Tugasgeofis Sea Level

8/2/2019 Tugasgeofis Sea Level

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Sea level

This marker indicating the sea level is placed on the path fromJerusalem to the Dead Sea.

Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as thehalfway point between the mean high tide and the mean low tide); used as a standard in reckoningland elevation.[1] MSL also plays an extremely important role in aviation, where standard sea levelpressure is used as the measurement datum of altitude at flight levels.

Measurement

Sea level measurements from 23 long tide gauge records in geologically stable environments showa rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).[citation needed]
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To an operator of a tide gauge, MSL means the "still water level"the level of the sea withmotions such as wind waves averaged outaveraged over a period of time such that changes insea level, e.g., due to thetides, also get averaged out. One measures the values of MSL in respectto the land. Hence a change in MSL can result from a real change in sea level, or from a change inthe height of the land on which the tide gauge operates.

In the UK, mean sea level has been measured at Newlyn inCornwalland Liverpool for decades,by tide gauges to provide Ordnance Datum for thezerometres height on UK maps.

In France, the Margraphe in Marseilles [1] measures continuously the sea level since 1883 andoffers the longest collapsed data about the sea level. It is used for a part of continental Europe andmain part of Africa as official sea level.

Satellite altimeters have been making precise measurements of sea level since the launch ofTOPEX/Poseidonin 1992. A joint mission ofNASA and CNES, TOPEX/Poseidon was followedby Jason-1 in 2001 and theOcean Surface Topography Mission on the Jason-2 satellite in 2008.

Difficulties in utilization

To extend this definition far from the sea means comparing the local height of the mean sea surfacewith a "level" reference surface, ordatum, called the geoid. In a state of rest or absence of externalforces, the mean sea level would coincide with this geoid surface, being an equipotential surface ofthe Earth's gravitationalfield. In reality, due to currents, air pressure variations, temperature andsalinity variations, etc., this does not occur, not even as a long term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as(stationary) ocean surface topography. It varies globally in a range of 2 m.

Traditionally, one had to process sea-level measurements to take into account the effect of the 228-monthMetonic cycle and the 223-month eclipse cycle on the tides. Mean sea level is not constantover the surface of the Earth. For instance, mean sea level at the Pacific end of the Panama Canalstands 20 cm (7.9 in) higher than at the Atlantic end.

Sea level and dry land
http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Tide_gaugehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Tidehttp://en.wikipedia.org/wiki/Tidehttp://en.wikipedia.org/wiki/Newlynhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Liverpoolhttp://en.wikipedia.org/wiki/Ordnance_Datum_Newlynhttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Metrehttp://www.ign.fr/institut/documentArticle.do?indexRoot=8&indexChild=7&idDoc=5732211http://en.wikipedia.org/wiki/TOPEX/Poseidonhttp://en.wikipedia.org/wiki/TOPEX/Poseidonhttp://en.wikipedia.org/wiki/NASAhttp://en.wikipedia.org/wiki/CNEShttp://en.wikipedia.org/wiki/Jason-1http://en.wikipedia.org/wiki/Ocean_Surface_Topography_Missionhttp://en.wikipedia.org/wiki/Ocean_Surface_Topography_Missionhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Ocean_surface_topographyhttp://en.wikipedia.org/wiki/Metonic_cyclehttp://en.wikipedia.org/wiki/Metonic_cyclehttp://en.wikipedia.org/wiki/Eclipse_cyclehttp://en.wikipedia.org/wiki/Pacific_Oceanhttp://en.wikipedia.org/wiki/Panama_Canalhttp://en.wikipedia.org/wiki/Atlantic_Oceanhttp://en.wikipedia.org/wiki/Tide_gaugehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Tidehttp://en.wikipedia.org/wiki/Newlynhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Liverpoolhttp://en.wikipedia.org/wiki/Ordnance_Datum_Newlynhttp://en.wikipedia.org/wiki/Metrehttp://www.ign.fr/institut/documentArticle.do?indexRoot=8&indexChild=7&idDoc=5732211http://en.wikipedia.org/wiki/TOPEX/Poseidonhttp://en.wikipedia.org/wiki/NASAhttp://en.wikipedia.org/wiki/CNEShttp://en.wikipedia.org/wiki/Jason-1http://en.wikipedia.org/wiki/Ocean_Surface_Topography_Missionhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Ocean_surface_topographyhttp://en.wikipedia.org/wiki/Metonic_cyclehttp://en.wikipedia.org/wiki/Eclipse_cyclehttp://en.wikipedia.org/wiki/Pacific_Oceanhttp://en.wikipedia.org/wiki/Panama_Canalhttp://en.wikipedia.org/wiki/Atlantic_Ocean


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Sea level sign (2/3 of the way up the cliff face) above Badwater Basin, Death Valley NationalPark, USA

Several terms are used to describe the changing relationships between sea level and dry land.When the term "relative" is used, it means change relative to a fixed point in the sediment pile. Theterm "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre ofthe earth, for example as a result of melting ice-caps. The term "steric" refers to global changes insea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes inthe level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy ortectoniceffects; it implies no change in the volume of water in the oceans. The melting ofglaciersat the end ofice ages is one example of eustatic sea level rise. The subsidence of land due to thewithdrawal ofgroundwater is an isostatic cause of relative sea level rise. Paleoclimatologists cantrack sea level by examining the rocks deposited along coasts that are very tectonically stable, likethe east coast of North America. Areas like volcanic islands are experiencing relative sea level riseas a result of isostatic cooling of the rock which causes the land to sink.

On other planets that lack a liquid ocean, planetologists can calculate a "mean altitude" byaveraging the heights of all points on the surface. This altitude, sometimes referred to as a "sealevel", serves equivalently as a reference for the height of planetary features.

Sea level change

Local and eustatic sea level
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Water cycles between ocean,atmosphere, andglaciers.

Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark,averaged over a period of time (such as a month or a year) long enough that fluctuations caused bywaves andtides are smoothed out. One must adjust perceived changes in LMSL to account forvertical movements of the land, which can be of the same order (mm/yr) as sea level changes.Some land movements occur because of isostatic adjustment of the mantle to the melting oficesheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, andwhen the ice melts away the land slowly rebounds. Changes in ground-based ice volume alsoaffect local and regional sea levels by the readjustment of the geoid and true polar wander.Atmospheric pressure, ocean currents and local ocean temperature changes can affect LMSL aswell.

Eustatic change (as opposed to local change) results in an alteration to the global sea levels due tochanges in either the volume of water in the world oceans or net changes in the volume of theocean basins.[2]

A more recent theory as to why sea levels have varied so dramatically throughout geologic historyis, in part, due to Isostatic Rebound. Part of this new theory states that as denser oceanic plate issubducted under less dense continental plate at a convergence plate boundary with a subductionzone, some of the oceanic plate is pulled downward briefly (geologically speaking) as the oceanicplate sinks into the Earth's Mantle. The Continental crust then bounces back up, restoring itsprevious position at equilibrium. Recent studies have suggested this was the case with thecontinent of Australia.

Short term and periodic changes
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There are many factors which can produce short-term (a few minutes to 14 months) changes in sealevel.

