Pular para o conteúdo principal

How surfers can predict wave height just by reading the wind

Swell: in the 1940s, Harald Sverdrup, Walter Munk, and Hans-Jürgen Bretschneider created an empirical method for predicting wave height based on wind speed | Photo: Shutterstock

The road to becoming a complete surfer is a long one. But if there's one skill that could set you apart, it's surf forecasting and basic meteorology.

Here's the good news.

You don't need a PhD in oceanography to guess how big or good the waves will be tomorrow.

With just a bit of wind data, an interactive map, and a few easy equations born from one of the most trusted wave prediction methods in history - the Sverdrup-Munk-Bretschneider (SMB) method or modification - you can do it old-school style.

The idea is to look at a purple fetch on Windy.com, measure its distance from your favorite surf break, and guess the wave height you might get when you paddle out.

And, trust us, you don't even need to be a math geek to estimate it quite well.

Walter Munk and Harald Sverdrup: the co-founders of the Sverdrup-Munk-Bretschneider (SMB) method for predicting wave height | Photo: UCSD

The Story Behind the SMB Method

Back in the 1940s, World War II strategists and planners faced a serious problem: how to predict ocean conditions for landing operations when no wave buoys or satellites existed.

The science of surf forecasting had yet to be invented, yet, interestingly, they ended up laying its foundations.

There is actually an amazing book titled "Surf Forecasting For Invasions During World War II" that details the story of the wave predictions for D-Day, when thousands of Allied troops disembarked on the Normandy beaches.

The SMB is a set of equations created by Harald Sverdrup, Walter Munk, and Hans-Jürgen Bretschneider.

The formulae connected wind speed, fetch (the distance the wind blows over open water), and duration (how long it blows) to wave height and period.

In other words, based on these empirical calculations, several fields of study, from naval operations to surf forecasting and coastal management, can determine the size (and quality) of the waves at the end of their travel, i.e., on coastlines.

The SMB Method: the updated version of the original wave height prediction nomogram

The Three Variables

According to SMB, wind-driven waves can exist in three "growth stages":

  1. Fetch-limited: the wind blows over a small or enclosed area, like a lake or bay. Waves stop growing once they run out of space;
  2. Duration-limited: the wind blows for only a short time, long enough to agitate the water, not long enough to build big waves;
  3. Fully arisen sea (fully developed sea): the wind has blown strong and long over a large fetch, and the sea has reached its maximum possible wave size for that wind speed;

Each case has its own formula for wave height (H) and wave period (T), too.

The Simplified Equations

Here's the wave forecaster's cheat sheet.

Use U in meters per second (m/s), F in meters (m), and t in seconds (s). You can convert the values this way:

  • 1 kilometers per hour = 0.2778 meter per s;
  • 1 kilometer = 1000 meters;
  • 1 hour = 3600 seconds;

1. Fetch-Limited Waves

In this case, wind runs out of ocean before it runs out of time.

H = 0.016 * U * F^0.5
T = 0.285 * F^0.33

Rule of thumb: a long fetch (hundreds of kilometers) means bigger, better-organized waves even if the wind stops before they reach shore.

2. Duration-Limited Waves

Now, this is the case of when the wind blows over enough ocean, but not for long enough.

H = 0.018 * U^1.25 * t^0.75
T = 0.83 * U^0.57 * t^0.37

Here, short-lived wind bursts create small, choppy waves, which could eventually be OK for windsurfing, but not clean enough to have nice surf lines.

3. Fully Arisen Sea

Finally, here's the equation for when the wind blows long and hard over a vast fetch and the sea maxes out.

H = 0.025 * U^2
T = 0.73 * U

It's the dream setup for open-ocean swell generation, that is, groundswells.

If you see strong winds (like 60 kilometers per hour or more) blowing for over a day across hundreds of kilometers, you may wax up your surfboard because there's a swell factory in action.

Swell: the SMB method is an old-school way of predicting wave height using wind speed and fetch data | Photo: Golara/Creative Commons

Don't Know Which One Applies? Here's a Trick

In real life, it might be tricky to know if the waves are fetch- or duration-limited, right?

