greek metron (μετρον), "a measure" via the French mètre. Its first recorded usage in English is from 1797. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum. In 1793, France adopted the metre, with this definition, as its official unit of length. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the earth, this length became the standard. So, the circumference of the Earth through the poles is approximately forty million metres. Earth in a vacuum.]] In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation would preserve the new prototype metre and kilogram when constructed, distribute national metric prototypes, and would maintain comparisons between them and non-metric measurement standards. This organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light: :The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.

Timeline of definition

- 1790 May 8 — The French National Assembly decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
- 1791 March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass.
- 1799 December 10 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
- 1889 September 28 — The first CGPM defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice.
- 1927 October 6 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
- 1960 October 20 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p¹⁰ and 5d⁵ quantum levels of the krypton-86 atom.
- 1983 October 21 — The seventeenth CGPM defines the length to be distance travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

External links

- [http://www.unitconversion.org/unit_converter/length.html?unit=meter&value=1 Length Converter: convert metre to other units, such as yard, mile, and so on]
- [http://physics.nist.gov/cuu/Units/meter.html History of the metre at the U.S. National Institute of Standards and Technology (NIST)]
- [http://www.mel.nist.gov/div821/museum/timeline.htm Timeline of history of the metre at the NIST]
- [http://www1.bipm.org/en/scientific/length/ Bureau International des Poids et Measures - Lengths] Category:SI base units Category:Units of length ko:미터 ms:Meter ja:メートル simple:Metre th:เมตร

Kilometre

A kilometre (American spelling: kilometer), symbol: km is a unit of length in the metric system equal to 1000 metres (from the Greek words χίλια (khilia) = thousand and μέτρο (metro) = count/measure). It is approximately equal to 0.621 miles, 1094 yards or 3281 feet. Slang terms for kilometre include "klick" (sometimes spelt "click" or "klik") and "kay" (or "k"). All these slang terms can also refer to kilometres per hour.

Metric system

:Main articles: Metric system and Metre Like the kilometre, all units of length in the metric system are based on the metre, by adding an SI prefix that stands for a power of ten, such as hecto for one hundred to form hectometre (= 0.1 kilometre) or mega for one million to form megametre (= 1,000 kilometre). The metre is not only the basis for all units of length in the metric system, but also of units of area (the square metre) and volume (the cubic metre). This extends to the kilometre, so one can have square and cubic kilometres. Unicode has symbols for "km" (㎞), for square kilometre (㎢) and for cubic kilometre (㎦); however, they are useful only in CJK texts, where they are equal in size to one Chinese character.

Pronunciation

In theory, the pronunciation of the word kilometre should have the stress placed on the first syllable, in line with other metric prefixes (as in kilogram, kilojoule and, analogous, kilobyte). However, pronunciation with the stress on the second syllable is usual in English.

Kilonewton

---- The newton (symbol: N) is the SI unit of force. It is named after Sir Isaac Newton in recognition of his work on classical mechanics.

Definition

A newton is the amount of force required to accelerate a mass of one kilogram at a rate of one metre per second squared. :1 N = 1 kg·m·s^–2

SI multiples

Explanation

The notions of mass and force are often confused in everyday life, but must be kept separate in science and engineering. The newton was first used around 1904, but not until 1948 was it officially adopted by the General Conference on Weights and Measures (CGPM) as the name for the MKS unit of force. Rather fittingly, given the story about how Newton arrived at his theory of gravity after contemplating why an apple falls downwards, the mass of a small apple exerts a force of about 1 newton on Earth.

Conversions

Density of air

The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics. In the SI system it is measured as the number of kilograms of air in a cubic meter (kg/m³). At sea level and at 20 °C dry air has a density of approximately 1.2 kg/m³. varying with pressure and temperature. Air density and air pressure decrease with increasing altitude.

