Prelab — Wave Dynamics: Hurricane Surf Forecasting

Learning Objectives

By the end of this prelab you should be able to describe what a wave buoy actually measures, explain why two waves of the same height can behave completely differently, and predict in plain language whether a given storm is going to make good surf or just a mess. You will also have opinions, which is arguably the most important lab skill.

Define swell height, swell period, swell direction, and wind direction, and explain what each one tells you about a wave.
Describe how NOAA buoys collect wave data in deep water, and why that data matters for coastlines thousands of miles away.
Predict, from a small set of buoy measurements, whether a storm will produce weak windswell, clean groundswell, or genuinely dangerous surf.
Use the simulator below to build an intuition for how changes in period and height change a wave's shape — not just its size.

Background: What Buoys Actually Measure

A NOAA weather buoy looks, from above, like a yellow mixing bowl that has lost its job. Inside that bowl is a set of accelerometers and tilt sensors that record how the buoy rises, falls, and pitches as waves pass underneath it. Every hour or so, the buoy radios those measurements back to shore, where they become the numbers you will be reading for this lab. Nothing about the buoy is measuring "surf." It is measuring open-ocean water rising and falling.

Two numbers from the buoy do most of the forecasting work. The first is swell height, which is the average height of the taller waves passing the buoy, reported in meters or feet. The second is swell period, the time in seconds between one wave crest and the next. Height tells you how much energy is in each wave. Period tells you how organized that energy is — and, critically, how far it has traveled.

When deep-water waves reach shallow water near a coastline, they slow down, bunch up, and grow taller. A tidy 6-foot swell offshore can show up as 8–10 feet of actual surf at the beach. This is why "4 feet at the buoy" sometimes means a fun afternoon and sometimes means "please do not enter the water," depending almost entirely on the period.

Rule of thumb: height is how hard the wave hits. Period is how far it traveled to get there. Long-period swells come from serious, distant storms; short-period swells were made by the wind that is blowing right now, probably in your face.

Key Terms (click to flip)

Tap a card to check your memory. No judgment if you flip them all at once — that's called studying.

Mini Wave Simulator

Drag the sliders to change wave height, period, and wind speed. Watch what happens to the shape of the wave and the forecast verdict. This is a simplified cartoon of what the full lab simulator does — you are allowed to have fun with it.

Swell height: 2.0 m Swell period: 10 sec Wind speed: 15 knots

Adjust the sliders to see a verdict.

If you find yourself in the “expert only” zone on a Monday morning, the correct forecasting response is to close the laptop.

Storm Dossiers You'll Meet in Lab

Five real storms, five different personalities. Glance through these now so the data table on lab day does not feel like a wall of numbers. You will be asked to decide which of these generated the best surf and which one generated mostly regrets.

Cat 2 · 1995

Hurricane Humberto

Swell: 1.3 m
Period: 12 sec
Direction: SE (135°)
Wind: E, 20–25 kt

Cat 4 · 1995

Hurricane Luis

Swell: 3.3 m
Period: 20 sec
Direction: E (90°)
Wind: NW, 8–12 kt

Cat 4 · 2007

Hurricane Felix

Swell: 6.6 m
Period: 18 sec
Direction: ENE (70°)
Wind: W, 10–15 kt

Cat 2 · 1995

Hurricane Marilyn

Swell: 3.6 m
Period: 14 sec
Direction: ESE (112°)
Wind: SW, 5–10 kt

TS · 2001

Tropical Storm Iris

Swell: 1.0 m
Period: 9 sec
Direction: NE (45°)
Wind: NE, 25–30 kt

Pre-Reading Questions

Answer these before you arrive at lab. You don't need to be right — you need to have a first guess. Being wrong on paper is free; being wrong in the ocean is not.

1. In your own words, what is the difference between swell height and swell period? Which one would you rather know about if you only had time to ask one question?

2. A buoy reports 2 m height with a 6-second period. A second buoy reports 2 m height with an 18-second period. Which will produce bigger, more powerful waves at the beach, and why?

3. Which of these sets of conditions would you expect to produce the cleanest surf at an East Coast break?

A. 4 m swell, 6 sec period, 30 kt onshore wind B. 2 m swell, 16 sec period, 5 kt offshore wind C. 5 m swell, 8 sec period, 20 kt onshore wind D. 1 m swell, 4 sec period, calm

4. Look at the dossiers above. Without doing any math, which hurricane do you predict produced the best surf, and which one produced the worst? You'll check your answer in lab.

5. NOAA buoys are expensive to build, maintain, and occasionally fish out of the Atlantic. Give one reason, beyond surfing, that it is worth paying for them.

Before You Walk In

Bring: this prelab (answered), a pencil that actually works, and a basic calculator or phone. You will not need a wetsuit. You will not need a board. You will, however, need to have flipped at least a few of those vocabulary cards, because the lab moves quickly and nobody wants to pause the simulation to explain what "groundswell" means.