Forecast wave period map

What the wave period map shows

The time between the main waves ahead, in seconds. Under 8 seconds they are wind waves raised close by; over 12, swell from a storm far away, which carries more power and breaks harder on the shore. The lines run the way the waves travel.

How to read it

Under 6 s

Wind waves: short, steep and choppy.

6 to 10 s

Wind waves and swell together.

10 to 14 s

Swell from a distant storm.

Over 14 s

A long groundswell, carrying great power; it breaks hard on the shore.

What it is for

• Tell a local chop from a swell come from far away.
• Judge how hard the waves will break: the longer the period, the more power each carries.
• Read with the wave height: the same height at a longer period makes bigger surf.

How it is made

The sea maps are drawn from the weather service’s wave model: the waves the wind raises over every ocean, worked out from the wind forecast, on fine grids along the coasts of the United States (a point about every 11 miles) and a coarser one over the open ocean (about every 17 miles). Each of its runs, four a day, is read whole, laid onto the same grid of latitude and longitude as the radar at 2.8 miles a cell, the finest grid first where they meet, and played forward from the hour it starts.

A frame every 3 hours to 3 days, then every 6 hours to 7 days, from the hour the forecast starts, played from now as a loop of 1 to 7 days. As data: /radar/now.json?show=fwper, the times of its frames.