Misplaced Pages

#380619

62-581: When climbers attempt to climb Everest via the North Ridge (Tibet), the first camp on the mountain itself (traditional Camp IV, modern Camp I) is established on the North Col. From this point at approximately 7,020 metres (23,030 ft) above sea level , climbers ascend the North Ridge to reach a series of progressively higher camps along the North Face of Everest. Climbers make their final push to

124-509: A couple of naughty schoolboys" went deep into Chinese territory, down to Rongbuk and round to the old prewar Camp III beneath the North Col. This map is inverted; south is up and north is down. The North Col is lower than South Col , and farther from the Everest peak. This Tibet location article is a stub . You can help Misplaced Pages by expanding it . Sea level Mean sea level ( MSL , often shortened to sea level )

186-409: A defined barometric pressure . Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an automatic terminal information service (ATIS). Since the terrain elevation is also referenced to MSL,

248-432: A few metres, in timeframes ranging from minutes to months: Between 1901 and 2018, the average sea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s. This was faster than the sea level had ever risen over at least the past 3,000 years. The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022. Climate change due to human activities

310-403: A given time and location using astrophysical data and formulas, to yield the tidal correction △ g tid . The local topography of the land surface affects the gravity measurement. Both terrain higher than the measurement point and valleys lower than the measurement point reduce the measured value of gravity. This is taken into account by the terrain correction △ g T . The terrain correction

372-450: A gravity anomaly is always specified with reference to a particular model. The Bouguer , free-air , and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. A gravity survey is conducted by measuring the gravity anomaly at many locations in a region of interest, using a portable instrument called a gravimeter . Careful analysis of the gravity data allows geologists to make inferences about

434-545: A gravity anomaly is always specified with reference to a particular model. The Bouguer , free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. The starting point for the model field is the International Reference Ellipsoid, which gives the normal gravity g n for every point on the Earth's idealized shape. Further refinements of

496-473: A local positive anomaly may indicate a body of metallic ores . Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. For example, the northeast-southwest trending high across central New Jersey represents a graben of Triassic age largely filled with dense basalts . Currently,

558-497: A location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity measured at every point on its surface would be given precisely by a simple algebraic expression. However, the Earth has a rugged surface and non-uniform composition, which distorts its gravitational field. The theoretical value of gravity can be corrected for altitude and

620-438: A reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level ), and in the atmospheric sciences , and in land surveying . An alternative is to base height measurements on a reference ellipsoid approximating the entire Earth, which is what systems such as GPS do. In aviation, the reference ellipsoid known as WGS84

682-470: A sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century. Local factors like tidal range or land subsidence will greatly affect the severity of impacts. For instance, sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century. Yet, of the 20 countries with

SECTION 10

#1732772942381

744-496: A theoretical model and using this to correct the Bouguer anomaly to yield an isostatic anomaly. Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Local measurements of the gravity of Earth help us to understand the planet's internal structure. The Bouguer anomaly over continents is generally negative, especially over mountain ranges. For example, typical Bouguer anomalies in

806-503: Is a measure of the local departure from isostatic equilibrium, due to dynamic processes in the viscous mantle. At the center of a level plateau, it is approximately equal to the free air anomaly. The isostatic correction is dependent on the isostatic model used to calculate isostatic balance, and so is slightly different for the Airy-Heiskanen model (which assumes that the crust and mantle are uniform in density and isostatic balance

868-496: Is accurate to 0.1 mgal at any latitude λ {\displaystyle \lambda } . When greater precision is needed, a more elaborate formula gives the normal gravity with an accuracy of 0.0001 mgal. The Sun and Moon create time-dependent tidal forces that affect the measured value of gravity by about 0.3 mgal. Two-thirds of this is from the Moon. This effect is very well understood and can be calculated precisely for

930-417: Is an average surface level of one or more among Earth 's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum  – a standardised geodetic datum  – that is used, for example, as a chart datum in cartography and marine navigation , or, in aviation, as the standard sea level at which atmospheric pressure

992-594: Is around +70 mgal along the Andes coast, and this is attributed to the subducting dense slab. The trench itself is very negative, with values more negative than −250 mgal. This arises from the low-density ocean water and sediments filling the trench. Gravity anomalies provide clues on other processes taking place deep in the lithosphere . For example, the formation and sinking of a lithospheric root may explain negative isostatic anomalies in eastern Tien Shan . The Hawaiian gravity anomaly appears to be fully compensated within

1054-488: Is because the sea is in constant motion, affected by the tides, wind , atmospheric pressure, local gravitational differences, temperature, salinity , and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as a datum . For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge . Still-water level or still-water sea level (SWL)

