Sea Level as a Point of Measurement

Discussion in 'Earth Science' started by Acitnoids, Sep 18, 2018.

  1. Acitnoids Registered Senior Member

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    1st I am referring to elevation and not height.

    If I stay to the theme of why I started this thread I would say there are two reasons that Everest would change six feet. I am not saying these are the only reasons, these are the reasons that pertain to the title and O.P. of this thread based on my generalized understanding heh

    One would be plate tectonics that is currently pushing the mountain range up in relation to the mean sea level.

    Two would be a rise or drop in the mean sea level.
     
    Last edited: Sep 20, 2018
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  3. iceaura Valued Senior Member

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    "Ideal sphere" is wrong. It would put Greenland under sea level.
    "Ideal circular-ish average" is wrong - the geoid is not an average.
    The rest of that sentence is also wrong - the geoid has almost nothing to do with the atmosphere, astrophysic influences, or surface tension of any kind.
    Air pressure is not the major factor in calculating the datum "sea level" for a given location - it is used for calculating storm surges and the like, but is a very small factor in establishing the sea level in the first place.
    All of these calculations "reflect reality" as precisely and rigorously as the best of us can manage - they have no other basis, and no other purpose. Sea water exists, elevations are physical distances, people deal with these physical features of their surroundings in real life.
    But you are referring to claims of a change in height - 4mm per year.
    That would be measured from a given datum - the "sea level" calculated at a given time and location, the center of the earth, some fixed base used for all measurements alike.
    The mean sea level of some other time or place than the one used to compare Everest's height from one year to the next would be irrelevant.
     
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  5. Acitnoids Registered Senior Member

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    I have remembered why I stopped posting here years ago. Casual conversation is nonexistent on the internet. ✋✋✋
     
    Last edited: Sep 20, 2018
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  7. sideshowbob Sorry, wrong number. Valued Senior Member

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    Somebody on the Internet has to be wrong. That's what keeps the rest of us here.

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  8. Acitnoids Registered Senior Member

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    These were my original questions.

    I understand that the boiling of water is given as 212°F ( 100°C) at sea level. The reason sea level is important is because of Atmospheric pressure. This is not insignificant.

    I also recognize that if sea level rises by, say, 100ft then this statement will remain the same.

    What my original post asked was; if this point we commonly call "sea level" is constant then how much higher is sea level now compared to when we agreed on that standard? If it is not fixed then why do mountains stay the same hight?

    Unfortunately these simple question fell victim to both of our egos.
     
    Last edited: Sep 21, 2018
  9. Write4U Valued Senior Member

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    I agree, in fact I believe it is the most important factor . It is a constant which can serve as a baseline?
     
  10. iceaura Valued Senior Member

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    The standard use of "sea level" for measuring and recording elevation is hundreds if not thousands of years old. This shows the changes in actual height of the surface since 1993: https://oceanservice.noaa.gov/facts/globalsl.html
    Any "sea level" used to measure elevation in a given area is a calculated, imposed, specified datum - it varies by location, its method of calculation varies according to need, and it is changed according to need in ways explicitly specified by those making the change.
    The height above sea level of my front yard - around 900 feet, iirc, and I don't know which if any actual sea is involved - I believe has been recalculated for satellite and air travel and setting grade and so forth from the early surveys, but what they used for base and what they used as a datum for sea level I have no idea. Anyone interested in the height changes of the land itself would of course convert the old numbers to the new base (or vice versa) in step one.
    It already is, where that's useful - such as weather reports.
    Here are wind speeds and humidity readings reported by level of atmospheric pressure - with one major addition: "surface" numbers are reported as a separate layer. There is no atmospheric pressure reliably corresponding to "surface", although "1013 hPa" is generally taken as close to sea level: https://earth.nullschool.net
    Air is inconstant - famously, metaphorically, even stereotypically. Barometric pressure varies by temperature, humidity, height, and even wind formations. Notice that even careful taking of averages won't help - the ring of lows around Antarctica are fairly stable in general location, but do not correspond reliably to extra sea height, for example. Neither does the characteristic high over Siberia correspond to a gigantic dent in the planet's surface. Illustration: https://en.wikipedia.org/wiki/File:Mslp-jja-djf.png
    Notice that the "sea level" west of the southern coast of Chile, calculated from these 15 year averages and the standard lapse rate of about 12 hPa per 100 meters (Wiki), would differ from the "sea level" north of the nearest Antarctic coast by more than 300 meters.
     