Periodic sea level changes

Diurnal and semidiurnal astronomical tides 1224 h P 0.210+ m

Long-period tidesRotational variations (Chandler wobble) 14 month P

Meteorological and oceanographic fluctuations

Atmospheric pressure Hours to months 0.7 to 1.3 m

Winds (storm surges) 15 days Up to 5 m

Evaporation and precipitation (may also follow long-termpattern)

Days to weeks

Ocean surface topography (changes in water density andcurrents)

Days to weeks Up to 1 m

El Nio/southern oscillation 6 mo every 510 yr Up to 0.6 m

Seasonal variationsSeasonalwater balance among oceans (Atlantic, Pacific, Indian)

Seasonal variations in slope of water surface

Riverrunoff/floods 2 months 1 m

Seasonal water density changes (temperature and salinity) 6 months 0.2 m

Seiches

Seiches (standing waves) Minutes to hours Up to 2 m

Earthquakes

Tsunamis(generate catastrophic long-period waves) Hours Up to 10 m

Abrupt change in land level Minutes Up to 10 m

Long term changes

Sea-level changes and relative temperatures
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Various factors affect the volume or mass of the ocean, leading to long-term changes in eustaticsea level. The primary influence is that of temperature on seawater density and the amounts ofwater in rivers, lakes, glaciers,polar ice caps andsea ice. Over much longergeological timescales,changes in the shape of the oceanic basins and in land/sea distribution will also affect sea level.

Observational and modelling studies ofmass loss from glaciers and ice caps indicate a contributionto sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.

Glaciers and ice caps

Each year about 8 mm (0.3 inch) of water from the entire surface of the oceans falls into theAntarctica and Greenlandice sheets as snowfall. If no ice returned to the oceans, sea level woulddrop 8 mm every year. To a first approximation, the same amount of water appeared to return tothe ocean in icebergs and from ice melting at the edges. Scientists previously had estimated whichis greater, ice going in or coming out, called the mass balance, important because it causes changesin global sea level. High-precision gravimetry from satellites in low-noise flight has since

determined Greenland is losing billions of tons per year, in accordance with loss estimates fromground measurement.

Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sealevel. Likewise, the melting of the northern polarice cap which is composed of floatingpack icewould not significantly contribute to rising sea levels. Because they are lower in salinity, however,their melting would cause a very small increase in sea levels, so small that it is generally neglected.

Scientists previously lacked knowledge of changes in terrestrial storage of water.Surveying of water retention by soil absorption and by reservoirs outright ("impoundment") atjust under the volume ofLake Superioragreed with a dam-building peak in the 1930s-1970s

timespan. Such impoundment masked tens of millimetres of sea level rise in that span. (Impact of Artificial Reservoir Water Impoundment on Global Sea Level.http://www.sciencemag.org/cgi/content/full/320/5873/212?rss=1. B. F. Chao,* Y. H. Wu, Y. S.Li). If small glaciers and polar ice caps on the margins of Greenland and the AntarcticPeninsula melt, the projected rise in sea level will be around 0.5 m. Melting of the Greenlandice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet wouldproduce 61.1 m of sea level rise.[3] The collapse of the grounded interior reservoir of the WestAntarctic Ice Sheetwould raise sea level by 56 m.[4] The snowline altitude is the altitude of the lowest elevation interval in which minimumannual snow cover exceeds 50%. This ranges from about 5,500 metres above sea-level at theequator down to sea level at about 70 N&S latitude, depending on regional temperatureamelioration effects.Permafrostthen appears at sea level and extends deeper below sea levelpolewards. As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of thepermafrost zone, they cannot melt in a timeframe much less than several millennia; therefore itis likely that they will not, through melting, contribute significantly to sea level rise in thecoming century. They can, however, do so through acceleration in flow and enhanced icebergcalving.
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Climate changes during the 20th century are estimated from modelling studies to have ledto contributions of between 0.2 and 0.0 mm/yr from Antarctica (the results of increasingprecipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation andrunoff). Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over

the 20th century as a result of long-term adjustment to the end of the last ice age.

The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimaterange from the combination of factors above[5] but active research continues in this field. Theterrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quitelarge.

Geological influences

Comparison of two sea level reconstructions during the last 500 Ma. The scale of change duringthe last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic

history, long-term average sea level has been significantly higher than today.

At times duringEarth's long history, the configuration of the continents and seafloor have changeddue toplate tectonics. This affects global sea level by determining the depths of the ocean basinsand how glacial-interglacial cycles distribute ice across the Earth.

The depth of the ocean basins is a function of the age of oceanic lithosphere: as lithospherebecomes older, it becomes denser and sinks. Therefore, a configuration with many small oceanicplates that rapidly recycle lithosphere will produce shallower ocean basins and (all other thingsbeing equal) higher sea levels. A configuration with fewer plates and more cold, dense oceaniclithosphere, on the other hand, will result in deeper ocean basins and lower sea levels.

When there were large amounts of continental crust near the poles, the rock record showsunusually low sea levels during ice ages, because there was lots of polar land mass upon whichsnow and ice could accumulate. During times when the land masses clustered around the equator,ice ages had much less effect on sea level.

Over most of geologic time, long-term sea level has been higher than today (see graph above).Only at thePermian-Triassic boundary ~250 million years ago was long-term sea level lower thantoday. Long term changes in sea level are the result of changes in the oceanic crust, with adownward trend expected to continue in the very long term.[6]

During the glacial/interglacial cycles over the past few million years, sea level has varied bysomewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets(mostly in the northern hemisphere) with water evaporated from the sea.

TheMediterranean Basin's gradual growth as the Neotethys basin, begun in theJurassic, did notsuddenly affect ocean levels. While the Mediterranean was forming during the past 100 millionyears, the average ocean level was generally 200 metres above current levels. However, the largestknown example of marine flooding was when the Atlantic breached the Strait of Gibraltarat the
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end of theMessinian Salinity Crisisabout 5.2 million years ago. This restored Mediterranean sealevels at the sudden end of the period when that basin had dried up, apparently due to geologicforces in the area of the Strait.

Long-term causesRange of

effectVertical effect

Change in volume of ocean basins

Plate tectonics and seafloor spreading (plate divergence/convergence) Eustatic 0.01 mm/yr
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and change in seafloor elevation (mid-ocean volcanism)

Marine sedimentation Eustatic < 0.01 mm/yr

Change in mass of ocean water

Melting or accumulation of continental ice Eustatic 10 mm/yr

Climate changes during the 20th century

Antarctica (the results of increasing precipitation) Eustatic-0.2 to0.0 mm/yr

Greenland (from changes in both precipitation and runoff) Eustatic0.0 to0.1 mm/yr

Long-term adjustment to the end of the last ice age

Greenland and Antarctica contribution over 20th century Eustatic0.0 to0.5 mm/yr

Release of water from earth's interior Eustatic

Release or accumulation of continental hydrologic reservoirs Eustatic

Uplift or subsidence of Earth's surface (Isostasy)

Thermal-isostasy (temperature/density changes in earth's interior) Local effectGlacio-isostasy (loading or unloading of ice) Local effect 10 mm/yr

