The science of forecasting wave heights is a complex one and is becoming more complex as more is learned about their formation and characteristics.
In short, waves are caused by wind, however there are a number of factors which determine their height and, equally important, their frequency. Wind speed is clearly a factor and essentially it is the wind strength relative to the existing wave speed. Additionally the width and length of the fetch ie how big an area over which the wind blows in a single direction, are big factors coupled with the depth of the water.
All these factors affect the characteristics of a wave:
- Wave height which is measured from the crest to the trough.
- Wave length which is measured from crest to crest.
- Wave frequency which is the time interval between crests over a single point.
All this makes perfect sense when you think about it - you can’t get 50ft waves in Coniston Water but you can see them in the Atlantic.
Studying wave behaviour gave rise to the Beaufort Scale where the windspeed at sea could be pretty accurately estimated from wave behaviour.
The trouble is that waves carry an enormous amount of energy and they can travel great distances.
That is when they become a swell - they travel so far they bear no relation to the local wind conditions. You may have a wonderful calm day in Cornwall but the beaches are being pounded by big Atlantic rollers - the swell created some time ago by a storm many miles away. (A Tsunami however is not a swell)
You can have the swell travelling in one direction (generally smooth topped waves) and wind waves (peaked) travelling in another.
A cape can cause treacherously confused sea conditions as differing swells collide at different angles from the prevailing wind waves.
Only recently have scientists admitted to the existence of rogue waves, where waves combine to produce monster waves which have long been known about by mariners. For years, scientists relied on a recognised formula to compute the size of waves and there is a maximum wave size in this formula which many mariners argued was too small compared to the reality.
It’s very difficult for a simple mariner to argue against a boffin with a slide rule. I have personal knowledge of this when giving evidence as to the size of waves one stormy night many years ago when a man was lost overboard from a nearby vessel. My evidence (and those of others in the same area of the Baltic at the time) was described as an exaggeration by a university professor. I described it as a short sea (waves coming at a high frequency) interspersed with bigger waves at a much lower frequency (a swell). If they combined (perhaps once an hour) you got a monster wave that would break over the vessel easily washing someone outside on a deck overboard. Anyway, I digress…
Forecasting wave heights has improved as they are now measured by satellite and computer models can be quickly adjusted to reflect actual conditions however, I don’t think the forecasting of swells per se has improved to the same degree.
We have all seen waves get larger as they approach a shore and so measurement of wave height can get complicated.
At the end of October a Briton applied to Guinness World Records for the title for the largest wave surfed, a monster over 80’ high off Portugal. This will probably come down to how the height of the wave is measured. For surfing, one method involves standing on the beach looking out to sea and another method takes an opposite view looking towards the beach from the sea. The large difference between the two methods is the trough size which is larger on the advancing side as the wave approaches the shore.