Swell period and kJ

Tomorrow's forecast says 4 ft at 8 s. The day after says 3 ft at 16 s. Which day is bigger? At most spots it's the second one, even though the number looks smaller. Period is the reason, and kJ is the one number that shows it: 190 for the first day, 420 for the second.

The rest of this page is the why, with four figures you can drag.

Period: count the seconds

Sit on your board out the back and count from one wave to the next. That count is the period. A buoy out at sea does the same thing, crest to crest.

Period tells you where a swell came from. Under about 8 s it's windswell: made by wind nearby, crests packed close together, not much push. From about 13 s up it's groundswell from a storm a long way off. That's the stuff that arrives as clean lines, in sets with lulls between them.

Period also sets the distance from one crest to the next, the wavelength. At 8 s the crests are 330 ft apart. At 16 s they're 1,300 ft apart. Twice the period, four times the wavelength.

And long-period swell travels faster. A 16 s swell covers about 670 miles a day, an 8 s swell half that. So when a storm far away sends waves your way, the long-period ones show up first.

Long waves feel the bottom first

Water doesn't travel with a wave. It goes round in a circle and ends up where it started, which is why the buoy bobs up and down and doesn't surf off to the beach. The circles get smaller as you go down. Half a wavelength below the surface, the water hardly moves at all.

An 8 s wave only stirs the top 160 ft. A 16 s swell reaches down 660 ft. So groundswell touches the bottom much farther out, and once a wave touches bottom it slows down, bends towards the shallower water and starts to grow.

You've seen what that does. It's why a long-period swell wraps round a headland into a bay that windswell marches straight past, and why some reefs do nothing until the period gets long.

kJ: height and period in one number

A taller wave carries more energy. So does a longer one. kJ counts both. It's the energy in a single wave of the swell, measured in a slice of that wave one metre wide.

Back to the two days. 4 ft at 8 s is the taller swell, but 3 ft at 16 s has four times the wavelength, so each of its waves carries more than twice the energy.

Height and period count the same. Double either one and the kJ goes up four times.

How much is 420 kJ? Enough to lift a small car about 100 ft into the air. And that's only the slice of the wave one metre wide.

At the break: same height, twice the wave

When a swell gets into shallow water it slows down. The waves behind catch up, the crests bunch together, and each wave gets shorter and taller until it's about three quarters as tall as the water is deep. Then it breaks.

On this beach the two days break at about the same height: 5.7 ft for 3 ft at 16 s and 5.4 ft for 4 ft at 8 s, both around head high. What's different is behind the crest. When it breaks, the 16 s wave is 240 ft from crest to crest and the 8 s wave is 120 ft. Same height, twice the wave behind it. That's the push you feel when one lands on you, and it's what kJ picks up and height doesn't.

That's on a dead-straight beach, though. Reefs, points and canyons gather long-period swell, because it bends more, so at plenty of breaks the 16 s day comes in clearly bigger as well. And longer isn't always better: on a straight beach break a long-period swell often turns up as one long wall and closes out.

Why Ripple doesn't give a face height

That last figure just turned a swell into a breaking height. So why doesn't Ripple print one for your spot?

Because the figure cheated. It took a dead-straight beach, an even slope, and a swell coming square on. Change one thing and the answer moves. Turn that same beach 60° away from the swell and 3 ft at 16 s breaks at about 4.3 ft, down from 5.7 ft. A reef or a point that gathers the swell can make it far bigger. A bay tucked behind a headland gets a fraction of it. Then add the tide, the sandbanks and a second swell.

A forecast can only give a face height if it knows each spot: the seabed, which way the break faces, how it handles each swell direction and tide. Ripple works for any point on the map and tunes nothing per spot, so any rule we picked would be wrong at most breaks, and you'd have no way to tell when. Surfers don't even agree on how to measure a face. Call it the Hawaiian way, from the back, and you get about half.

So Ripple shows what the models actually forecast: each swell's height, period and direction out at sea, and its kJ. The last step is yours, and you only have to learn it once. Note the numbers on a few days you surf, and you'll know what they mean at your break.

What to do with the number

Windguru and Surf-Forecast print a kJ figure for the swell, and theirs match the formula above. Ripple gives the kJ of every swell in the water and each one's share of the total. Is it worth watching? We think so. If you only look at one number, kJ is a better one than height.

Some swells to get a feel for it. The last column is the same straight beach as before.

Look at the two rows with 420 kJ. Same energy, different wave: 4 ft at 12 s breaks a bit taller, 3 ft at 16 s is thicker.

• There's no magic kJ. 400 might be perfect at your beach break and nowhere near enough for the reef up the coast.
• Two 3 ft swells don't make 6 ft. They make about 4.2 ft, because it's the energy that adds up. That's why Ripple splits a mixed sea by kJ.
• The forecast height is an average of the bigger waves. Most waves are smaller than it, and the best sets of the hour run about one and a half times it.