Hydro-isostasy (loading or unloading of water) Local effect

Volcano-isostasy (magmatic extrusions) Local effect

Sediment-isostasy (deposition and erosion of sediments) Local effect < 4 mm/yr

Tectonic uplift/subsidence

Vertical and horizontal motions of crust (in response to fault motions) Local effect 13 mm/yr

Sediment compaction

Sediment compression into denser matrix (particularly significant inand nearriver deltas)

Local effect

Loss of interstitial fluids (withdrawal ofgroundwateroroil) Local effect 55 mm/yr Earthquake-induced vibration Local effect

Departure from geoid

Shifts in hydrosphere, aesthenosphere, core-mantle interface Local effect

Shifts in earth's rotation, axis of spin, and precession ofequinox Eustatic

External gravitational changes Eustatic

Evaporation and precipitation (if due to a long-term pattern) Local effect

Changes through geologic time
http://en.wikipedia.org/wiki/Isostasyhttp://en.wikipedia.org/wiki/Volcanohttp://en.wikipedia.org/wiki/River_deltahttp://en.wikipedia.org/wiki/River_deltahttp://en.wikipedia.org/wiki/Groundwaterhttp://en.wikipedia.org/wiki/Groundwaterhttp://en.wikipedia.org/wiki/Petroleumhttp://en.wikipedia.org/wiki/Petroleumhttp://en.wikipedia.org/wiki/Hydrospherehttp://en.wikipedia.org/wiki/Aesthenospherehttp://en.wikipedia.org/wiki/Earth's_rotationhttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Isostasyhttp://en.wikipedia.org/wiki/Volcanohttp://en.wikipedia.org/wiki/River_deltahttp://en.wikipedia.org/wiki/Groundwaterhttp://en.wikipedia.org/wiki/Petroleumhttp://en.wikipedia.org/wiki/Hydrospherehttp://en.wikipedia.org/wiki/Aesthenospherehttp://en.wikipedia.org/wiki/Earth's_rotationhttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Gravity


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Comparison of two sea level reconstructions during the last 500 Ma. The scale of change duringthe last glacial/interglacial transition is indicated with a black bar. Note that over most of geologichistory long-term average sea level has been significantly higher than today.

Sea level change since the end of the last glacial episode. Changes displayed in metres.
http://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Metre


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Sea level has changed overgeologic time. As the graph shows, sea level today is very near thelowest level ever attained (the lowest level occurred at the Permian-Triassic boundary about 250million years ago).

During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was

about 130 m lower than today, due to the large amount of sea waterthat had evaporated and beendeposited as snow and ice, mostly in the Laurentide ice sheet. The majority of this had melted byabout 10,000 years ago.

Hundreds of similar glacial cycles have occurred throughout the Earth's history. Geologists whostudy the positions of coastal sediment deposits through time have noted dozens of similarbasinward shifts of shorelines associated with a later recovery. This results in sedimentary cycleswhich in some cases can be correlated around the world with great confidence. This relatively newbranch of geological science linking eustatic sea level to sedimentary deposits is called sequencestratigraphy.

The most up-to-date chronology of sea level change during the Phanerozoic shows the followinglong term trends:[7]

Gradually rising sea level through theCambrian Relatively stable sea level in the Ordovician, with a large drop associated with the end-Ordovician glaciation Relative stability at the lower level during the Silurian A gradual fall through the Devonian, continuing through the Mississippian to long-termlow at the Mississippian/Pennsylvanian boundary A gradual rise until the start of the Permian, followed by a gentle decrease lasting until theMesozoic.

Recent changes

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm per year.[8] The majority of this rise can be attributed to the increase in temperature of the sea and theresulting thermal expansion of sea water. Additional contributions come from water sources onland such as melting snow and glaciers (seeglobal warming).[9]

Aviation

Using pressure to measure altitude results in two other types of altitude. Distance above true orMSL (mean sea level) is the next best measurement to absolute. MSL altitude is the distance abovewhere sea level would be if there were no land. If one knows the elevation of terrain, the distanceabove theground is calculated by a simple subtraction.

An MSL altitudecalled pressure altitude by pilotsis useful for predicting physiologicalresponses in unpressurized aircraft (see hypoxia). It also correlates with engine, propeller, andwing performance, which all decrease in thinner air.
http://en.wikipedia.org/wiki/Geologic_timehttp://en.wikipedia.org/wiki/Permianhttp://en.wikipedia.org/wiki/Triassichttp://en.wikipedia.org/wiki/Sea_waterhttp://en.wikipedia.org/wiki/Snowhttp://en.wikipedia.org/wiki/Icehttp://en.wikipedia.org/wiki/Laurentide_ice_sheethttp://en.wikipedia.org/wiki/History_of_Earthhttp://en.wikipedia.org/wiki/Geologistshttp://en.wikipedia.org/wiki/Sedimenthttp://en.wikipedia.org/wiki/Sequence_stratigraphyhttp://en.wikipedia.org/wiki/Sequence_stratigraphyhttp://en.wikipedia.org/wiki/Phanerozoichttp://en.wikipedia.org/wiki/Phanerozoichttp://en.wikipedia.org/wiki/Sea_level#cite_note-6http://en.wikipedia.org/wiki/Cambrianhttp://en.wikipedia.org/wiki/Cambrianhttp://en.wikipedia.org/wiki/Ordovicianhttp://en.wikipedia.org/wiki/Ordovicianhttp://en.wikipedia.org/wiki/Silurianhttp://en.wikipedia.org/wiki/Devonianhttp://en.wikipedia.org/wiki/Mississippianhttp://en.wikipedia.org/wiki/Mississippianhttp://en.wikipedia.org/wiki/Pennsylvanianhttp://en.wikipedia.org/wiki/Pennsylvanianhttp://en.wikipedia.org/wiki/Permianhttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Sea_level#cite_note-7http://en.wikipedia.org/wiki/Global_warminghttp://en.wikipedia.org/wiki/Global_warminghttp://en.wikipedia.org/wiki/Sea_level#cite_note-IPCC4-8http://en.wikipedia.org/wiki/Sea_level#cite_note-IPCC4-8http://en.wikipedia.org/wiki/Earth's_surfacehttp://en.wikipedia.org/wiki/Earth's_surfacehttp://en.wikipedia.org/wiki/Pressure_altitudehttp://en.wikipedia.org/wiki/Pressure_altitudehttp://en.wikipedia.org/wiki/Hypoxia_(medical)http://en.wikipedia.org/wiki/Geologic_timehttp://en.wikipedia.org/wiki/Permianhttp://en.wikipedia.org/wiki/Triassichttp://en.wikipedia.org/wiki/Sea_waterhttp://en.wikipedia.org/wiki/Snowhttp://en.wikipedia.org/wiki/Icehttp://en.wikipedia.org/wiki/Laurentide_ice_sheethttp://en.wikipedia.org/wiki/History_of_Earthhttp://en.wikipedia.org/wiki/Geologistshttp://en.wikipedia.org/wiki/Sedimenthttp://en.wikipedia.org/wiki/Sequence_stratigraphyhttp://en.wikipedia.org/wiki/Sequence_stratigraphyhttp://en.wikipedia.org/wiki/Phanerozoichttp://en.wikipedia.org/wiki/Sea_level#cite_note-6http://en.wikipedia.org/wiki/Cambrianhttp://en.wikipedia.org/wiki/Ordovicianhttp://en.wikipedia.org/wiki/Silurianhttp://en.wikipedia.org/wiki/Devonianhttp://en.wikipedia.org/wiki/Mississippianhttp://en.wikipedia.org/wiki/Pennsylvanianhttp://en.wikipedia.org/wiki/Permianhttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Sea_level#cite_note-7http://en.wikipedia.org/wiki/Global_warminghttp://en.wikipedia.org/wiki/Sea_level#cite_note-IPCC4-8http://en.wikipedia.org/wiki/Earth's_surfacehttp://en.wikipedia.org/wiki/Pressure_altitudehttp://en.wikipedia.org/wiki/Hypoxia_(medical)


