START WITH A QUESTION
Does the water travel in the same way as a wave crest?
LOOK CLOSELYFind a high point—the crest—and a low point—the trough. Then try changing the height with the slider.
This model lets you compare shapes. Its wave heights have no real-world units. It does not calculate wind conditions or predict the sea.
Wind transfers energy to water
NOTICE THISWhat is passed along from one place to the next?
A travelling wave and moving water
WHAT THIS TELLS USA wave transfers energy through the movement of water.
Wind blowing over the sea transfers energy to the water, generating waves. Waves growing under the local wind are called wind waves. Out at sea, waves of different shapes and sizes overlap.
Our diagram shows one regular wave so that differences are easier to see. Move the slider and watch the vertical distance between a crest and a trough change.
Go deeper: does water stay exactly in place?
In a simplified model of small waves in deep water, water particles move in roughly circular paths. Real water also experiences currents and wave-related drift. This does not mean that water never travels, or that a floating person will stay in one place.
Sources for this explanation: G1
Height, length and time
NOTICE THISHow is the distance between crests different from the wait between them?
Keep the observation point fixed
WHAT THIS TELLS USWavelength is the distance between crests. Period is the time between crests passing one fixed point.
Wave height is the vertical distance from trough to crest. Wavelength is the distance from one crest to the next. Wave period is the time between successive crests passing a fixed point.
A high wave and a wave with widely spaced crests describe different things. This model changes wave height while keeping wavelength the same.
Go deeper: what do you keep fixed?
To compare wavelengths, look across the water at one moment. To measure period, keep the observation point fixed and measure time. Following a crest would not measure its period. This model keeps height and wavelength constant and shows the sequence of crests passing.
Sources for this explanation: G2
Swell, forecasts and individual waves
NOTICE THISIs the forecast wave height the tallest wave you might meet?
Select the tallest third, then average
WHAT THIS TELLS USIndividual waves can be higher than the forecast wave height.
Waves that travel beyond the wind that generated them, or remain after that wind has weakened, are called swell. Waves from a distant typhoon can reach a coast far from the storm.
Swell can grow higher as it approaches shallow coastal water. Sunny weather and light local winds alone do not tell you the shore is safe. Check the latest weather and sea information before your visit.
Go deeper: what does significant wave height summarise?
Significant wave height is the average height of the highest third of observed waves. In this nine-wave example, select the three tallest. It is neither the maximum nor the average of all waves. An offshore forecast also cannot tell you the exact height of each wave at the water's edge.
Sources for this explanation: G2
CONNECT THE PICTURE AND THE WORD
Words to remember
- Wave height
- The vertical distance from trough to crest.
- Wavelength
- The horizontal distance between neighbouring crests.
- Period
- The time between successive crests passing a fixed point.
- Swell
- Waves that travel beyond their generating winds, or remain after those winds weaken or change.
CAN YOU EXPLAIN IT IN YOUR OWN WORDS?
Check your understanding
Think first, then open each question to read the explanation. You can look back at the diagrams.
Q1Which could be measured as five seconds: wavelength or period?
Explanation: Period. Wavelength is a distance, measured in units such as metres. Period is a time, measured in seconds.
Q2If the wind drops at your beach, does swell disappear too?
Explanation: No. Waves can arrive from winds far away. Local wind alone does not tell you the wave conditions.
LET’S THINK
A question to explore further
When you make the wave bigger in the diagram, which changes: wave height or wavelength?
Show a hint and explanation
Wave height changes; wavelength stays the same. Compare the distance between successive crests to check. In the real sea, several characteristics can change at once.
Explore together, at home or in class
Draw a side view of a wave. Explain the difference between height, distance and time. A still picture can show height and wavelength; measuring period needs an observation over time.
Sources & further reading
We developed our own explanations, diagrams and learning questions using these sources. Inclusion does not imply review or endorsement by the source organisation. Links marked “Japanese” lead to Japanese-language material.
Sources for the added diagrams and explanations
Additional sources checked: 2026-09-21. Numerical diagrams use invented teaching values.
- [G1] NOAA: Why does the ocean have waves? ↗Wave energy and the basic movement of water.
- [G2] JMA: understanding ocean waves (Japanese) ↗Wavelength, period, significant wave height, swell and shallow-water effects.
Original lesson sources
- [1] Japan Meteorological Agency: understanding waves (Japanese) ↗Wave height, wavelength, period, wind waves, swell and changes in shallow water.
First published: 2026-09-20 · Updated: 2026-09-21 · Original sources checked: 2026-09-20 · Editorial policy