Whether heading out for a day of fishing or taking departure for a bluewater passage, most mariners both professional and recreational, will keep close tabs on the current and forecasted weather both before leaving the dock and while actually out on the water.
For most of us in North America, the “National Weather Service” (NWS) and “NOAA Weather Radio” are the go to resources for marine weather forecasts. The problem is in interpreting what the forecasts are telling us. Here, we are going to concentrate on wave heights.
When either reading a printed copy or listening to marine weather forecasts, very often you will hear either “waves,” “swells,” or “seas” referenced in the forecast. Understanding the differences between these terminologies is important.
Always remember, unless you are a skilled observer, most people tend to over-estimate the real heights of waves. Once that happens, they become very much like fish tales; they tend to get larger every time the story is told.
With the exception of tsunami like events; waves are created by winds. As the wind blows across a calm (and relatively smooth) water surface, the friction between the air and the water's surface tends to elongate the surface, creating waves. As these small waves form, the surface becomes less and less smooth thus increasing that friction on the water's surface. The result; larger and larger waves.
There is a very high degree of correlation between winds and wave heights. Since this correlation exists, the following can be deduced; when all other factors remain the same:
1. The higher the wind speed, the higher the waves.
2. The longer the wind blows, the higher the waves.
3. The greater the distance over which the wind blows, the higher the waves.
"Waves" are generated from the action of the wind from locally driven weather events.
"Swells" are also waves generated by winds that are formed from distant weather events.
"Seas" are used to describe the combination of wave heights and swell heights when superimposed on one another.
Specifically, “Seas” are defined as the square root of the square of the Swell Height (S) plus the square of the Wave height (W) or:
While there are a number of defining characteristics to waves, swells, and seas, the (2) primary ones of concern to the mariner are "Wave Heights" and "Wave Periods."
Whether we are discussing "Waves, Swells, or Seas," wave heights are all measured the same: From the crest to the trough (typically in feet or meters).
When considering "Waves or Swells" independently, the wave period is measured from the crest of a wave or swell to the crest of the following wave or swell (in seconds.)
When it comes to "Seas" however, things become a bit more complicated. Simply put, waves and swells can have different periods. So when superimposed on one another such as in a coastal forecast, the term "Dominant Period" is often used. The Dominant Period (in seconds) is the wave period associated with highest energetic waves. It will always be equal to either the swell period or the wind-wave period.
To determine which one they may be referring to, a good rule of thumb is:
The bottom line is – The longer the period the smoother the ride.
So just what is NOAA Weather Radio telling us when they give us wave heights during the marine forecast? The first thing when listening is to determine if they are talking about waves, swells, or seas. Often times they may all be used in the same area forecast and as noted above, there is a difference.
In all NWS marine weather forecasts, the heights of "Seas, Swells, or Waves" will always be the "Significant Height," whether stipulated or not in the forecast.
Significant Wave Height is used because larger waves are more "significant" (Read Important) than smaller waves.
So what does that mean exactly?
"Significant Wave Height is the average of the highest one-third (33%) of waves (measured from trough to crest) that occur over a given time period within the forecast area."
Because the "Significant Wave Height" is an average of the largest waves; be aware that any individual wave can be much smaller and some will be larger and occasionally some may be much larger.
If the forecast calls for seas or waves of 5 to 7 feet, this implies that the average of the highest one-third of will have a significant wave height of 5 to 7 feet, but beware. . .
It does not mean that all waves encountered will be within the forecasted significant wave height; some will be less and some will be higher, occasionally much higher!
The NWS presumes that individual wave heights can be statistically derived. The model used has been proven reliable in showing that with a "Significant Wave Height" of 7 feet - roughly one of every ten waves will likely be greater than 8 feet; one in every one hundred waves likely will be greater than 11 feet; and one in every 1000 waves will be greater than 13.0 feet.
Below is a table showing the probability of encountering waves greater than the Significant Wave Height forecasted.
Significant Wave Height Vs. Expected Wave Height
Significant Wave Height Vs. Expected Wave Height
|Significant Wave Height (in feet)||1 in every 10 waves||1 in every 100 waves||1 in every 1000 waves|
As a general rule, the largest individual wave one may encounter will be slightly less than twice the Significant Wave Height.
Typically the term "Seas" are used on coastal and offshore forecasts while the term "Waves" are used on inshore waters such as bays, harbors, and lakes.
TODAY...S winds 20 to 25 kt. Seas 5 to 6 ft. Dominant period 4 seconds.
TONIGHT...SW winds 15 to 20 kt, becoming W 10 to 15 kt in the late evening and overnight. Seas 5 to 6 ft, subsiding to 3 to 4 ft late. Dominant period 6 seconds.
"Wave Heights" and "Swell Heights" are given separately.
TUE NIGHT...SE wind 15 to 25 kt easing to 5 to 15 kt after midnight. Wind waves 2 to 4 ft subsiding to 1 to 2 ft after midnight. W swell 9 ft at 16 seconds building to 11 ft at 17 seconds after midnight.
The term "Seas" is used on coastal, offshore, and inland waters.
WED NIGHT … Coastal Waters - S gale to 45 kt. Seas 23 ft. Rain.
WED NIGHT … Inland Waters - S wind 30 kt. Gusts to 45 kt. Seas 6 ft, except 16 ft near ocean entrances.
Typically the term "Waves" is used.
THU … Southeast winds 15 to 25 knots veering south in the afternoon. Waves building to 4 to 8 feet occasionally to 10 feet.