1116-438: Is calculated from knowledge of the local topography and estimates of the density of the rock making up the high ground. In effect, the terrain correction levels the terrain around the measurement point. The terrain correction must be calculated for every point at which gravity is measured, taking into account every hill or valley whose difference in elevation from the measurement point is greater than about 5% of its distance from

1178-427: Is due to change in either the volume of water in the world's oceans or the volume of the oceanic basins . Two major mechanisms are currently causing eustatic sea level rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands. Many factors can produce short-term changes in sea level, typically within

1240-483: Is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and local mean sea level. Another alternative is to use a geoid -based vertical datum such as NAVD88 and the global EGM96 (part of WGS84). Details vary in different countries. When referring to geographic features such as mountains, on a topographic map variations in elevation are shown by contour lines . A mountain's highest point or summit

1302-512: Is measured to calibrate altitude and, consequently, aircraft flight levels . A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location. Sea levels can be affected by many factors and are known to have varied greatly over geological time scales . Current sea level rise is mainly caused by human-induced climate change . When temperatures rise, mountain glaciers and polar ice sheets melt, increasing

SECTION 20

#1732772942381

1364-477: Is positive over ocean basins and negative over high continental areas. This shows that the low elevation of ocean basins and high elevation of continents is compensated by the thickness of the crust at depth. The higher terrain is held up by the buoyancy of thicker crust "floating" on the mantle. The isostatic anomaly is defined as the Bouger anomaly minus the gravity anomaly due to the subsurface compensation, and

1426-541: Is provided by changes in crust thickness), the Pratt-Hayford model (which assumes that the bottom of the crust is at the same depth everywhere and isostatic balance is provided by lateral changes in crust density), and the Vening Meinesz elastic plate model (which assumes the crust acts like an elastic sheet). Forward modelling is the process of computing the detailed shape of the compensation required by

1488-800: Is the gravity anomaly. The normal gravity accounts for the bulk gravitation of the entire Earth, corrected for its idealized shape and rotation. It is given by the formula: g n = g e ( 1 + β 1 sin 2 ⁡ λ + β 2 sin 2 ⁡ 2 λ ) {\displaystyle g_{n}=g_{e}(1+\beta _{1}\sin ^{2}\lambda +\beta _{2}\sin ^{2}2\lambda )} where g e {\displaystyle g_{e}} = 9.780 327  m⋅s ; β 1 {\displaystyle \beta _{1}} = 5.302 44 × 10 ; and β 2 {\displaystyle \beta _{2}} = −5.8 × 10 . This

1550-450: Is the level of the sea with motions such as wind waves averaged out. Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides , also have zero mean. Global MSL refers to a spatial average over the entire ocean area, typically using large sets of tide gauges and/or satellite measurements. One often measures the values of MSL with respect to

1612-727: Is the main cause. Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise , with another 42% resulting from thermal expansion of water . Sea level rise lags behind changes in the Earth 's temperature by many decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened. What happens after that depends on human greenhouse gas emissions . If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. It could then reach by 2100 slightly over 30 cm (1 ft) from now and approximately 60 cm (2 ft) from

1674-531: Is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such as Death Valley, California , the elevation AMSL is negative. It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid . In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of

1736-599: The Central Alps are −150 milligals. By contrast, the Bouguer anomaly is positive over oceans. These anomalies reflect the varying thickness of the Earth's crust. The higher continental terrain is supported by thick, low-density crust that "floats" on the denser mantle, while the ocean basins are floored by much thinner oceanic crust. The free-air and isostatic anomalies are small near the centers of ocean basins or continental plateaus, showing that these are approximately in isostatic equilibrium. The gravitational attraction of

1798-570: The geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean , whose surface dips as much as 106 m (348 ft) below the global mean sea level (excluding minor effects such as tides and currents). Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level. Instantaneous sea level varies substantially on several scales of time and space. This

1860-418: The height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today. Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at the poles and 6,371.001 km (3,958.756 mi) on average. This flattened spheroid , combined with local gravity anomalies , defines

1922-562: The 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES , TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level ( AMSL ) is the elevation (on the ground) or altitude (in the air) of an object, relative to

North Col - Misplaced Pages Continue

1984-973: The 19th century. With high emissions it would instead accelerate further, and could rise by 1.0 m ( 3 + 1 ⁄ 3  ft) or even 1.6 m ( 5 + 1 ⁄ 3  ft) by 2100. In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F). Rising seas affect every coastal and island population on Earth. This can be through flooding, higher storm surges , king tides , and tsunamis . There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves . Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade. The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without

