Movements of Ocean Water

Waves and their parts, why tides rise and fall and their kinds, and the forces, types and effects of ocean currents.

Key ideas

Most waves are made by wind; energy moves across the ocean while the water only moves in small circles.
A wave breaks when the water is shallower than half its wavelength.
Tides are the regular rise and fall of the sea, caused mainly by the moon’s pull, with the sun’s pull and the centrifugal force.
Spring tides (higher) come at new and full moon, with sun, moon and earth in a line; neap tides (lower) about seven days later, with sun and moon at right angles.
Currents are started by solar heating, wind, gravity and the Coriolis force, and kept going by differences in density.
Cold currents usually flow along west coasts in low and middle latitudes; warm currents along east coasts.
Where warm and cold currents mix, plankton thrives and the best fishing grounds lie.

Amplitude	wave height ÷ 2
Wave speed	wavelength ÷ period (m/s)
Waves passing	60 ÷ period in seconds × minutes
Semi-diurnal tide	two highs and two lows a day: about 6 hours from high to low, 12 hours from high to high
Rise or fall per hour	tidal range ÷ 6 hours
Highest tides	Bay of Fundy
Surface currents	upper 400 m, about 10% of ocean water; deep currents the other 90%

At a place with semi-diurnal tides, high tide is at 12:30. Roughly when will the next low tide and the next high tide be?

Working: about 6 hours to low tide, 12 hours to the next high tide

Answer: low tide about 18:30, high tide about 00:30