Time for round 2. Last week, we published an Instagram story inviting you to ask our forecasters your most burning surf-related questions.
The result? A whole load of inquisitive surf hounds wrote in, a lot of repeated questions, meaning there's certainly demand for explaining some of the terminology – all flecked with joke responses and 'what's the forecast for X spot on X day' etc etc. Anyway, we read them. All of them. And sent them over to our resident wave wizard, Tony Butt, who took the time to answer a few. All names are the IG handles that submitted the q.
Omar Tsidon: What is the difference between a high pressure and a low pressure?
The terms ‘high pressure’ and ‘low pressure’ refer to the amount that the atmosphere is pressing down onto the surface of the Earth. Atmospheric pressure is measured in millibars (mb) or hectopascals (hPa) which are the same thing.
Due to differences in things like temperature, which affect the density of the air, the pressure varies from place to place. The convention in meteorology is that anything below 1013 mb is considered a low pressure and anything above that value is considered a high pressure. On the surface, the air will try to flow from areas of higher pressure to areas of lower pressure (‘down the pressure gradient’). However, due to the rotation of the Earth, it can’t flow in a straight line. The rotation of the Earth makes it turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. That is why the air around highs and lows circulates the way it does. More info HERE and HERE.
Cam Hayward: Why do the waves tend to close out at places such as Newquay Bay?
If you have a look at the map you will see that Newquay Bay is situated almost directly opposite the prevailing swell direction, in contrast to Fistral, which faces directly into the oncoming swell. Imagine a swell line approaching from the west. One end of that swell line hits the Cribber reef and Towan Headland, but the rest of the swell line doesn’t.
The end that hits those obstacles is slowed down by the shallow water, whereas the rest carries on at the same speed. As a result, the swell line is ‘stretched’ into a long, bowed-out line that is more or less the same height along its length. If there are no reefs or sandbars to make the waves peel, they will close out. This is known as bathymetric defocusing. It is the opposite effect from bathymetric focusing, where a swell line is compressed into a peak. More info HERE.
Quefotóncachón: What is the hardest spot to forecast?
There isn’t really one hardest spot to forecast, but I can give you some clues as to why some spots are harder to forecast than others. For example, places a long way from the storm centres tend to be easy to forecast. The swell is generated several days before it reaches the coast, and, once the swell is on its way, it is relatively straightforward to predict.
These areas also tend to have more stable wind regimes, so the local wind can be predicted more reliably. In contrast, places where the storms develop closer to the coastline, are much more difficult to forecast. The time taken for the swell to reach the coast is much shorter, plus any slight change in the trajectory or strength of the storm can seriously affect local wind conditions or swell direction.
Finally, the sensitivity of a particular spot to different combinations of swell size, direction and period can make that spot difficult to forecast. At some places, only a slight change in swell direction or period can greatly affect the wave height, which makes it especially tricky to forecast. More info HERE.
Matiascb and Jersey Simon: What are the swell and wind angles in degrees relative to?
This might seem like a very simple question, but the conventions for wind and swell direction can sometimes be quite confusing. To start with, when we talk about wind direction or swell direction, we mean the direction from which the wind or swell is coming. For example, a northwest swell is coming from the northwest, therefore it is travelling towards the southeast.
Same with a northwest wind. Now, when we start talking about degrees, things can also be confusing, because people sometimes use different reference points. In most cases – and in the case of the MSW forecasts – the ‘zero’ reference is north (twelve o'clock), and the angle increases as you go round clockwise. Therefore, an east swell is 90 degrees (three o'clock); a south swell is 180 degrees (six o'clock) and a west swell is 270 degrees (nine o'clock). Where I live in the north of Spain, for example, most swells come from the more or less the northwest, i.e. between about 300 and 325 degrees. Hope that makes things a bit clearer.
Questions fromSeptember 3 2020
Daniel Paterson: How does period affect different breaks, e.g. pointbreaks v. beachbreaks etc.
