Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Sea Level shopping experience:
1. Compare - without doubt the biggest advantage that the Sea Level offers shoppers today is the ability to compare thousands of Sea Level at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.
2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about
3. Testimonials - don't know anybody that has bought a Sea Level? Wrong! If the Sea Level is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.
4. Questions - Got a question about Sea Level then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....
5. Reputation - Never heard of the company selling Sea Level? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Sea Level and build up a picture of their reputation for sales, returns, customer service, delivery etc.
6. Returns - still worried that even after all of the above your Sea Level wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.
7. Feedback - happy with your Sea Level then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.
8. Security - check for the yellow padlock on the Sea Level site before you buy, and the s after http:/ /i.e. https:// = a secure site
9. Contact - got a question about Sea Level, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.
10. Payment - ready to pay for your Sea Level, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.
Mean
sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level , however, involves complex measurement, and accurately determining MSL can prove difficult.
Mean Sea Level as used by military Flight Surgeons and Aerospace Units: Using pressure to measure altitude results in two other types of altitude. True or MSL (mean sea level) is the next best measurement to absolute - and in some ways better. MSL tells you how far you are above an imaginary line at sea level. If you then know the elevation of terrain, the next step is to determine how far you are above ground. It also tells you how thin the air is, which determines your physiological response to that altitude. True Altitude (MSL) has been adjusted for local high or low pressure conditions. FL or Flight Level is another related term that is measured in hundreds of feet. At a standard pressure that correlates to 18,000 feet, the flight level is one-eight-zero.
Measurement
records in geologically stable environments show a rise of around 20 centimeters (8 inches) during the 20th century (2 millimeters/year).
To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind
waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in 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.
In the
UK, mean sea level has been measured at
Newlyn in Cornwall and Liverpool on Merseyside for decades, by tide gauges to provide Ordnance Datum Newlyn for the
zero metres height on UK maps.
Difficulties in utilization
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or 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
gravity field. In reality, due to currents, air pressure variations, temperature and salinity 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) sea 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-month
Metonic cycle and the 223-month eclipse cycle on the tides. Mean sea level does
not remain constant over the surface of the entire earth. For instance, mean sea level at the
Pacific Ocean end of the Panama Canal stands 20 cm higher than at the Atlantic Ocean end.
Despite the difficulties,
aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below - see Schiphol Airport) above mean sea level, and the altitude of the
airports where they intend to land. That problem can compound when landing on an
aircraft carrier in a gravitational anomaly. In aviation mean sea level is increasingly being defined according to the
reference ellipsoid defined by the
World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the
Global Positioning System.
Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it connotes change that is not attributed to any specific cause. The term "eustatic" refers to global changes in the sea level due to water mass added (or removed from) the
oceans (e.g. melting of
ice sheets). 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 masses due to thermal buoyancy or tectonic effects and implies no real change in the volume of water in the oceans. The melting of
glaciers at the end of
ice ages is an example 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 stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as 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", serves equivalently as a reference for the height of planetary features.
Changes through geologic time
Sea level has changed over geologic time. As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the
Permian-
Triassic boundary about 250 million years ago). For this reason, sea level is more prone to rise than fall today, and small changes in
climate can have noticeable effects during human lifetimes.
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 water that had evaporated and been deposited as snow and
ice in northern hemisphere glaciers. The majority of the glaciers had melted by about 10,000 years ago, but minor glacial melting has continued (with occasional reversals) throughout recorded human history. More detail about the changes in sea level for the past 140,000 years can be seen by accessing this chart.
Hundreds of similar glacial cycles have occurred throughout the History of Earth. Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. This results in
sedimentary cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called
sequence stratigraphy.
See also
External links
- Permanent Service for Mean Sea Level
- Global sea level change: Determination and interpretation
- Environment Protection Agency Sea level rise reports
- Properties of isostasy and eustasy
Mean
sea level (MSL) is the average (mean) height of the
sea, with reference to a suitable reference surface. Defining the reference level , however, involves complex measurement, and accurately determining MSL can prove difficult.
Mean Sea Level as used by military Flight Surgeons and Aerospace Units: Using pressure to measure altitude results in two other types of altitude. True or MSL (mean sea level) is the next best measurement to absolute - and in some ways better. MSL tells you how far you are above an imaginary line at sea level. If you then know the elevation of terrain, the next step is to determine how far you are above ground. It also tells you how thin the air is, which determines your physiological response to that altitude. True Altitude (MSL) has been adjusted for local high or low pressure conditions. FL or Flight Level is another related term that is measured in hundreds of feet. At a standard pressure that correlates to 18,000 feet, the flight level is one-eight-zero.
