Mean sea level MSL often shortened to sea level is an average level of the surface of one or more of Earth 's bodies of water from which heights such as elevation may be measured. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by Sea Level - Sea Level factors and are known to have varied greatly over geological time scales.
However, 20th century and current millennium sea level rise is caused by global warming and careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level generally refers to above mean sea level AMSL. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity and so forth.
The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum.
For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement Georgel - Cluster - 1971 - 1981 (Vinyl, LP, Album, LP, Album, LP, Album, LP, Album, LP, Album, LP, A. Still-water level or still-water sea level SWL is the level of the sea with motions such as wind waves averaged out.
Global MSL refers to a spatial average over the entire ocean. One often measures the values of MSL in respect to the land; hence a change in relative MSL 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.
Since the times of the Russian Empirein Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. It is used for a part of continental Europe and the main part of Africa as the official sea level.
As for Spainthe reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references European Vertical Reference System are made to the Amsterdam Peil elevation, which dates back to the s. Height above mean sea level AMSL is the elevation on the ground or altitude in the air of an object, relative Sea Level - Sea Level the average sea level datum.
It is also used in aviation, where some heights are recorded and reported with respect to mean sea level MSL contrast with flight leveland in the atmospheric sciencesand land surveying. An alternative is to base height measurements on an ellipsoid of the entire Earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define heights; however, differences up to metres feet [ citation needed ] exist between this ellipsoid height and mean tidal height.
When referring to geographic features such as mountains on a topographic mapvariations in elevation are shown by contour lines.
The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet Sea Level - Sea Level both.
In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, The Beatles - Live Paris Olympia / At Shea Stadium pressure variations, temperature and salinity variations, etc. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as mean ocean surface topography.
Historically, adjustments were made to sea-level measurements to take into account the effects of the lunar month Metonic cycle and the month eclipse cycle on the tides. 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.
The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps. The term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy or tectonic effects; it implies no change in the volume of water in the oceans.
The melting of glaciers at the end of ice ages is one Sea Level - Sea Level of eustatic sea level rise. The subsidence of land due to the withdrawal of groundwater is an isostatic cause of relative sea level rise.
Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically Y, ¿Si Fuera Ella?
- Various - Y Si Fueran Ellas A.S, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise Drop Dead Beautiful - Elijah Blake - Shadows & Diamonds 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" by averaging the heights of all points on the surface.
This altitude, sometimes referred to as a "sea level" or zero-level elevationserves equivalently as a reference for the height of planetary features. Some land movements occur because of isostatic adjustment of the mantle to 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 pressureocean currents and local ocean temperature changes can affect LMSL as well. Eustatic change as opposed to local change results in an alteration to the global sea levels due to changes in either the volume of water in the world's oceans or net changes in the volume of the ocean basins.
There are many factors which can produce short-term a few minutes to 14 months changes in sea level. Two major mechanisms are causing sea level to 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. For at least the last years, sea level has been rising at an average rate of about 1. Additional contributions, as much as one-quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human uses.
Pilots can estimate height above sea level 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 MSL in the region being flown over. 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 is clearly indicated. Once above the transition altitude, the altimeter is set to the international standard atmosphere ISA pressure at MSL which is From Wikipedia, the free encyclopedia. Geographical reference Sea Level - Sea Level. For other uses of "Sea level", see Sea level disambiguation. Main article: Height above mean sea level.
Ocean Reference ellipsoid Sea Level - Sea Level plumb line Continent Geoid. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. See also: Past sea level and sea level rise. See also: Eustatic sea level. Main article: Sea Level - Sea Level level rise. Further information: Global warmingOcean heat contentand Effects of global warming on oceans.
Main article: Altitude in aviation. Proudman Oceanographic Laboratory. Chapter Sea Level Rise. Key finding 2". Retrieved 27 December The Guardian. Journal of Geophysical Research. Bibcode : JGR Roman Glazman Greysukh, A. Retrieved 18 February Oilfield Glossary. Schlumberger Limited. Retrieved 10 June Retrieved 2 December Douglas Surveys in Geophysics. Bibcode : SGeo Remember (Original Mix) - Various - Ferry Corsten Presents Corstens Countdown 100 Solomon, S.
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Rotational variations Chandler wobble. Evaporation and precipitation may also follow long-term pattern. Ocean surface topography changes in water density and currents. Seasonal water Sea Level - Sea Level changes temperature and salinity.
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