Tides and Waves
Moon and wind drive tides and waves.
About this topic
Tides are the rise and fall of sea level caused mainly by the Moon's gravity (and the Sun's). Most coasts have two high and two low tides daily. Spring tides (large range) occur at new and full moon; neap tides (small range) at quarter moon.
Waves are generated by wind transferring energy to the water. Wave size depends on wind speed, duration and fetch. Near shore, waves slow, grow taller and break. Tsunamis are long waves from undersea earthquakes. Tidal energy can be harnessed.
Spring tides do not have anything to do with the season. A useful picture is that the Moon's gravity pulls more strongly on the near side of Earth and less on the far side, creating two bulges, and as Earth spins under them most places meet two highs and two lows daily; the cycle takes 24 hours 50 minutes because the Moon also moves, so high tide comes about 50 minutes later each day. The Bay of Fundy in Canada has a tidal range of up to about 16 m due to its funnel shape and resonance.
Wave energy depends on wind speed, how long it blows and the fetch, the distance over open water. Water in a wave moves in circles while the shape travels on, which is why a floating bottle bobs up and down rather than moving with the wave. When a wave nears the shore, the base drags on the bottom, the wavelength shortens and the height grows until it topples and breaks. Surfers read these conditions. Tidal barrages and turbines generate predictable electricity, as at La Rance in France. A common misconception is that a tsunami is a giant breaking wave; it is more like a fast-rising flood. It is also wrong to say tides are caused only by the Moon's pull on water on the near side.
Key ideas
- Two tides a day
- Tidal range
- Waves carry energy not water
Quick check
Use cases: where Tides and Waves helps
- Homework: review the key idea behind Tides and Waves before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Tides and Waves without notes.
- Visual learning: play with the interactive lab to see how Tides and Waves changes when you adjust its inputs.
- Teaching a friend: explain Tides and Waves in your own words, then compare your version with the key ideas.
Earth science helps with weather forecasts, natural hazards, energy and climate planning.
Frequently asked questions
What is Tides and Waves?
Moon and wind drive tides and waves.
What will I learn in Tides and Waves?
Two tides a day. Tidal range. Waves carry energy not water.
Is Tides and Waves part of Earth & Space?
Yes. It is one of 60 Earth & Space topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Tides and Waves lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Tides and Waves?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.