When the wind keeps blowing

Ripples grow into waves

If the wind keeps blowing, the ripples grow bigger and bigger until they become waves. Water that has been lifted up is pulled back down by gravity. As it comes down, it pushes up the water next to it, and that water comes down too. This rising and falling, passed along from place to place, is a wave.

Why?Three things decide how big waves get: how strong the wind is, how long it blows, and how much open sea it blows across. If any one of them is too small, the waves cannot finish growing.

Learn moreWhen wind blows long enough over a big enough stretch of sea, the waves reach a size where they stop growing. A formula for that size (Pierson–Moskowitz) gives about 2.1 m for a 10 m/s wind, about 4.8 m for 15 m/s and about 8.6 m for 20 m/s. Real waves are uneven, so their height is given as the average of the highest third (significant wave height).

Wave height grown by the wind

4.8 m

Wind 10 m/s

about 2.1 m · 7-second waves

Wind 15 m/s

about 4.8 m · 11-second waves

Wind 20 m/s

about 8.6 m · 15-second waves

calculated from the textbook equations for water waves (linear wave theory)