Swell Direction & Swell Windows

Swell direction is the compass bearing swell arrives FROM, in degrees. A swell window is the range of those bearings a particular break can actually receive, given the land, islands and seafloor around it.

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Direction is the most under-read number on a forecast, and the one that most often explains a disappointing session after a big reading.

Degrees, and the FROM convention

Marine convention reports the direction energy comes FROM, not where it is going. 270° is a westerly swell — moving east, arriving from the west. This is the same convention used for wind, and mixing them up inverts every conclusion.

Rough compass equivalents: 0° north, 90° east, 180° south, 270° west.

What a swell window is

Draw the arc of open ocean visible from a break and you have its swell window. Anything outside that arc is shadowed by a landmass, an island chain or a shallow bank, and arrives sharply reduced or not at all.

Windows can be startlingly narrow. A break tucked behind a headland may accept only 40° of the compass, which is why it is empty on days when everywhere else is firing — and why it is perfect on the handful of days each year when a swell lines up.

Refraction complicates it

Windows are not hard edges. Waves bend toward shallow water, so energy from just outside a window still wraps in, weaker and from a slightly different angle than the open-ocean bearing suggests. Long-period swell refracts more, which is why it reaches into bays and around points that short-period swell never finds.

PierMonkey models this as a falloff with a soft shoulder rather than a cutoff, and every covered break has its window documented with the storm corridors that feed it.

See it on live data

The fastest way to make any of this concrete is to go and look at the numbers being described.

Common questions

Does swell direction matter more than size?▾

For most breaks, yes. A modest swell from the right direction beats a large one from the wrong direction, because the wrong direction is largely blocked before it arrives.

What is the difference between swell direction and wind direction?▾

Both are reported as the bearing the energy comes FROM, but they describe different things. Swell direction is where the waves originate; wind direction determines whether the surface is clean or blown out.

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