Few things in nature are as dependable as the tide. Just as the sun rises in the east and the stars come out at night, the ocean rises and falls along our shores on a schedule we can count on. Tides are among the most reliable phenomena in the world — and they are, at heart, waves.
The NOAA San Francisco Tide Station, in operation for more than 150 years. Credit: NOAA.
Very long waves
Tides are very long-period waves that travel through the ocean in response to the forces exerted by the moon and the sun. They begin in the open ocean and move toward the coast, where they show up as the regular rise and fall of the sea surface.
Crest and trough
| When this part of the wave arrives | The coast has |
|---|---|
| The crest, the highest part | high tide |
| The trough, the lowest part | low tide |
The difference in height between high tide and low tide is called the tidal range.
Why tides matter
The rise and fall of the tides plays an important role in the natural world, and it can have a marked effect on everything people do at sea. For how often tides come and go, see how frequent tides are.
Sources
Based on "What are tides?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
收录于这些专栏Tides and Water Levels: a NOAA Ocean Service tutorial
许可协议： CC0 1.0（公有领域） · 改编自 oceanservice.noaa.gov
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