Effects of temperature and pressure

The formula for the density of air is given by: :

\rho = \frac

where ρ is the air density, p is pressure, R is the gas constant, and T is temperature. The individual gas constant R for dry air is: :

R_\mathrm = 287.05 \frac

Therefore:
- At standard temperature and pressure (0 °C and 101.325 kPa), dry air has a density of ρ_STP = 1.293 kg/m³.
- At standard ambient temperature and pressure (25 °C and 100 kPa) dry air has a density of ρ_SATP = 1.168 kg/m³.

Effect of water vapor

For moist air, the partial pressure of the water vapor must be considered as well. In this case, the density of the air is the sum of the density of the dry air and the density of the water vapor: :

\rho = \frac + \frac

The gas constant for water vapor is: :

R_\mathrm = 461.495 \frac

Effects of altitude

To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using the universal gas constant instead of the specific one:
- sea level atmospheric pressure p₀ = 101325 Pa = 1013.25 mbar or hPa = 101.325 kPa (= 101325 kg/m²)
- sea level standard temperature T₀ = 288.15 K
- Earth-surface gravitational acceleration g = 9.80665 m/s².
- dry adiabatic lapse rate L = −0.0065 K/m
- universal gas constant R = 8.31447 J/(mol·K)
- molecular weight of dry air M = 0.0289644 kg/mol Temperature at altitude h metres above sea level is given by the following formula (only valid below the tropopause): :

T = T_0 + L \cdot h

The pressure at altitude h is given by: :

p = p_0 \cdot \left(1 + \frac \right)^\frac

Density can then be calculated according to a molar form of the original formula: :

\rho = \frac

Importance of temperature

The below table demonstrates that the properties of air change significantly with temperature. Table - speed of sound in air c, density of air ρ, acoustic impedance Z vs. temperature °C

External links

- [http://www.sengpielaudio.com/ConvDensi.htm Conversions of density units ρ]
- [http://wahiduddin.net/calc/density_altitude.htm Air density and density altitude calculations] Category:Meteorology Category:Weather

Kilopascal

---- The pascal (symbol: Pa) is the SI unit of pressure. It is equivalent to one newton per square metre. The same unit is also used for stress, Young's modulus, and tensile strength.

Definition

1 pascal (Pa) = 1 N/m² = 1 J/m³ = 1 kg·m^–1·s^–2

SI multiples

Origin

The unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher.

Explanation

1 megapascal (MPa) = 1 000 000 Pa = 1 N/mm². Standard atmospheric pressure is 101 325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25 mbar = 760 Torr (ISO 2533). Meteorologists worldwide have for a long time measured atmospheric pressure in millibars. After the introduction of SI units, many preferred to preserve the customary pressure figures. Therefore, meteorologists use hectopascals today for air pressure, which are equivalent to millibars, while similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. : 1 hectopascal (hPa) = 100 Pa = 1 mbar. : 1 kilopascal (kPa) = 1000 Pa = 10 hPa. In the former Soviet mts system, the unit of pressure is the pieze, which is equivalent to one kilopascal. The Unicode computer character set has dedicated symbols ㎩ for Pa and ㎪ for kPa, but these exist merely for backward-compatibility with some older ideographic character-sets.

Comparison to other units of pressure

Millimetre

To help compare different orders of magnitude this page lists lengths between 10^-3 m and 10^-2 m (1 mm and cm).
- Distances shorter than 1 mm
- 1.0 mm is equal to
- 1/1000^th of a metre
- 0.039 inches
- side of square of area 1 mm²
- edge of cube of volume 1 mm³
- 2.54 mm — distance between pins on old DIP (dual-inline-package) electronic components
- 5 mm — length of average red ant
- 7.62 mm — common military ammunition size
- Distances longer than 1 cm

