A land bridge.
the gradient
-197 ft.
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217 - 20 = 197 Therefore, it is 197 feet between the seabed (and anchor) and the sea level.
The further you are from sea level, the thinner the atmosphere becomes. Thinner air is less capable of trapping heat, so the temperature drops as you gain altitude. This is why mountains, which are higher above sea level, tend to be colder than low-lying areas.
During an ice age, Earth's sea level generally drops because water is locked up in the polar ice caps and glaciers. This causes less water to be in the oceans, leading to lower sea levels compared to during interglacial periods.
elevation
ELEVATION :)
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The highest point of elevation is Mount Davis at 3,213 feet above sea level and it drops to zero at the Delaware's exit from the state. The average elevation of the state is 1,099 feet above sea level.
For every thousand (1000) feet above sea level, the boiling point of water drops by two (2) degrees. Water at sea level boils at 212 degrees Fahrenheit or 100 degrees celsius. Fifteen (15) feet above sea level will make no real difference to the temp needed for boiling.