2046-504: The Airy isostatic anomaly is zero over regions where there is complete isostatic compensation. The free-air anomaly is also close to zero except near boundaries of crustal blocks. The Bouger anomaly is very negative over elevated terrain. The opposite is true for the theoretical case of terrain that is completely uncompensated: The Bouger anomaly is zero while the free-air and Airy isostatic anomalies are very positive. The Bouger anomaly map of

2108-749: The Alps shows additional features besides the expected deep mountain roots. A positive anomaly is associated with the Ivrea body, a wedge of dense mantle rock caught up by an ancient continental collision. The low-density sediments of the Molasse basin produce a negative anomaly. Larger surveys across the region provide evidence of a relict subduction zone. Negative isostatic anomalies in Switzerland correlate with areas of active uplift, while positive anomalies are associated with subsidence. Over mid-ocean ridges ,

2170-448: The Earth's gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits gravity anomalies such as those measured by NASA's GRACE satellites . In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and

2232-535: The French astronomer Jean Richer established an observatory on the island of Cayenne . Richer was equipped with a highly precise pendulum clock which had been carefully calibrated at Paris before his departure. However, he found that the clock ran too slowly in Cayenne, compared with the apparent motion of the stars. Fifteen years later, Isaac Newton used his newly formulated universal theory of gravitation to explain

2294-705: The Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and the main part of Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante , while the European Vertical Reference System is calibrated to the Amsterdam Peil elevation, which dates back to

2356-456: The amount of water in the oceans, while the existing seawater also expands with heat. Because most of human settlement and infrastructure was built in response to a more-normalized sea level with limited expected change, populations affected by sea level rise will need to invest in climate adaptation to mitigate the worst effects or, when populations are at extreme risk, a process of managed retreat . The term above sea level generally refers to

2418-441: The anomaly. Newton showed that the measured value of gravity was affected by the rotation of the Earth, which caused the Earth's equator to bulge out slightly relative to its poles. Cayenne, being nearer the equator than Paris, would be both further from the center of Earth (reducing the Earth's bulk gravitational attraction slightly) and subject to stronger centrifugal acceleration from the Earth's rotation. Both these effects reduce

2480-401: The effects of nearby terrain, but it usually still differs slightly from the measured value. This gravity anomaly can reveal the presence of subsurface structures of unusual density. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore. Different theoretical models will predict different values of gravity, and so

2542-489: The figure of an ellipsoid of revolution. Gravity on the surface of this reference ellipsoid is then given by a simple formula which only contains the latitude . For Earth, the reference ellipsoid is the International Reference Ellipsoid , and the value of gravity predicted for points on the ellipsoid is the normal gravity , g n . Gravity anomalies were first discovered in 1672, when

North Col - Misplaced Pages Continue

2604-577: The free-air anomalies are small and correlate with the ocean bottom topography. The ridge and its flanks appear to be fully isostatically compensated. There is a large Bouger positive, of over 350 mgal, beyond 1,000 kilometers (620 mi) from the ridge axis, which drops to 200 over the axis. This is consistent with seismic data and suggests the presence of a low-density magma chamber under the ridge axis. There are intense isostatic and free-air anomalies along island arcs . These are indications of strong dynamic effects in subduction zones. The free-air anomaly

2666-482: The geoid is referred to as (mean) ocean surface topography . It varies globally in a typical range of ±1 m (3 ft). Several terms are used to describe the changing relationships between sea level and dry land. The melting of glaciers at the end of ice ages results in isostatic post-glacial rebound , when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic subsidence from

2728-688: The gravitational attraction of the bulk mass of the earth is slightly reduced. The free-air correction is simply 0.3086 mgal m times the elevation above the reference ellipsoid. The remaining gravity anomaly at this point in the reduction is called the free-air anomaly . That is, the free-air anomaly is: Δ g F = g m + ( Δ g F A + Δ g T + Δ g tide ) − g n {\displaystyle \Delta g_{F}=g_{m}+(\Delta g_{FA}+\Delta g_{T}+\Delta g_{\text{tide}})-g_{n}} The free-air anomaly does not take into account

2790-634: The greatest exposure to sea level rise, twelve are in Asia , including Indonesia , Bangladesh and the Philippines. The resilience and adaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts. The greatest impact on human populations in the near term will occur in the low-lying Caribbean and Pacific islands . Sea level rise will make many of them uninhabitable later this century. Pilots can estimate height above sea level with an altimeter set to

2852-555: The height of planetary features. 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 long enough that fluctuations caused by waves and tides are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to sea level changes (millimetres per year). Some land movements occur because of isostatic adjustment to

2914-433: The high terrain is balanced by the reduced gravitational attraction of its underlying low-density roots. This brings the free-air anomaly, which omits the correction terms for either, close to zero. The isostatic anomaly includes correction terms for both effects, which reduces it nearly to zero as well. The Bouguer anomaly includes only the negative correction for the high terrain and so is strongly negative. More generally,

2976-591: The land; hence a change in relative MSL or ( relative sea level ) can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates, or both. In the UK, the ordnance datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool . Since

3038-406: The layer of material (after terrain leveling) outside the reference ellipsoid. The gravitational attraction of this layer or plate is taken into account by the Bouguer plate correction, which is −0.0419 × 10 ρ h mgal m kg . The density of crustal rock, ρ, is usually taken to be 2670 kg m so the Bouguer plate correction is usually taken as −0.1119 mgal m h . Here h is the elevation above

3100-548: The lithosphere, not within the underlying aesthenosphere, contradicting the explanation of the Hawaiian rise as a product of aesthenosphere flow associated with the underlying mantle plume. The rise may instead be a result of lithosphere thinning: The underlying aesthenosphere is less dense than the lithosphere and it rises to produce the swell. Subsequent cooling thickens the lithosphere again and subsidence takes place. Local anomalies are used in applied geophysics . For example,

3162-399: The measurement point. This is tedious and time-consuming but necessary for obtaining a meaningful gravity anomaly. The next correction is the free-air correction. This takes into account the fact that the measurement is usually at a different elevation than the reference ellipsoid at the measurement latitude and longitude. For a measurement point above the reference ellipsoid, this means that

SECTION 50

#1732772942381

3224-481: The melting of ice sheets at the end of the last ice age . The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds . Changes in ground-based ice volume also affect 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 as well. Eustatic sea level change (global as opposed to local change)

3286-411: The model field are usually expressed as corrections added to the measured gravity or (equivalently) subtracted from the normal gravity. At a minimum, these include the tidal correction △ g tid , the terrain correction △ g T , and the free air correction △ g FA . Other corrections are added for various gravitational models. The difference between the corrected measured gravity and the normal gravity

3348-436: 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 is clearly indicated. Once above the transition altitude, the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg. Gravity anomalies The gravity anomaly at

3410-612: The reference ellipsoid. The remaining gravity anomaly at this point in the reduction is called the Bouguer anomaly . That is, the Bouguer anomaly is: Δ g B = g m + ( Δ g B P + Δ g F A + Δ g T + Δ g tide ) − g n {\displaystyle \Delta g_{B}=g_{m}+(\Delta g_{BP}+\Delta g_{FA}+\Delta g_{T}+\Delta g_{\text{tide}})-g_{n}} The Bouguer anomaly

3472-457: The static and time-variable Earth's gravity field parameters are determined using modern satellite missions, such as GOCE , CHAMP , Swarm , GRACE and GRACE-FO . The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined from satellite laser ranging . Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions, e.g., GOCE . These satellite missions aim at

3534-399: The subsurface geology. The gravity anomaly is the difference between the observed acceleration of an object in free fall ( gravity ) near a planet's surface, and the corresponding value predicted by a model of the planet's gravitational field . Typically the model is based on simplifying assumptions , such as that, under its self-gravitation and rotational motion , the planet assumes

3596-536: The summit from Camp VI at 8,230 metres (27,001 ft) altitude. The North Col was first climbed by George Mallory , Edward Oliver Wheeler , and Guy Bullock on 24 September 1921, during the British reconnaissance expedition . This was the first time a Westerner had set foot on Mount Everest. Although long credited to Mallory, discovery of the North Col was in fact made by Wheeler about a week before Mallory confirmed its existence while searching for possible routes to

3658-523: The summit of Mount Everest. All subsequent expeditions in the 1920s and 1930s attempted to reach the summit of Everest by using the North Col. Before 1950, most Everest expeditions went from Tibet and via the North Col, but most now go from Nepal via the South Col . In 1951, two mountaineers on the 1952 British Cho Oyu expedition , Edmund Hillary and George Lowe , crossed the Nup La Col, and "like

3720-518: The times of the Russian Empire , in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) above chart datum and 1.304 m (4 ft 3.3 in) below the average sea level. In France,

3782-414: The value of gravity, explaining why Richer's pendulum clock, which depended on the value of gravity, ran too slowly. Correcting for these effects removed most of this anomaly. To understand the nature of the gravity anomaly due to the subsurface, a number of corrections must be made to the measured gravity value. Different theoretical models will include different corrections to the value of gravity, and so

SECTION 60

#1732772942381

3844-411: The weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise. On planets that lack a liquid ocean, planetologists can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level" or zero-level elevation , serves equivalently as a reference for

#380619