Marine / Waves
Sea state for coastal locations
What is it?
Sea state describes wave height, period and direction — critical for swimmers, fishers and small craft. Wave energy grows with the square of height: a 2 m wave carries four times the force of a 1 m wave.
How we measure it
Significant wave height (m) and sea-surface conditions from the Open-Meteo marine feed, where your location is near enough to the coast to have data.
Significant wave height guide (WMO sea state)
- < 0.5 mCalm–smoothSafe for swimming and small boats
- 0.5–1.25 mSlightFine for most craft; watch children
- 1.25–2.5 mModerateSmall-craft caution; strong swimmers only
- 2.5–4 mRoughSmall craft stay in port; no swimming
- 4+ mVery rough–highDangerous for all coastal activity
Within safe range
Below ~1.25 m most beach and small-craft activity is comfortable, though rip currents can exist even on calm days.
Beyond the range
Above ~2.5 m breaking waves can knock adults off jetties and rocks, and rip currents strengthen. Most coastal drownings involve rips: the escape is to swim parallel to shore, not against the current.
Precautions
- Check conditions before entering — heed local flags and lifeguards.
- Caught in a rip: stay calm, float, swim parallel to shore until free.
- Keep well back from rocks and seawalls in rough conditions; sneaker waves reach dry ledges.
- Small craft: file a plan, wear lifejackets, respect small-craft advisories.