    Last edited: Sep 21, 2018
  11. Gawdzilla Sama Valued Senior Member

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    And how would you measure that change in elevation?
     
  12. sideshowbob Sorry, wrong number. Valued Senior Member

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    Both the zero point on the thermometer and the zero point on the elevation scale are arbitrarily chosen for convenience. Neither has any inherent significance.

    If "sea level" was to change drastically, the atmospheric pressure at sea level would also change significantly - but I doubt that we would change the boiling point of water accordingly.
     
  13. sculptor Valued Senior Member

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    fyi
    sea level is down over a meter from 3500 years ago
     
  14. Acitnoids Registered Senior Member

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    I agree with almost everything you have said. My one question would be:

    If there was a drastic rise in sea level, would the atmosphere rise in elevation and keep atmospheric pressure relatively the same or would the atmosphere condense do to the Earths gravity?

    I assume it would condence but I do not know to say for sure.
     
  15. RainbowSingularity Valued Senior Member

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    this question has been addressed at length by several physicists, biologists, climatologists(these are multi discipline scientists) and various other highly studied and qualified scientists
    some years ago.

    what is the mechanism ?
    heat exchange point
    what is the item moving/exchanging ? = water
    what is the change process ? = liquid/solid into aqueous water vaopour and dissipation

    what is the scientific mean point of human expereince/impact ? condensation level where liquid water turns into water vapour.

    how does this direclty impact humans ? = because it lifts the liquid water level up by moving the exchange/conversion point to a higher altitude making previousely very wet areas into more wet or liquid areas.

    it is called "basic science" by the scientists who have been studying it for around 20 years.
    (because it is simple/fundermental physics)

    the direct answer to your question is quite complicated though the answer is fairly simple.
    the atmosphere is likely to become thinner and more compressed(if you define atmosphere as being the tiny thin layer that humans need to survive in)
     
  16. Michael 345 New year. PRESENT is 69 years old Valued Senior Member

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    Guess work mode in operation here combined with some simplifycation

    Earth perfect smooth ball at sea floor level
    Hence sea level equal world wide
    Sea level rises 10 metres
    Atmosphere rises 10 metres
    Gravity remains constant
    Hence gravity exerts same pull on atmosphere
    Earth losses 10 metres atmosphere
    Gravity has less atmosphere to pull down
    Atmospheric pressure drops

    Discuss

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  17. iceaura Valued Senior Member

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    A rise in sea level in consequence of melting land ice would lower, not raise, the average height of the surface of the earth - ice occupies more volume than water.
     
  18. Michael 345 New year. PRESENT is 69 years old Valued Senior Member

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    Should that read sea ice not land ice since land ice would have no impact on sea levels

    If melted and flows into the sea it would then affect the sea level by raising it

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  19. iceaura Valued Senior Member

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    It should read land ice - which melts into the sea, flows into the sea and melts, etc. Water flows into the sea, generally. Before, it was adding to the height of the surface of the earth; after, it was adding less.
    The land surface is part of the surface of the earth. When the glaciers melt into the ocean, the land height is less as the sea height is more, the net change is in this case negative.
    Same is true of sea ice, but less likely to be overlooked imho - not a factor in the thread above.
     
  20. sideshowbob Sorry, wrong number. Valued Senior Member

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    I was wondering the same thing myself.

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    My simplistic guess is that farther from the earth's center the pull of gravity would be less so the same amount of air would exert less pressure per square inch. On the other hand, the surface area would also change, so....
     
  21. Acitnoids Registered Senior Member

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    This should read land ice, not sea ice.

    There is a simple experiment you can do at home to see why:

    Fill a glass full of ice then fill the glass with water. Set the glass aside and let all of the ice melt. The melted ice will not rise the level of water and make it spill over the brim.

    Solid ice is less dense than liquid water. This is an unusual property. Most solids are more dense than their liquid state and science is still trying to figure out why water behaves this way.

    If you add ice to a full glass of water the glass will overflow the brim. So if land ice moves to take the place of the collapsing northwest Antarctica sea shelf then yes, sea levels will rise.
     
    Last edited: Sep 22, 2018
  22. Michael 345 New year. PRESENT is 69 years old Valued Senior Member

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    I took land ice to be - ice ON land NOT ice acting AS land

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  23. Acitnoids Registered Senior Member

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    Sea Ice acts as a buttress. It holds back the land ice. If sea ice breaks away then land Ice will take it's place.
     
    Last edited: Sep 22, 2018

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