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A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location. [1] The term above sea level generally refers to 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.
A tide gauge is a device for measuring the change in sea level relative to a vertical datum. [2] [3] It is also known as a mareograph, [4] marigraph, [5] and sea-level recorder. [6] When applied to freshwater continental water bodies, the instrument may also be called a limnimeter. [7] [8]
Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun, by Earth's rotation and by centrifugal force caused by Earth's progression around the Earth-Moon barycenter.
One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured in pounds per square inch (psi) or in standard atmospheres (atm). The ambient pressure at sea level is approximately one atmosphere, which is equal to 1.01325 bars (14.6959 psi), which is close enough for bar and atm to be ...
The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm. In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
Over geologic time sea level has fluctuated by more than 300 metres, possibly more than 400 metres. The main reasons for sea level fluctuations in the last 15 million years are the Antarctic ice sheet and Antarctic post-glacial rebound during warm periods. The current sea level is about 130 metres higher than the historical minimum.
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Map of pressure systems across North America. A pressure system is a peak or lull in the sea level pressure distribution, a feature of synoptic-scale weather.The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg).