Understanding the sea state
"2 metres" means nothing without the period. Learn to read swell and wind sea to anticipate comfort — and safety.
Wind sea vs swell
Two very different things hide behind "the sea state":
| Wind sea | Swell | |
|---|---|---|
| Origin | local wind, here and now | distant or past wind |
| Shape | short, disorganised, breaking crests | long, regular, rounded |
| Feel | slamming, uncomfortable | wide roll, more manageable |
The real sea is often the sum of the two, sometimes coming from different directions (cross sea), which makes handling harder.
Significant height and period
The significant height (Hs) is the average of the highest one-third of the waves. Remember that roughly one wave in ten exceeds Hs, and you can meet waves ~1.5 to 2× Hs. The period (in seconds) is the time between two crests: it's what tells a manageable sea from a hard one.
The role of fetch and shallows
The longer the fetch (the distance over which the wind blows), the more the sea builds. And when swell reaches shallow water or runs against a current, it steepens and breaks: that's where the sea becomes dangerous, even in moderate wind.