Measurement
records in geologically stable environments show a rise of around 20 centimeters (8 inches) during the 20th century (2 millimeters/year).
To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind
waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the
tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in 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.
In the UK, mean sea level has been measured at Newlyn in Cornwall and
Liverpool on Merseyside for decades, by tide gauges to provide
Ordnance Datum Newlyn for the
zero metres height on UK maps.
Difficulties in utilization
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or 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 gravity field. In reality, due to currents, air pressure variations, temperature and salinity 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)
sea 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-month Metonic cycle and the 223-month eclipse cycle on the tides. Mean sea level does
not remain constant over the surface of the entire earth. For instance, mean sea level at the
Pacific Ocean end of the Panama Canal stands 20 cm higher than at the Atlantic Ocean end.
Despite the difficulties, aviators flying under
instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below - see
Schiphol Airport) above mean sea level, and the altitude of the airports where they intend to land. That problem can compound when landing on an
aircraft carrier in a gravitational anomaly. In aviation mean sea level is increasingly being defined according to the reference ellipsoid defined by the World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System.
Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it connotes change that is not attributed to any specific cause. The term "eustatic" refers to global changes in the sea level due to water mass added (or removed from) the oceans (e.g. melting of ice sheets). 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 masses due to thermal buoyancy or
tectonic effects and implies no real change in the volume of water in the oceans. The melting of glaciers at the end of
ice ages is an example 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 stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as 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", serves equivalently as a reference for the height of planetary features.
Changes through geologic time
Sea level has changed over geologic time. As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the Permian-
Triassic boundary about 250 million years ago). For this reason, sea level is more prone to rise than fall today, and small changes in
climate can have noticeable effects during human lifetimes.
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 water that had evaporated and been deposited as snow and
ice in northern hemisphere glaciers. The majority of the glaciers had melted by about 10,000 years ago, but minor glacial melting has continued (with occasional reversals) throughout recorded human history. More detail about the changes in sea level for the past 140,000 years can be seen by accessing this chart.
Hundreds of similar glacial cycles have occurred throughout the
History of Earth. Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. This results in sedimentary cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called
sequence stratigraphy.
See also
External links
- Permanent Service for Mean Sea Level
- Global sea level change: Determination and interpretation
- Environment Protection Agency Sea level rise reports
- Properties of isostasy and eustasy
BBC - Weather Centre - Climate Change - Sea Level Rises
BBC Weather's climate change site. All the issues and key topics including global warming, greenhouse effect, ozone, kyoto, politics and the environment.
Sea level networks
African tide gauge network Location of tide gauge Inhambane (Mozambique) Location of tide gauge Pemba (Mozambique)
National Tidal and Sea Level Facility Home
National Tidal and Sea Level Facility based at the Proudman Oceanographic Laboratory, POL, NERC,monitoring, forecasting, tidal predictions, Storm surges, Storm surge forecasts ...
Ocean Surface Topography from Space
Ocean Surface Topography seeks to study the sea surface height. Maintaining a database of ocean surface topography can help predict short-term changes in weather and longer-term ...
Sea level - Wikipedia, the free encyclopedia
Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level [1], however, involves complex ...
Sea level rise - Wikipedia, the free encyclopedia
Sea-level rise is an increase in sea level. Multiple complex factors may influence this change. Sea-level has risen about 130 metres (400 ft) since the peak of the last ice age ...
BBC NEWS | Science/Nature | Sea level rise 'is accelerating'
Global sea-levels could rise by about 30cm by the end of this century if current trends continue, a study warns.
CRU Information Sheet no. 10: Sea level rise
Climatic Research Unit: Information sheets 10: Sea level rise Sarah Raper Introduction On the decade-to century timescale, changes in global mean temperature result in changes in ...
Sea Level Change Resources
SEA LEVEL CHANGE & ASSOCIATED FEATURES WEB SITES RELATING TO SEA LEVEL CHANGE BELOW... THE place to go for Sea Level Change information is the website of MIDDLESEX UNIVERSITY's ...
Sea Level
Sea Level. The global sea level has already risen by around between 10 to 25 centimetres during the last 100 years, at the rate of 1 to 2 millimetres per year.