Torr

The torr (symbol: Torr) or millimetre of mercury (mmHg) is a non-SI unit of pressure. It is the atmospheric pressure that supports a column of mercury 1 millimetre high. The unit is named after Evangelista Torricelli, Italian physicist and mathematician, for his discovery of the principle of the barometer. One way to define pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. Although a manometer may use any fluid in principle, common fluids like water give heights that cannot be contained in a normal room. A water column needs to be of the order of 10 metres high to exert 1 atmosphere of pressure. Therefore a very dense fluid is required—mercury. Normal atmospheric pressure can support around 760 mm of mercury; hence 1/760 of an atmosphere, or 1 mm of mercury (mmHg), has been a convenient measure of pressure for a long time, and is sometimes also called a torr. Because the standard atmosphere has been precisely defined (10th CGPM, 1954), and the standard atmosphere had previously been defined as 760 mmHg exactly, those two definitions are now combined to define the torr as exactly 101325/760 ≈ 133.3223684 pascals. Although the pascal is now the more commonly used unit of pressure, the torr is still used in high vacuum engineering, particularly where pressures are low enough that viscosity is absent. The torr, usually under the millimetre of mercury name, remains a common unit for the measurement of blood pressure in much of the world. Although they are synonyms in practice, the torr and millimetre of mercury are very slightly different by virtue of their definitions in British Standard BS 2520 ([http://www.sizes.com/units/mmHg.htm], [http://www.npl.co.uk/pressure/punits.html]). While the torr is defined as given above, the millimetre of mercury (called the "conventional millimetre of mercury") is defined by the World Meteorological Organization [http://stommel.tamu.edu/~baum/paleo/paleogloss-old/node38.html] as "the pressure exerted at the bottom of a vertical column exactly 1 mm deep of a fluid whose density is exactly 13.5951 g/cm³, at a location where the acceleration due to gravity is exactly 980.665 cm/s²" [http://home.att.net/~numericana/answer/constants.htm]. The "conventional density of mercury" used makes 760 mmHg equal a pressure of exactly 101325.0144354 Pa, a percentage difference from the standard atmosphere of about 0.14 μPa/Pa (i.e., 0.000014 %). Such a small difference is utterly negligible in most practical applications.

External links

- [http://www.npl.co.uk/pressure/punits.html NPL - pressure units] ja:トル

Millibar

:Note that Mbar redirects here; if used with a capital M, it would be megabar. See SI prefix. A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. It is equivalent to 1 hPa. The millibar was introduced by Sir Napier Shaw in 1909, and internationally adopted in 1929. Unicode has a symbol for "mb": (㏔). The millibar is not an SI unit. The SI unit is the pascal (Pa), with 1 mbar = 100 Pa = 1 hPa = 0.1 kPa. Meteorologists worldwide have long measured air pressure in a variety of units including millibars. It has taken some time after the introduction of SI units for people to change to pascals. The unit millibar is still used although official use is gradually changing to hPa which is the numerically equivalent SI unit. Similar pressures are given in kilopascals in practically all other fields where the hecto prefix is hardly ever used. In Canadian weather reports, the norm is kPa. Americans are familiar with the millibar in US reports of hurricanes and other cyclonic storms, where lower central pressure generally means higher winds and a stronger storm.

Typical pressure

The average sea-level pressure is 1013.25 hPa (mbar) . Air pressure decreases exponentially with increased altitude, at a rate of about 12% per 1000 m.

Pound-force per square inch

:The abbreviation psi has multiple meanings; see Psi for other possibilities. ---- Pound-force per square inch (symbol: lbf/in²) is a non-SI unit of pressure.

Definition

1 lbf/in² = 6 894.757 29 Pa

Explanation

The language-dependent abbreviation psi is common in regions where English is spoken. At 1 lbf/in², a force of one pound-force is applied to an area of one square inch. Other abbreviations are used:
- psia (pounds-force per square inch absolute) - gauge pressure plus local atmospheric pressure.
- psid (psi difference) - difference between two pressures.
- psig (pounds-force per square inch gauge)
- psivg (psi vented gauge) - difference between the measuring point and the local pressure.
- psisg (psi sealed gauge) - difference between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point. The ksi (kludge) is defined as 1000 psi, combining the prefix kilo with the psi abbreviation. It is occasionally seen in materials science and mechanical engineering, where it is used to specify stress and Young's modulus.

