The sea, as high as it is right now

What you are looking at

The colour is the significant wave height: the average of the highest third of the waves, which is what a sailor would call the height of the sea. Deep blue is under a metre and a half, bright blue a moderate sea, pale blue a rough one, magenta a high sea of six metres and more. Storms far out at sea send swell across whole oceans, which is why a calm day can still bring big surf: follow the particles from a Southern Ocean low and they arrive, days later, on the coasts of Indonesia, California or Portugal.

How to read it

• Brighter, faster particles mean a higher sea; magenta is six metres and more.
• The particles move the way the waves travel, not the way the water flows — a wave passes through the water, it does not carry it along.
• Switch on Wind with it: the highest seas sit under the strongest winds, and the swell runs on long after the wind has dropped.

Questions

Where are the biggest waves right now?

The figure at the top of this page gives the highest sea on the model's grid this hour and where it is. Most days it is in the Southern Ocean, where the wind blows round the world with no land in its way — unless a hurricane or a North Atlantic winter storm is doing better.

What does wave height mean?

The figure is the significant wave height: the average of the highest third of the waves. Individual waves vary around it — about one in a hundred is one and a half times as high, and one in a few thousand is twice as high, which is where rogue waves come from.

Why are there waves when there is no wind?

Because waves outlast the wind that made them. A storm raises a sea, and the longest waves leave it as swell, crossing thousands of kilometres of ocean with little loss. The surf on a windless beach was often made by a storm a week ago and half an ocean away.