Waves: height, period and swell
Significant wave height, wind sea and swell, and why the period matters as much as the height.
Wind sea and swell
- Wind sea is made by the wind blowing where you are, now or in the last few hours. It is short and steep and changes when the wind does.
- Swell was made by wind far away, perhaps days ago and thousands of miles off. It travels as long, regular waves and can arrive in calm weather. A swell from the wrong direction can make an anchorage or a harbour entry dangerous even with light local wind.
The numbers in a wave forecast
- Significant wave height (Hs) is the average height of the highest third of the waves. It matches what an observer estimates by eye. The largest wave in a series is typically close to twice Hs, and rarely a freak wave is higher.
- Period (Tp) is the time between crests of the dominant waves, in seconds. A longer period means a longer, more powerful wave.
- Direction is the direction the waves come from. Wave models often give wind sea and one or two swells separately.
| Height | Period | What it feels like |
|---|---|---|
| 1 m | 4 s | Short and choppy; uncomfortable on a small boat |
| 1 m | 10 s | Gentle, long swell; barely noticeable offshore |
| 2.5 m | 7 s | Steep and rough; hard going, spray over the deck |
| 2.5 m | 13 s | Long, powerful swell; comfortable at sea, dangerous breaking on bars and entrances |
A rough guide only: the effect depends on the boat, the course relative to the waves and the depth.
How a wave forecast is made
A wave model, such as GFS-Wave or ECMWF WAM, takes the wind forecast from an atmospheric model and calculates how much wave energy is created, how it travels in each direction and how it decays. Two consequences:
- A wind error becomes a wave error, and a larger one: a small mistake in the wind speed over a long stretch of sea grows into a clear mistake in wave height. Compare wind forecasts first.
- Swell is forecast well, because it depends on storms days earlier and far away, which are observed from satellites. Local wind sea is the part that depends on the local wind forecast.
Where wave forecasts go wrong
- Sheltered and narrow waters. A coarse wave grid cannot see fjords, bays, archipelagos or the lee of an island, so the forecast there is often too high or does not reflect the shelter. Finer coastal models help.
- Shallow water. Waves slow, steepen and bend as the depth falls, so near the coast and on banks the sea is higher and steeper than offshore.
- Currents and tide. Wind against current makes waves shorter, steeper and higher. Few wave forecasts include this fully.