Context

- atmospheric pressure on the ocean's surface - 14.7 psia
- automobile tire - 32 psig
- air brake reservoir - 90 to 120 psig
- full scuba tank - 3,000 psig

External links

- [http://www.ex.ac.uk/trol/scol/ccpress.htm Conversion Calculator for Units of Pressure]
- [http://www.aeroconsystems.com/electronics/Pressure_transducer_basics/Transducer_primer.htm Pressure measurement primer] Category:Customary units in the United States Category:Imperial units ja:重量ポンド毎平方インチ

Technical atmosphere

A technical atmosphere (symbol: at) is a non-SI unit of pressure equal to 1 kilogram-force per square centimeter, i.e. 98.066 5 kilopascals (kPa) or about 0.96784 standard atmospheres.

External links

- NIST Guide for the Use of the International System of Units (SI), [http://physics.nist.gov/Pubs/SP811/appenB8.html Appendix B: Conversion Factors] Category:Atmosphere Category:Obsolete units of measure ja:工学気圧

Isobar

An isobar is a line of equal or constant pressure on a graph, plot, or map; an isopleth of pressure. In meteorology, the pressures shown are reduced to sea level, not the surface pressures at the map locations. Also, in nuclear physics, two nuclides with the same mass number are called isobars. For example, Boron-12 and Carbon-12 are isobars. Isobar has also occasionally been used as a synonym for a heat pipe.

External links

- [http://www.nsdl.arm.gov/Library/glossary.shtml#Isobar National Science Digital Library - Isobar]
- [http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl?Drawing_Contour_Plots Drawing Contour Plots]. A lesson plan that deals with drawing various isopleths including isobars. Category:Meteorology

QNH

QNH is a Q code used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain defined region. This region may be fairly widespread, or apply only to the airfield for which the QNH was given. The QNH for a particular airport landing area is QFE and will give the runway elevation above sea level on landing. In the UK the lowest forecast value of QNH for an altimeter setting region is called the "Regional Pressure Setting" and may be used to ensure safe terrain separation when cruising at lower altitudes. In some parts of the world a similar procedure is adopted and this is known as "Regional QNH" however this name has been modified to the above in the UK for reasons of ambiguity. The mnemonic for the code is "Query Newlyn Harbour". Newlyn Harbour in Cornwall, UK is home to the National Tidal and Sea Level Facility which is a reference for mean sea level. Another mnemonic sometimes used is "Q - Not Here" meaning it refers to the pressure setting that applies away from the airfield. This is to distinguish it from QFE, which novices sometimes confuse. ATC may update pilots with the QNH on a regular basis. A typical radio conversation might go:
- Pilot: Golf Whiskey Alpha Charlie Foxtrot, requesting regional QNH
- ATC: Golf Charlie Fox, Cotswold QNH one-zero-one-three
- Pilot: QNH one-zero-one-three, Golf Charlie Fox Here, the pilot of G-WACF requests the regional air pressure, which is given as 1013 millibars for the Cotswold region. The pilot reads back the safety-critical part of the transmission (in this case the QNH), as he is required to do. In most parts of the world, QNH is given in millibars (or hectopascals). In North America, QNH is given in hundredths of inches of mercury (in the example, ATC would say "Golf Charlie Fox, baro two niner niner two" as in 29.92 inches of mercury).