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Pilots can estimate height above terrain with an altimeter set to a defined barometric pressure.Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSLin the region being flown over. This pressure is referred to as either QNH or "altimeter" and istransmitted to the pilot by radio from air traffic control (ATC) or an Automatic TerminalInformation Service (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can

estimate height above ground by subtracting the terrain altitude from the altimeter reading.Aviation charts are divided into boxes and the maximum terrain altitude from MSL in each box isclearly indicated. Once above the transition altitude (see below), the altimeter is set to theinternational standard atmosphere (ISA) pressure at MSL which is 1013.2 HPa or 29.92 inHg.[10]

Flight level

MSL is useful for aircraft to avoid terrain, but at high enough altitudes, there is no terrain to avoid.Above that level, pilots are primarily interested in avoiding each other, so adjust their altimeter tostandard temperature and pressure conditions (average sea level pressure and temperature) anddisregard actual barometric pressureuntil descending below transition level. To distinguish from

MSL, such altitudes are called flight levels. Standard pilot shorthand is to express flight level ashundreds of feet, so FL 240 is 24,000 feet (7,300 m). Pilots use the international standard pressuresetting of 1013.25 hPa (29.92 inHg) when referring to Flight Levels. The altitude at which aircraftare mandated to set their altimeter to flight levels is called "transition altitude". It varies fromcountry to country. For example in the U.S. it is 18,000 feet, in many European countries it is3,000 or 5,000 feet.

Notes

1. ^ What is "Mean Sea Level"?Proudman Oceanographic Laboratory

2. ^ "Eustatic sea level". Oilfield Glossary. Schlumberger Limited.

http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20level. Retrieved 10June 2011.3. ^ "Some physical characteristics of ice on Earth". Climate Change 2001: The Scientific

Basis. http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113.4. ^ Geologic Contral on Fast Ice Flow - West Antarctic Ice Sheet. by Michael Studinger,

Lamont-Doherty Earth Observatory5. ^ GRID-Arendal. "Climate Change 2001: The Scientific Basis".

http://www.grida.no/climate/ipcc_tar/wg1/428.htm. Retrieved 2005-12-19.6. ^ Mller, R. Dietmar; et al. (2008-03-07). "Long-Term Sea-Level Fluctuations Driven by

Ocean Basin Dynamics". Science 319 (5868): 13571362. doi:10.1126/science.1151540.PMID18323446.

7. ^ Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes".

Science 322 (5898): 648. doi:10.1126/science.1161648. PMID18832639.http://www.sciencemag.org/cgi/content/full/322/5898/64.8. ^ Bruce C. Douglas (1997). "Global Sea Rise: A Redetermination". Surveys in

Geophysics18 (2/3): 279292.doi:10.1023/A:1006544227856.9. ^ Bindoff, N.L.; Willebrand, J.; Artale, V.; Cazenave, A.; Gregory, J.; Gulev, S.;

Hanawa, K.; Le Qur, C. et al. (2007). "Observations: Oceanic Climate Change and Sea Level".In Solomon, S.; Qin, D.; Manning, M. et al.. Climate Change 2007: The Physical Science Basis.Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel
http://en.wikipedia.org/wiki/Altimeterhttp://en.wikipedia.org/wiki/QNHhttp://en.wikipedia.org/wiki/Air_traffic_controlhttp://en.wikipedia.org/wiki/Air_traffic_controlhttp://en.wikipedia.org/wiki/Automatic_Terminal_Information_Servicehttp://en.wikipedia.org/wiki/Automatic_Terminal_Information_Servicehttp://en.wikipedia.org/wiki/International_standard_atmospherehttp://en.wikipedia.org/wiki/Sea_level#cite_note-9http://en.wikipedia.org/wiki/Flight_levelhttp://en.wikipedia.org/wiki/Flight_levelhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-0http://www.straightdope.com/columns/read/148/what-is-sea-level#1http://en.wikipedia.org/wiki/Proudman_Oceanographic_Laboratoryhttp://en.wikipedia.org/wiki/Proudman_Oceanographic_Laboratoryhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-Oilfield_Glossary_1-0http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20levelhttp://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20levelhttp://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20levelhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-412.htmtab113_2-0http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113http://en.wikipedia.org/wiki/Sea_level#cite_ref-3http://www.ldeo.columbia.edu/~mstuding/wais.htmlhttp://en.wikipedia.org/wiki/Lamont-Doherty_Earth_Observatoryhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-428.htm_4-0http://en.wikipedia.org/wiki/UNEP/GRID-Arendalhttp://en.wikipedia.org/wiki/UNEP/GRID-Arendalhttp://www.grida.no/climate/ipcc_tar/wg1/428.htmhttp://www.grida.no/climate/ipcc_tar/wg1/428.htmhttp://www.grida.no/climate/ipcc_tar/wg1/428.htmhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-5http://en.wikipedia.org/wiki/Science_(journal)http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1126%2Fscience.1151540http://dx.doi.org/10.1126%2Fscience.1151540http://en.wikipedia.org/wiki/PubMed_Identifierhttp://en.wikipedia.org/wiki/PubMed_Identifierhttp://www.ncbi.nlm.nih.gov/pubmed/18323446http://www.ncbi.nlm.nih.gov/pubmed/18323446http://en.wikipedia.org/wiki/Sea_level#cite_ref-6http://www.sciencemag.org/cgi/content/full/322/5898/64http://www.sciencemag.org/cgi/content/full/322/5898/64http://en.wikipedia.org/wiki/Digital_object_identifierhttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1126%2Fscience.1161648http://en.wikipedia.org/wiki/PubMed_Identifierhttp://www.ncbi.nlm.nih.gov/pubmed/18832639http://www.ncbi.nlm.nih.gov/pubmed/18832639http://www.ncbi.nlm.nih.gov/pubmed/18832639http://www.sciencemag.org/cgi/content/full/322/5898/64http://www.sciencemag.org/cgi/content/full/322/5898/64http://en.wikipedia.org/wiki/Sea_level#cite_ref-7http://en.wikipedia.org/wiki/Digital_object_identifierhttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1023%2FA%3A1006544227856http://dx.doi.org/10.1023%2FA%3A1006544227856http://en.wikipedia.org/wiki/Sea_level#cite_ref-IPCC4_8-0http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://en.wikipedia.org/wiki/Altimeterhttp://en.wikipedia.org/wiki/QNHhttp://en.wikipedia.org/wiki/Air_traffic_controlhttp://en.wikipedia.org/wiki/Automatic_Terminal_Information_Servicehttp://en.wikipedia.org/wiki/Automatic_Terminal_Information_Servicehttp://en.wikipedia.org/wiki/International_standard_atmospherehttp://en.wikipedia.org/wiki/Sea_level#cite_note-9http://en.wikipedia.org/wiki/Flight_levelhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-0http://www.straightdope.com/columns/read/148/what-is-sea-level#1http://en.wikipedia.org/wiki/Proudman_Oceanographic_Laboratoryhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-Oilfield_Glossary_1-0http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20levelhttp://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20levelhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-412.htmtab113_2-0http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113http://en.wikipedia.org/wiki/Sea_level#cite_ref-3http://www.ldeo.columbia.edu/~mstuding/wais.htmlhttp://en.wikipedia.org/wiki/Lamont-Doherty_Earth_Observatoryhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-428.htm_4-0http://en.wikipedia.org/wiki/UNEP/GRID-Arendalhttp://www.grida.no/climate/ipcc_tar/wg1/428.htmhttp://www.grida.no/climate/ipcc_tar/wg1/428.htmhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-5http://en.wikipedia.org/wiki/Science_(journal)http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1126%2Fscience.1151540http://en.wikipedia.org/wiki/PubMed_Identifierhttp://www.ncbi.nlm.nih.gov/pubmed/18323446http://en.wikipedia.org/wiki/Sea_level#cite_ref-6http://www.sciencemag.org/cgi/content/full/322/5898/64http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1126%2Fscience.1161648http://en.wikipedia.org/wiki/PubMed_Identifierhttp://www.ncbi.nlm.nih.gov/pubmed/18832639http://www.sciencemag.org/cgi/content/full/322/5898/64http://en.wikipedia.org/wiki/Sea_level#cite_ref-7http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1023%2FA%3A1006544227856http://en.wikipedia.org/wiki/Sea_level#cite_ref-IPCC4_8-0http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdf


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on Climate Change. Cambridge University Press. http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdf.10. ^ US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121

Standard sea level

Standard Sea Level (SSL) (also known as Sea Level Standard(SLS)) defines a set of conditionsfor physical calculations. The term standard sea level is used to indicate that values of properties
http://en.wikipedia.org/wiki/Cambridge_University_Presshttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-9http://en.wikipedia.org/wiki/Federal_Aviation_Administrationhttp://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocumenthttp://en.wikipedia.org/wiki/Sea_levelhttp://en.wikipedia.org/wiki/Cambridge_University_Presshttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdfhttp://en.wikipedia.org/wiki/Sea_level#cite_ref-9http://en.wikipedia.org/wiki/Federal_Aviation_Administrationhttp://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocumenthttp://en.wikipedia.org/wiki/Sea_level


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are to be taken to be the same as those standard at sea level, and is done to define values for use ingeneral calculations.

Atmospheric properties

At SSL some atmospheric properties are:

Pressure,P= 101,300Pa = 2116 lbf/ft2

Density, = 1.225 kg/m3 = 0.07647 lb/ft3 = 0.002377 slug/ft3 Temperature, T= 288.16K= 518.69R Gas constant of air,Rair= 286.97 m

2/(s2K) = 1716 ft2/(s2R)

Dynamic viscosity, = 1.79105Pas = 3.737107 slug/sft

Sources

Aerodynamics, aeronautics, andflight mechanics by Barnes Warnock McCormick Edition:illustrated Published by Wiley, 1979 Original from the University of Michigan Digitized Dec14, 2007 ISBN 0-471-03032-5, 978-0-471-03032-4 652 pages

Above mean sea level

The term above mean sea level (AMSL) refers to the elevation (on the ground) oraltitude (in theair) of any object, relative to the averagesea leveldatum. AMSL is used extensively in radio (both

inbroadcasting and othertelecommunications uses) by engineers to determine the coveragearea astation will be able to reach. It is also used in aviation, where all heights are recorded and reportedwith respect to AMSL (though also see flight level), and in theatmospheric sciences.

Definition

The concept of a "mean sea level" is in itself rather artificial, because it is not possible to determinea figure for mean sea level for the entire planet, and it varies quite a lot even on a much smallerscale. This is because the sea is in constant motion, affected by the high and low pressure zonesabove it, the tides, local gravitational differences, and so forth. The best one can do is to pick aspot and calculate the mean sea level at that point and use it as a datum. For example, theOrdnanceSurveyuses a height datum based on the measurements of mean sea level at a particular gauge atNewlyn,Cornwall from 1915 to 1921[1] for their maps of Great Britain, and this datum is actuallysome 80 cm different from the mean sea level reading obtained on the other side of the country. Analternative is to base height measurements on an ellipsoid of the entire earth, which is whatsystems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 isincreasingly used to define mean sea level. Another alternative is to use a geoid based datum suchasNAVD88.
http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pound-forcehttp://en.wikipedia.org/wiki/Square_foothttp://en.wikipedia.org/wiki/Square_foothttp://en.wikipedia.org/wiki/Square_foothttp://en.wikipedia.org/wiki/Pound_(mass)http://en.wikipedia.org/wiki/Slug_(mass)http://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Rankine_scalehttp://en.wikipedia.org/wiki/Rankine_scalehttp://en.wikipedia.org/wiki/Gas_constanthttp://en.wikipedia.org/wiki/Viscosityhttp://en.wikipedia.org/wiki/Pascal-secondhttp://en.wikipedia.org/wiki/Aeronauticshttp://en.wikipedia.org/wiki/Special:BookSources/0471030325http://en.wikipedia.org/wiki/Elevationhttp://en.wikipedia.org/wiki/Altitudehttp://en.wikipedia.org/wiki/Altitudehttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Averagehttp://en.wikipedia.org/wiki/Averagehttp://en.wikipedia.org/wiki/Sea_levelhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Broadcastinghttp://en.wikipedia.org/wiki/Broadcastinghttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Engineerhttp://en.wikipedia.org/wiki/Coveragehttp://en.wikipedia.org/wiki/Areahttp://en.wikipedia.org/wiki/Aviationhttp://en.wikipedia.org/wiki/Aviationhttp://en.wikipedia.org/wiki/Flight_levelhttp://en.wikipedia.org/wiki/Atmospheric_scienceshttp://en.wikipedia.org/wiki/Atmospheric_scienceshttp://en.wikipedia.org/wiki/Atmospheric_scienceshttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Newlynhttp://en.wikipedia.org/wiki/Newlynhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Above_mean_sea_level#cite_note-0http://en.wikipedia.org/wiki/Reference_ellipsoidhttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/World_Geodetic_Systemhttp://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/NAVD88http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pound-forcehttp://en.wikipedia.org/wiki/Square_foothttp://en.wikipedia.org/wiki/Pound_(mass)http://en.wikipedia.org/wiki/Slug_(mass)http://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Rankine_scalehttp://en.wikipedia.org/wiki/Gas_constanthttp://en.wikipedia.org/wiki/Viscosityhttp://en.wikipedia.org/wiki/Pascal-secondhttp://en.wikipedia.org/wiki/Aeronauticshttp://en.wikipedia.org/wiki/Special:BookSources/0471030325http://en.wikipedia.org/wiki/Elevationhttp://en.wikipedia.org/wiki/Altitudehttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Averagehttp://en.wikipedia.org/wiki/Sea_levelhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Broadcastinghttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Engineerhttp://en.wikipedia.org/wiki/Coveragehttp://en.wikipedia.org/wiki/Areahttp://en.wikipedia.org/wiki/Aviationhttp://en.wikipedia.org/wiki/Flight_levelhttp://en.wikipedia.org/wiki/Atmospheric_scienceshttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Ordnance_Surveyhttp://en.wikipedia.org/wiki/Newlynhttp://en.wikipedia.org/wiki/Cornwallhttp://en.wikipedia.org/wiki/Above_mean_sea_level#cite_note-0http://en.wikipedia.org/wiki/Reference_ellipsoidhttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/World_Geodetic_Systemhttp://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Datum_(geodesy)http://en.wikipedia.org/wiki/NAVD88


