What makes the waves crash in on a beach?
The waters of the oceans and lakes are always moving.
On the oceans, this is caused by the tides –
periodic water movement caused by the Moon’s gravitational pull.
On smaller lakes and ponds, it’s usually caused by wind. And some HUGE waves –
we’re talking Tsunamis here – are caused by underwater earthquakes.
Far out into the ocean, regular waves (not during a
tropical storm or hurricane) tend to stay pretty consistent in size,
since they travel along the surface of the water.
But once they get close to shore, their shape changes as the water gets shallower.
The wave is traveling at a certain speed along the surface,
but the bottom of the wave starts to lose speed as the the sea floor slows it down.
At the same time, the top of the wave is still moving quickly,
unaffected by the drag at the bottom, and it starts to topple over,
which forms the crest of the wave and causes it to eventually CRASH down with great force.
See for yourself: Catch the waves!
If you live near or travel to a beach, watch the waves.
If your grownups say it’s safe, wade out to feel the waves crashing.
Only a few steps out, you can probably stay standing, but close to the breaking point
(right when they topple over and crash) it might be enough to knock you over, so be careful!
Can you feel how the tops of the waves speed up as they get closer to shore?
What happens if you duck under the crest of the wave and let it pass you by?
If there is a lifeguard at the beach, or if you know someone who works as a lifeguard,
ask them about “riptides” and how to avoid them.