QFE

Aviation Acronym

QFE is a Q code used by pilots and air traffic control (ATC) to refer to the barometric altimeter setting which will cause an altimeter to read height above a particular runway threshold. An altimeter set to QFE will therefore read zero when on the ground at the beginning of the runway. It can be thought of as the QNH for a particular airfield. This setting may be used during take off and landing and when flying in the circuit. A mnemonic for the code is "Q Field Elevation". ATC will update pilots with the QFE when necessary. A typical radio conversation might go:
- Pilot: Golf Whiskey Alpha Charlie Foxtrot, requesting taxi clearance for local VFR.
- ATC: Golf Charlie Fox, taxi to Alpha for 25 right hand, QFE niner-niner-eight millibars.
- Pilot: To Alpha, 25 right, QFE niner-niner-eight, Golf Charlie Fox. Here, the pilot of G-WACF (who is on the ground) requests a taxi clearance and is told to taxi to holding point A for runway 25, the circuit is right-handed and QFE is 998 millibars. The pilot acknowledges the information by repeating it back to ATC. In most parts of the world, QFE is given in millibars (or hectopascals, which is the same-sized SI unit). Whilst the Royal Air Force (RAF) and some European private pilots still use QFE, it is largely obsolete in commercial aviation, where QNH is preferred for take off and landing. In the USA and Canada QFE is rarely used.

Microsoft Acronym

QFE stands for Quick Fix Engineering. This is the Microsoft term for a 'Bug patch' or as it was previously known, a 'Hotfix'. Many software modules related to Microsoft products return a QFE number indicating a patch number in the version/build information. QFE's are often bundled together to make a 'Service Pack'.

Internet Acronym

:QFE is also a popularly used internet forum acronym for "quoted for emphasis."

Altimeter

An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The traditional altimeter found in most aircraft works in measuring the air pressure from a static port in the airplane. Air pressure decreases with an increase of altitude - about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) close to sea level. The altimeter is calibrated to show the pressure directly as altitude in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). The reference pressure can be adjusted by a setting knob. This is necessary since sea level air pressure varies with the weather. In pilot's jargon, the regional or local air pressure at mean sea level is called the QNH, and the pressure which will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field. An altimeter cannot however be adjusted for variations in air temperature. Difference in temperature from the ISA model will therefore cause error in indicated altitude. The calibration formula for an altimeter, up to 36,090 feet (11,000 m), can be written as: :

h = \frac

where h is the indicated altitude in feet,

P_0

is the static pressure and

P_

is the reference pressure (use same units for both). Other types of altimeter are the radar altimeter that measures the altitude more exactly using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure the exact height during the landing procedure of commercial aircraft. Mountaineers use wrist-mounted altimeters when on high-altitude expeditions, as do skydivers.

Scientific Uses

A number of satellites (See links) use exotic dual-band radar altimeters to measure height from a spacecraft. That measurement, coupled with orbital elements (possibly from GPS), enables determination of the topography. The two lengths of radio waves permit the altimeter to automatically correct for varying delays in the ionosphere. Over water, detailed satellite altitude information has proven amazingly useful. Humps in the water indicate gravit

Air Pressure

Standard atmospheric pressure

Mean sea level pressure (MSLP or SLP)

Atmospheric pressure variation

Intuitive feeling for atmospheric pressure based on height of water

See also

References

External links

Experiments

Pressure

Example

Relative or gauge pressure

Scalar nature of pressure

Hydrostatic pressure

Stagnation pressure

Units

See also

External links

Earth's atmosphere

Temperature and the atmospheric layers

Various atmospheric regions

Pressure

Thickness of the atmosphere

Composition

Heterosphere

Density and mass

The evolution of the Earth's atmosphere

References

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External links

Weight

Weight and mass

Weight as a force

Comparative weights on bodies of the solar system

Human weight in the medical sciences and ordinary language

Sports usage

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Air mass

Anti-cyclone

Origin

Dry Air

Cool or cold dry air type

High-latitudes maritime type

Warm, dry air type

References

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External links

Depression (meteorology)

Links

Molecule

Chemical bond

Size

Empirical formula

Chemical formula

Molecular geometry

Molecular spectroscopy

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Related lists

Metre

Conversions

History

Timeline of definition

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External links

Kilometre

Metric system

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Kilonewton

Definition

SI multiples

Explanation

Conversions

See also

Density of air

Effects of temperature and pressure

Effect of water vapor

Effects of altitude

Importance of temperature

See also