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Usage

When referring to geographic features such as mountains on a topographic map, variations inelevation are shown by contour lines. The elevation of a mountain denotes the highest point orsummit and is typically illustrated as a small circle on a topo map with the AMSL height shown in

either metres or feet or both.

Theheight above average terrain (HAAT) for a station is determined from topographic maps byaveraging the elevation AMSL at points along severalradials orradii. This is subtracted from theelevation AMSL of the antenna, including both the tower itself and the ground it is on, todetermine the difference.Negative numbers for HAAT sometimes result from this when the stationorairportis in a valley, which is significantly lower AMSL than the surrounding mountains. In therare case that a location is below sea level, AMSL itself is a negative number. For one such casesee Amsterdam Airport Schiphol.

AMSL is also important to engineers in high-elevation areas because some equipment is not

designed with enough airflow for sufficient cooling in the thin air, which can cause overheating,damage andfailure of the electronic components within a transmitter.

References

1. ^Newlyn Tidal Observatory National Oceanography Centre - NERC

Geopotential heightGeopotential height is a vertical coordinate referenced to Earth's mean sea level an adjustmentto geometric height (elevationabove mean sea level) using the variation of gravity with latitudeand elevation. Thus it can be considered a "gravity-adjusted height". One usually speaks of thegeopotential height of a certain pressure level, which would correspond to the geopotential heightnecessary to reach the givenpressure.

At an elevation ofh, the geopotential is defined as

where g(,z) is the acceleration due to gravity, is latitude, and z is the geometricelevation. Thus, it is the gravitational potential energy per unit mass at that level. Thegeopotential height is
http://en.wikipedia.org/wiki/Geographyhttp://en.wikipedia.org/wiki/Geographyhttp://en.wikipedia.org/wiki/Mountainhttp://en.wikipedia.org/wiki/Topographic_maphttp://en.wikipedia.org/wiki/Topographic_maphttp://en.wikipedia.org/wiki/Contour_linehttp://en.wikipedia.org/wiki/Heighthttp://en.wikipedia.org/wiki/Height_above_average_terrainhttp://en.wikipedia.org/wiki/Height_above_average_terrainhttp://en.wikipedia.org/wiki/Radio_stationhttp://en.wikipedia.org/wiki/Radial_(radio)http://en.wikipedia.org/wiki/Radial_(radio)http://en.wikipedia.org/wiki/Radiushttp://en.wikipedia.org/wiki/Antenna_(electronics)http://en.wikipedia.org/wiki/Towerhttp://en.wikipedia.org/wiki/Negative_numberhttp://en.wikipedia.org/wiki/Airporthttp://en.wikipedia.org/wiki/Airporthttp://en.wikipedia.org/wiki/Valleyhttp://en.wikipedia.org/wiki/Amsterdam_Airport_Schipholhttp://en.wikipedia.org/wiki/Amsterdam_Airport_Schipholhttp://en.wikipedia.org/wiki/Failurehttp://en.wikipedia.org/wiki/Failurehttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Above_mean_sea_level#cite_ref-0http://www.pol.ac.uk/ntslf/tgi/ntobs.htmlhttp://en.wikipedia.org/wiki/Mean_sea_levelhttp://en.wikipedia.org/wiki/Elevationhttp://en.wikipedia.org/wiki/Elevationhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/wiki/Geographyhttp://en.wikipedia.org/wiki/Mountainhttp://en.wikipedia.org/wiki/Topographic_maphttp://en.wikipedia.org/wiki/Contour_linehttp://en.wikipedia.org/wiki/Heighthttp://en.wikipedia.org/wiki/Height_above_average_terrainhttp://en.wikipedia.org/wiki/Radio_stationhttp://en.wikipedia.org/wiki/Radial_(radio)http://en.wikipedia.org/wiki/Radiushttp://en.wikipedia.org/wiki/Antenna_(electronics)http://en.wikipedia.org/wiki/Towerhttp://en.wikipedia.org/wiki/Negative_numberhttp://en.wikipedia.org/wiki/Airporthttp://en.wikipedia.org/wiki/Valleyhttp://en.wikipedia.org/wiki/Amsterdam_Airport_Schipholhttp://en.wikipedia.org/wiki/Failurehttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Above_mean_sea_level#cite_ref-0http://www.pol.ac.uk/ntslf/tgi/ntobs.htmlhttp://en.wikipedia.org/wiki/Mean_sea_levelhttp://en.wikipedia.org/wiki/Elevationhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Atmospheric_pressure


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whereg0 is the standard gravity at mean sea level.

Geophysical scientists often use geopotential height rather than geometric height, becausedoing so in many cases makes analytical calculations more convenient. For example, theprimitive equations which weather forecast models solve are more easily expressed interms of geopotential than geometric height. Using the former eliminates centrifugal forceandair density (which is very difficult to measure) in the equations.

A plot of geopotential height for a single pressure level shows the troughs and ridges,Highs and Lows, which are typically seen on upper air charts. The geopotential thicknessbetween pressure levels difference of the 850 hPa and 1000 hPa geopotential heightsfor example is proportional to mean virtual temperature in that layer. Geopotential

height contours can be used to calculate the geostrophic wind, which is faster where thecontours are more closely spaced and tangential to the geopotential height contours.

References

Hofmann-Wellenhof, B. and Moritz, H. "Physical Geodesy", 2005. ISBN3211235841 Eskinazi, S. "Fluid Mechanics and Thermodynamics of our Environment", 1975.ISBN 0122425405

Normal heightNormal heights are heightsabove sea level, one of several types of height which are all computedslightly differently. Alternatives are: orthometric heights anddynamic heights.

The normal heightH* of a point is computed from geopotential numbers by dividing the point'sgeopotential number, i.e. its geopotential difference with that of sea level, by the average, normalgravitycomputed along the plumbline of the point. (More precisely, along the ellipsoidal normal,

averaging over the height range from 0 the ellipsoid toH* ; the procedure is thus recursive.