No problem. It's acceptable if you calculate all three and take the smallest value of wave height H.

The reason is that the ocean naturally limits itself by whichever factor - fetch or duration - is smaller.

For most oceanic situations, fetch is the limiter; for small lakes or short wind bursts, duration is.

A Quick Example: Your Local Surf Spot

Let's say a wind map shows a 30 kilometers per hour wind (that's 8.3 m/s) blowing across 200 kilometers of open ocean for several hours.

Using the fetch-limited formula:

H = 0.016 * 8.3 * (200,000)^0.5 = 1.5 m

So, you'll probably get around head-high surf, give or take, but only if that swell travels toward your coast and the local winds don't ruin it.

If the wind only blew for an hour or two, plug those numbers into the duration formula, and you'd probably get 0.5 meters or less.

In other words, small wind chop, not surfable lines.

Putting It All Together

The SMB method is a perfect middle ground between raw weather charts and advanced surf forecasts.

It tells surfers how wind speed, fetch, and time interact to shape wave energy before it even reaches your beach.

So next time you see a pressure system spinning across the map, grab your calculator (or just your phone), plug in a few numbers, and get a first-glance read on what the ocean might deliver.

It's a great math exercise and will definitely surprise you.


Words by Luís MP | Founder of SurferToday.com



por Surfing | News, Headlines and Top Stories https://ift.tt/56PBDbC

Postagens mais visitadas deste blog

Duke Kahanamoku reflects on surfing, Olympics, and old Hawaii in 1966 interview

Duke Kahanamoku is the most influential surfer of all time and is often hailed as the father of modern surfing. There is nearly no one questioning these titles. Recently, Public Broadcasting Service (PBS) Hawaii unveiled a never-before-seen interview with the legendary surfer and Olympic swimmer. In the 1966 episode of Pau Hana Years, a seminal Hawaii television program that aired on KHET-TV (now PBS Hawaii) for 16 years, running from 1966 until 1982, Bob Barker chats with Duke Kahanamoku, then 76. The conversation drifts from royal ancestry to Olympic lanes, from Hollywood sets to a surfboard shaped by hand, tracing the outline of a life that helped define modern surfing and Hawaii's public image in the 20th century. And if you know little about the man who dreamed of getting surfing into the Olympic Games, this is a precious piece of history. A name with history, worn casually The interview starts with Kahanamoku explaining that "Duke" is not a title but his giv...
Leia mais

The hydrodynamics of surfboard fins

Have you ever wondered why a surfboard fin looks like that? It is a single or a set of fixed blades or keels located under a board, near the tail, often no bigger than a hand. Yet that small surface is where much of the surfboard's behavior takes place. Speed, hold, looseness, and the feeling of control all trace back to how water moves around fins. The physics of surfboard fins falls under hydrodynamics, the study of how fluids behave in motion. So, according to science, they feature a shape designed to turn flowing water into several forces. Let's take a look at what's at stake when fins and water interact. Lift and the feeling of control One of the key variables in hydrodynamic terms involving surfboard fins is lift. When a surfer leans into a turn, the board tilts and the fins meet the water at an angle. The angle is enough to create a pressure difference between the two sides of the fin. Water speeds up on one side and slows on the other. The result is a sidewa...
Leia mais

How paddleboarding transforms your body and mind

Adventure is on our doorstep. With so many different bodies of water available to paddleboarders, from city canals to coastal routes, we can find adventure in places much closer to home than people might initially expect. According to the Canal and River Trust, 50 percent of people in England and Wales live within just eight kilometers of a canal or river, and eight million people live less than one kilometer away. I had lived within just a few kilometers of the Leeds and Liverpool Canal for years and never really explored it before stand-up paddleboarding (SUP) came into my life . The challenge created both a new perspective and a deeper love for where I lived and the areas which I passed through. On my coast-to-coast journey, I slept in my own bed for two nights as the route passed through my then hometown of Skipton, yet I felt I was on a grand journey of discovery. We are braver, stronger, and more resilient than we think. SUP not only helps us feel more connected to our va...
Leia mais