Curious Kids: Why are there waves?

By Mark Hemer

28 May 2019

3 minute read

Graphic image of waves

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky! You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

Thanks for a great question, Evie.

When you look at the waves breaking at the beach, those waves might be at the end of a long journey. The waves might have been created thousands of kilometres away, or they could have been created near you.

There are lots of types of waves in the ocean, but the waves you usually see at a beach are created by the wind. When the wind blows over a smooth ocean, it creates little waves or ripples on the surface. If the wind continues to blow, the waves grow bigger.

Wave breaking on rock edge

A big wave lands at Dee Why Beach in Sydney. Taro Taylor/Flickr, CC BY

Faster, bigger, longer

The faster the wind blows (like in a strong storm out at sea), the bigger the waves will grow.

The further the wind blows (or the bigger the area of the storm), the bigger the waves will grow.

And the longer the wind blows (like in a storm that lasts a long time), the bigger the waves will grow.

If the wind stops, or changes direction, the waves will stop growing, but they won’t stop travelling.

They will keep travelling away from where they were created in a straight line, sometimes for days, until they run into something like a beach where they are stopped because they break. That’s why there are still waves at the beach, even when it is not windy.

Waves trip over themselves

Imagine you were running really quickly. But then suddenly, you ran into thick gloopy mud. Your feet would slow down, but the top half of your body would still be going fast. You’d trip over.

Waves do the same thing and that is when they break.

As waves approach the shore, the water is shallower, and the bottom of the wave starts to feel the sand and rocks and seaweed. The bottom of the wave slows down, and soon, the top of the wave is going faster than the bottom part of the wave, so the top spills forward and topples over in a big splash.

Surfer inside barrel of a wave

This wave is breaking over the top of the surfer because the top half of the wave is travelling faster than the bottom half. Flickr/Duncan Rawlinson – Duncan.co – @thelastminute, CC BY

Waves can travel a long way

Scientists who study the ocean (called oceanographers) have measured waves created in the Southern Ocean, and seen them travel all the way across the Pacific Ocean and break on the beaches of North America more than a week later.

Try counting the seconds between waves breaking on the beach. If the time between waves is 10 seconds or more, the waves have come from a long way away. If the waves were created nearby, the time between waves will be short, perhaps five seconds or fewer.

Sometimes when we look at the sea we might see different waves (some big, some small) all happening at the same time. These waves were created at different places, perhaps by different storms, but ended up in the same spot at the same time.

Freak waves

During big storms, waves can get very big. If big waves from two different storms meet together, that can create enormous waves that we call “freak waves”. The largest waves measured are around 25 metres high (that’s five giraffes standing on top of each other!) and they can tip over ships.

Evie’s question was answered by our Oceans and Atmosphere expert, Mark Hemer. Mark is making waves as leader of the Sea-Level, Waves and Coastal Extremes team within our Ocean and Atmosphere Climate Science Centre.

This article was originally published in The Conversation. Read the original article.