Normal heights are thus dependent upon the reference ellipsoid chosen. The Soviet Union andmany other Eastern European countries have chosen a height system based on normal heights,determined by geodetic precise levelling. Normal gravity values are easy to compute and"hypothesis-free", i.e., one does not have to know, as one would for computing orthometricheights, the density of the Earth's crust around the plumbline.

Normal heights figure prominently in the theory of the Earth's gravity field developed by theschool ofM.S. Molodenskii. The reference surface that normal heights are measured from is called
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the quasi-geoid, a representation of "mean sea level" similar to the geoid and close to it, butlacking the physical interpretation of an equipotential surface.

Orthometric height

The orthometric height is the distance H along a line of force from a given point P at the physicalsurface of an object to the geoid.

Orthometric heights are what are usually used in the US for ordinary engineering work. Values formeasured points can be obtained from the National Geodetic Survey data sheets [1]. This data wasgathered over many years by massive projects of precise optical leveling.

Orthometric heights relate approximately to the "height above sea level" but the currentNAVD88datum is tied to a specific elevation at a chosen point rather than to any location's exact mean sealevel.

GPS measurements give earth-centered coordinates, usually displayed as height above thereference ellipsoid, which cannot be related accurately to orthometric height above the geoidunless accurate gravity data is available for that location. NGS is undertaking the GRAV-Dprogram to obtain such data.[2]

Alternatives to orthometric height include dynamic height and normal height.

References

1. ^ http://www.ngs.noaa.gov

2. ^ http://www.ngs.noaa.gov/GRAV-D/

Dynamic height

Dynamic height is a way of specifying the height of a point above a reference, as opposed toorthometric heightornormal height.

Dynamic height is constant if one follows the same gravity potential as they move from place toplace. Because of variations in gravity, surfaces having a constant difference in dynamic heightmay be closer or further apart in various places. Dynamic heights are usually chosen so that zerocorresponds to the geoid.
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When optical leveling is done, the path corresponds closely to following a value of dynamic heighthorizontally, but to orthometric height for vertical changes measured on the leveling rod. Thussmall corrections must be applied to field measurements to obtain the orthometric height usuallyused in engineering. US National Geodetic Survey data sheets [1] give both dynamic andorthometric values.

References

1. ^ http://www.ngs.noaa.gov

Theoretical gravity

In geodesy and geophysics, theoretical gravity is a means to compare the true gravity on theEarth's surface with a physically smoothed model. The most common model of a smoothed Earthis the Earth ellipsoid.

Despite of the fact that the exact density layers in the Earth's interior are still unknown, thetheoretical gravitygof its level surface can be computed by a relative simple formula, which iscalled the International Gravity Formula. It refers to a mean Earth ellipsoid, the parameters ofwhich are set by international convention. It shows the gravity at a smoothed Earth's surface as afunction ofgeographic latitude; the actual formula is

The term 0.0516323 is calledgravity flattening (abbreviated ). As a physically defined formparameter it corresponds to the geometrical flatteningfof the earth ellipsoid.

Up to the 1960s, the formula either of theHayford ellipsoid (1924) or of the famous Germangeodesist Helmert (1906) was used. Hayford has an axis difference[clarification needed] to modernvalues of 250 m, Helmert only 70 m. The Helmert formula is

A slightly different formula forgas a function of latitude is the WGS (World GeodeticSystem) 1984 EllipsoidalGravity Formula:
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The difference between the WGS-84 formula and Helmert's equation is less than 0.68ppm or 6.8107 ms2.

Current sea level rise

Sea level measurements from 23 long tide gauge records in geologically stable environments showa rise of around 200 millimetres (8 inches) per century, or 2 mm/year.
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Changes in sea level since the end of the last glacial episode

Sea levels are rising. Current sea level rise potentially impacts human populations (e.g., thoseliving in coastal regions and on islands)[1] and the natural environment (e.g., marine ecosystems).[2]

Global average sea level rose at an average rate of around 1.7 +-.3mm per year over 1950 to 2009and at a satellite-measured average rate of about 3.3 +-.4mm per year from 1993 to 2009, [3] anincrease on earlier estimates.[4] It is unclear whether the increased rate reflects an increase in theunderlying long-term trend.[5]

Two main factors contributed to observed sea level rise. [6] The first is thermal expansion: as oceanwater warms, it expands.[7] The second is from the contribution of land-based ice due to increasedmelting. The major store of water on land is found in glaciers and ice sheets.

Sea level rise is one of several lines of evidence that support the view that the climate has recentlywarmed.[8] It is likely that human-induced (anthropogenic) warming contributed to the sea levelrise observed in the latter half of the 20th century.[9]

Sea level rise is expected continue for centuries.[10] The Intergovernmental Panel on ClimateChange (IPCC) projected sea level rise of 18 and 59 cm (7.1 and 23 in) during the 21st century,

estimating that the rate would further increase.[11] IPCC explicitly refrained from assessing thelikelihood of this rise, providing an upper limit, or projecting whether ice sheet flow fromAntarctica and Greenland might change.[12] For this century, one meter of sea level rise is wellwithin the range of more recent projections.[13]

On the timescale of centuries to millennia, the melting ofice sheets could result in even higher sealevel rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic icesheet, could contribute 46 metres (13 to 20 ft) or more to sea level rise.[14]
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Overview of sea-level change

Local and eustatic sea level

Water cycles between ocean,atmosphere, andglaciers.

Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark,

averaged over a period of time (such as a month or a year) long enough that fluctuations caused bywaves andtides are smoothed out. One must adjust perceived changes in LMSL to account forvertical movements of the land, which can be of the same order (mm/yr) as sea level changes.Some land movements occur because of isostatic adjustment of the mantle to the melting oficesheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, andwhen the ice melts away the land slowly rebounds. Atmospheric pressure,ocean currents and localocean temperature changes also can affect LMSL.

Eustatic change (as opposed to local change) results in an alteration to the global sea levels, suchas changes in the volume of water in the world oceans or changes in the volume of an ocean basin.[15]

Short term and periodic changes

Many factors can produce short-term (a few minutes to 18.6 years) changes in sea level.
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Short-term (periodic) causesTime scale

(P = period)Vertical effect

Periodic sea level changes

Diurnal and semidiurnal astronomical tides 1224 h P 0.210+ m

Long-period tides

Rotational variations (Chandler wobble) 14 month PLunar Node astronomical tides 18.613 year

Meteorological and oceanographic fluctuations

Atmospheric pressure Hours to months 0.7 to 1.3 m

Winds (storm surges) 15 days Up to 5 m

Evaporation and precipitation (may also follow long-termpattern)

Days to weeks
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Ocean surface topography (changes in water density andcurrents)

Days to weeks Up to 1 m

El Nio/southern oscillation6 mo every 510yr

Up to 0.6 m

Seasonal variations

Seasonalwater balance among oceans (Atlantic, Pacific, Indian)Seasonal variations in slope of water surface

Riverrunoff/floods 2 months 1 m

Seasonal water density changes (temperature and salinity) 6 months 0.2 m

Seiches

Seiches (standing waves) Minutes to hours Up to 2 m

Earthquakes

Tsunamis(generate catastrophic long-period waves) Hours Up to 10 m

Abrupt change in land level Minutes Up to 10 m

Longer term changes

Various factors affect the volume or mass of the ocean, leading to long-term changes in eustaticsea level. The two primary influences are temperature (because the density of waterdepends ontemperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes,glaciers,polar ice caps, and sea ice. Over much longergeological timescales, changes in the shapeof oceanic basins and in land/sea distribution affect sea level.

