Surf & Oceanography Glossary

Plain-language definitions of the terms behind every forecast on this site.

Waves & Period

Wave Height: The vertical distance from a wave's trough to its crest. Surf forecasts typically report significant wave height (Hs), not the face height you see at the beach — face heights can be 1.5–2× larger at the moment of breaking due to shoaling.
Wave Period: Time in seconds between successive wave crests passing a fixed point. Longer periods mean more powerful, well-organized waves. The single most important number in a surf forecast after height.
Peak Period (Tp): Period of the most energetic component of the wave spectrum — the "classic" period number reported by NDBC buoys and most surf forecasts. This site standardizes on Tp across all sources.
Mean Period (Tm): The energy-weighted average period across all frequency components. Usually 1–2 s shorter than Tp. Open-Meteo's wave_period field is Tm; this site requests wave_peak_period instead.
Significant Wave Height (Hs): The average height of the largest one-third of waves in a record. The standard wave-height metric used by NOAA, NDBC, and every operational wave model.
Wavelength: The horizontal distance between two successive crests. In deep water, wavelength L = gT²/(2π) ≈ 1.56 × T² in meters. A 14 s swell has a 306 m wavelength.
Group Velocity: Speed at which wave energy (and thus the swell itself) travels across the ocean. In deep water, cg ≈ 0.78 × T m/s. A 14 s swell travels ~10.9 m/s = 39 km/h.
Wave Steepness: Ratio of wave height to wavelength (H/L). Low steepness (under 0.008) indicates long-decayed clean swell; high steepness (over 0.025) indicates locally-generated wind sea. This site classifies swells by both period and steepness.

Swells & Sea State

Ground Swell: Long-period waves (typically 12 s or more) that have traveled far from a distant storm. Clean, powerful, well-organized lines. The most desirable surf.
Wind Swell: Short-period waves (typically under 9 s) generated by local winds. Less organized than ground swell; breaks rapidly. Often what you ride day to day.
Mixed Seas: Wave field with multiple distinct swell trains plus local wind sea, often arriving from different directions. Common in winter and during transitional seasons.
Swell Train: A coherent group of waves from one storm source, characterized by a single peak period and direction. A "stack" of swells means multiple trains overlapping.
Set Wave: The largest wave in a cluster. Surfers wait for sets; lulls between sets are the gaps. Average set frequency is 6–10 waves per minute in organized swell.
Decay: Gradual loss of wave height as a swell propagates away from its source. Long-period swells decay slowly and can cross entire oceans intact (the Hawaiian winter season is fed by storms in the Gulf of Alaska).
Spectral Partitioning: Decomposing a complex wave field into separate components (primary swell, secondary swell, wind sea). Sophisticated forecasts publish all three; simple ones just report a single "Hs".

Wind & Beach Orientation

Beach Orientation: The compass direction the beach faces — perpendicular to the shoreline, pointing toward the ocean. Determines which swell angles reach the beach unobstructed.
Offshore Wind: Wind blowing from land out to sea. Holds up wave faces and creates clean, glassy conditions. Surfers' favorite wind direction.
Onshore Wind: Wind blowing from sea onto land. Creates choppy, disorganized surf. Strong onshore destroys wave shape completely.
Cross-shore Wind: Wind blowing parallel to the shoreline. Adds surface texture without destroying wave shape — often surfable, sometimes preferable when very light.
Glassy: Conditions with virtually no wind, often at dawn before the sea breeze kicks in. Mirror-flat between waves; perfect for clean rides.

Coastal Effects

Shoaling: The amplification of wave height as a swell moves into shallower water. Why a 4 ft offshore swell can become a 6 ft wave at the beach.
Refraction: Bending of waves as they cross varying water depth, turning swell direction toward shore-normal. Why a NE swell can wrap into a south-facing cove.
Diffraction: Spreading of waves around obstacles such as headlands or jetties. Allows energy to reach areas in a swell's geometric shadow.
Sandbar: Underwater ridge of sand that focuses where waves break. Beach breaks live and die by their sandbars, which shift with storms and seasons.
Fetch: The over-water distance the wind blows in one direction. Long fetch + sustained high winds = bigger, longer-period swells. A 1,000 nm fetch is how Pacific groundswells get their power.

Tides

Tide: Periodic rise and fall of sea level driven by the gravitational pull of the moon and sun. Most US coasts see two highs and two lows per day (semi-diurnal).
Slack Tide: Brief period at high or low tide when the water effectively stops moving. Many breaks lose shape during slack; moving water keeps sandbars working.
Spring Tide: Extra-large tide range during full and new moons, when the sun and moon align. Bigger high-low difference; stronger tidal currents.
Neap Tide: Smaller-than-average tide range during quarter moons. Less variation between high and low.
MLLW: Mean Lower Low Water — the average of the lower of the two daily low tides over 19 years. NOAA's tidal datum for predictions.
Optimal Tide: The tide stage at which a particular break works best. Some spots fire on the push (incoming), some on the drop (outgoing); reef breaks tend to be more tide-sensitive than beach breaks.

Models & Forecasts

WaveWatch III (WW3): Third-generation spectral wave model run operationally by NOAA. Powers most public US wave forecasts at 0.16°–0.5° resolution.
GFS-Wave: Global Forecast System wave product — NOAA's implementation of WaveWatch III coupled to GFS winds. Updated every 6 hours, available via NOMADS and ERDDAP.
NDBC Buoy: National Data Buoy Center station. Moored or drifting buoys measuring waves, wind, and water properties in near-real time. The ground truth used to validate models.
CDIP: Coastal Data Information Program — a network of nearshore wave buoys (mostly West Coast) producing high-quality directional wave spectra.
Wave Spectrum: Distribution of wave energy across frequencies. A 1D spectrum shows energy vs frequency; a 2D directional spectrum adds direction-of-arrival.
Hindcast: A model run using past observations to reconstruct historical wave conditions. Used to validate forecasts and study extreme events.
Nowcast: A model run using current observations to estimate present conditions. Distinct from forecast (future) and hindcast (past).
Forecast Skill: Measure of how well a forecast matches observations. Wave-model skill (r²) typically falls from ~0.85 at lead 0 to ~0.55 at +144 h. The confidence sparkline on this site reflects this decay.
Model Bias: Systematic over- or under-prediction by a forecast model. Can be corrected against co-located buoy observations — a planned future improvement here.

Break Types

Beach Break: Surf break where waves break over a sandy bottom. Sandbars shift over time so shape varies with the seasons. Most US coast surf is beach breaks.
Reef Break: Surf break where waves break over a coral or rock reef. More predictable and shape-consistent than beach breaks but consequences for falling are higher.
Point Break: Surf break where waves wrap around a headland or rocky point, producing long peeling rides in one direction.
Jetty / Pier Break: Sandbar deposition at the up-current side of a man-made jetty or pier, focusing wave energy. Common on the East Coast (Manasquan, Wrightsville, etc.).

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