How to read a surf forecast

news · 2026-04-19

A surf forecast looks like a wall of numbers. Most of them don’t matter. Four of them do.

Height

The “wave height” on Harper is significant wave height — roughly the average of the largest third of waves. The biggest sets will be bigger; the smaller waves will be smaller. A 1.5 m forecast means there will be plenty of 2 m waves in the mix.

Period

Period is seconds between wave crests. More seconds means more energy and better-organised swell.

• < 7 s — wind chop. Short-lived, breaks mushy.
• 8–11 s — local wind swell to mid-period. Fun, but not lined up.
• 12–15 s — classic groundswell. Lines from the horizon.
• 16 s + — long-range storm swell. Packs a punch; the 1 m on the chart will feel bigger than it reads.

Period matters more than height for how a wave breaks.

Direction

Harper reports the direction the swell is coming from — a south swell means the energy is travelling up from the south. The arrow on the chart points where the swell is going, which is the opposite (marine convention).

Match the swell direction against your spot. A beach facing SW needs a S–W swell window; swell from the N will wrap around and lose most of its energy.

Wind

Wind sits outside the swell itself but decides whether the waves arrive clean or messy.

• Offshore (blowing from land to sea) — grooms the wave face, holds it up longer.
• Onshore (sea to land) — chops the surface, pushes waves to close out.
• Cross-shore — in between.

Light is almost always better than strong, regardless of direction.

Putting it together

A good session is usually decent height, long period, swell lined up with your spot, light offshore wind, mid-tide.

A bad session: short period, wind onshore, or swell hitting the coast at a bad angle — even if the height is big.

Start with period and wind. The other numbers fall out from there.