Observational and modelling studies ofmass loss from glaciers and ice caps indicate a contributionto sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.

Glaciers and ice caps

Each year about 8 mm of ocean water falls on the Antarctica and Greenland ice sheets assnowfall.If no ice returned to the oceans, sea level would drop 8 mm every year. To a first approximation,the same amount of water appeared to return to the ocean in icebergs and from ice melting at theedges. Scientists previously had estimated which is greater, ice going in or coming out, called themass balance, important because a non-zero balance causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight determined that Greenland was losing morethan 200 billion tons of ice per year, in accord with loss estimates from ground measurement. [16]

The rate of ice loss was accelerating, having grown from 137 gigatons in 20022003.[17]

Ice shelves float on the surface of the sea and, if they melt, to a first order they do not change sealevel. Likewise, shrinkage/expansion of the northern polarice cap which is composed of floatingpack ice do not significantly affect sea level. Because ice shelf water is fresh, however, meltingwould cause a very small increase in sea levels, so small that it is generally neglected.

The melting of small glaciers and polar ice caps on the margins of Greenland and theAntarctic Peninsula melt, would increase sea level around 0.5 m. Melting of the Greenland ice
http://en.wikipedia.org/wiki/Topographyhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/El_Ni%C3%B1ohttp://en.wikipedia.org/wiki/Southern_oscillationhttp://en.wikipedia.org/wiki/Seasonhttp://en.wikipedia.org/wiki/Seasonhttp://en.wikipedia.org/wiki/Riverhttp://en.wikipedia.org/wiki/Floodhttp://en.wikipedia.org/wiki/Salinityhttp://en.wikipedia.org/wiki/Seichehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Fresh_waterhttp://en.wikipedia.org/wiki/Lakehttp://en.wikipedia.org/wiki/Polar_ice_caphttp://en.wikipedia.org/wiki/Sea_icehttp://en.wikipedia.org/wiki/Geological_timescalehttp://en.wikipedia.org/wiki/Geological_timescalehttp://en.wikipedia.org/wiki/Retreat_of_glaciers_since_1850http://en.wikipedia.org/wiki/Geography_of_Greenlandhttp://en.wikipedia.org/wiki/Snowfallhttp://en.wikipedia.org/wiki/Snowfallhttp://en.wikipedia.org/wiki/Iceberghttp://en.wikipedia.org/wiki/Glacier_mass_balancehttp://en.wikipedia.org/wiki/Gravimetryhttp://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experimenthttp://en.wikipedia.org/wiki/Sea_level_rise#cite_note-15http://en.wikipedia.org/wiki/Sea_level_rise#cite_note-16http://en.wikipedia.org/wiki/Sea_level_rise#cite_note-16http://en.wikipedia.org/wiki/Ice_shelfhttp://en.wikipedia.org/wiki/North_polehttp://en.wikipedia.org/wiki/Ice_caphttp://en.wikipedia.org/wiki/Pack_icehttp://en.wikipedia.org/wiki/Glacierhttp://en.wikipedia.org/wiki/Glacierhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Antarctic_Peninsulahttp://en.wikipedia.org/wiki/Greenland_ice_sheethttp://en.wikipedia.org/wiki/Topographyhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/El_Ni%C3%B1ohttp://en.wikipedia.org/wiki/Southern_oscillationhttp://en.wikipedia.org/wiki/Seasonhttp://en.wikipedia.org/wiki/Riverhttp://en.wikipedia.org/wiki/Floodhttp://en.wikipedia.org/wiki/Salinityhttp://en.wikipedia.org/wiki/Seichehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Fresh_waterhttp://en.wikipedia.org/wiki/Lakehttp://en.wikipedia.org/wiki/Polar_ice_caphttp://en.wikipedia.org/wiki/Sea_icehttp://en.wikipedia.org/wiki/Geological_timescalehttp://en.wikipedia.org/wiki/Retreat_of_glaciers_since_1850http://en.wikipedia.org/wiki/Geography_of_Greenlandhttp://en.wikipedia.org/wiki/Snowfallhttp://en.wikipedia.org/wiki/Iceberghttp://en.wikipedia.org/wiki/Glacier_mass_balancehttp://en.wikipedia.org/wiki/Gravimetryhttp://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experimenthttp://en.wikipedia.org/wiki/Sea_level_rise#cite_note-15http://en.wikipedia.org/wiki/Sea_level_rise#cite_note-16http://en.wikipedia.org/wiki/Ice_shelfhttp://en.wikipedia.org/wiki/North_polehttp://en.wikipedia.org/wiki/Ice_caphttp://en.wikipedia.org/wiki/Pack_icehttp://en.wikipedia.org/wiki/Glacierhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Antarctic_Peninsulahttp://en.wikipedia.org/wiki/Greenland_ice_sheet


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sheet or the Antarctic ice sheet would produce 7.2 m or 61.1 m of sea level rise, respectively.[18] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raisesea level by 56 m.[19] The interior of the Greenland and Antarctic ice sheets was as of 2009 sufficiently high (andtherefore cold) enough that direct melt would require several millennia.[citation needed] They could

do so through acceleration in flow and enhanced iceberg calving. Also, melt of the fringes ofthe ice caps could be significant, as could be sub-ice-shelf melting in Antarctica. [citation needed] Climate changes during the 20th century were estimated from modelling studies to have ledto contributions of between 0.2 and 0.0 mm/yr from Antarctica (the results of increasingprecipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation andrunoff).[citation needed] Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr overthe 20th century as a result of long-term adjustment to the end of the last ice age [citation needed].

The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimaterange from the combination of factors above [20] but active research continues in this field. The

terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quitelarge.

In 1992, satellites began recording the change in sea level;[21][22] they display an acceleration in therate of sea level change, but they have not been operating for long enough to work out whether thissignals a permanent rate change, or an artifact of short-term variation. [citation needed]

Short term variability and long-term trends

On the timescale of years and decades, sea level records contain a considerable amount ofvariability.[23] For example, approximately a 10 mm rise and fall of global mean sea level

accompanied the 19971998 El Nio-Southern Oscillation (ENSO) event. Interannual or longervariability is a major reason why no long-term acceleration of sea level has been identified using20th century data alone. However, a range of evidence clearly shows that the rate of sea level riseincreased between the mid-19th and mid-20th centuries.[24] Evidence for this includes geologicalobservations, the longest instrumental records and the observed rate of 20th century sea level rise.For example, geological observations indicate that during the last 2,000 years, sea level changewas small, with an average rate of only 0.0 to 0.2 mm per year. This compares to an average rate of1.7 mm per year for the 20th century, with a range of plus or minus 0.5 mm per year.[25]

Past changes in sea level
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