Tony Butt: Period is really important when it comes to breaking waves for two basic reasons. Firstly, longer-period waves travel faster and carry more energy for a given height than shorter-period ones. Secondly, longer-period waves are thicker from front to back, and the water movements reach down further into the water column.
As a result, a longer-period wave hitting a shallow reef or sandbar will unleash more energy and be pushed up higher than a shorter-period one. The slowing effect of the seabed also extends to deeper water with longer period waves, which means they are focused and defocused more than shorter-period ones. More info HERE.
Googaloogs: With beachbreaks, there seems to be a season when there are always good banks. Any idea why?
The behaviour of beachbreaks and sandbars – the coastal morphodynamics – is a vast subject. However, there are a few basic principles, one of which is the Summer-Winter Profile.
This is where a beach responds to the incoming wave energy by changing the size and position of the sandbars. In winter, when you get big swells and storms, the sand moves offshore to form a long, continuous bar, which protects the coast from erosion. In the summer, when you get small, short-period waves, the sandbars gradually move onshore and split up along the beach. Therefore, depending on the beach in question, the sandbars might be good at a certain time of year, and bad at another. More in my book The Surfer’s Guide to Waves, Coasts and Climates
Ipicado30: Does the swell period increase with long distances between the surfbreak and the storm?
The period doesn’t actually increase, but this is basically what happens: As a swell propagates away from the storm centre, all the different period waves that have been generated by the storm are all mixed up, very close to each other, like runners in a marathon at the beginning of the race.
Then, as the swell moves away from the storm centre, the longer-period waves gradually move out in front because they travel faster than the shorter-period ones. This is called radial dispersion. When the swell arrives at the coast, the longer-period ones arrive first, followed by all the others. More info HERE.
Steven Bekker: What happens when you have two swells at the same time from two different angles?
This can make an enormous difference to the surf, depending on the type of spot and the difference between the two swell directions. Generally, reefs and points behave better with a single, good-quality swell from one direction.
If you have a secondary swell from a different angle, it can cross over the principle swell, breaking up the swell lines and making the waves section or back off. Beachbreaks, on the other hand, sometimes prefer a combination of two swells, as long as the angle difference between the two isn’t very much. This can allow a beachbreak that would normally close out, to have peeling waves; and in some cases it can create epic A-frame peaks. Look out for a future article on this.
Eric Remneback: What is the difference between groundswell and windswell?
This is a very basic concept, but often misunderstood. In some wave models you might see some period threshold, like 7 secs, above which is considered swell and below which is considered windsea.
This is just arbitrary, and not meant to be taken as a rule. The proper definition is this: Windswell, or windsea (it isn’t really swell) is what you get when the wind is still blowing over a particular area and pumping energy into the surface of the ocean and generating the waves; like a car travelling along with its engine running, pumping energy into the wheels. Groundswell, or just swell, is what you get when the waves have moved away from the generating area on their own, with energy no longer being input to them by the wind; like a car freewheeling with its engine turned off.
Aaron Gimpel: Can the water temp at a beach and reef next to each other have a noticeable difference?
The water temperature at the coast can be affected by several factors including the depth of the water and the degree of stratification, or layering, which, in turn, is affected by a phenomenon called upwelling.
This is where the wind blows the surface water away from the coast and allows colder water to rise up from underneath, creating a constant vertical cycling and keeping the surface water cold. Sometimes, in a horseshoe-shaped bay or similar, the wind might be offshore on one side of the bay and onshore on the other. This can lead to upwelling on one side and stagnant, stratified water on the other, with a noticeable temperature difference between the two. More info HERE
Turnbull09: If Ireland wasn’t in the way, would Blackpool have surf?
When I lived in Cornwall in the 1980s, we used to say that Cornwall would get much more swell, particularly from the north, if Ireland wasn’t there. At the time I didn’t know anything about offshore bathymetry or coastal geology, but somebody did suggest that, instead of getting rid of Ireland, it would be much easier just to move there. In the end, some of my friends did indeed end up moving there.
Even if you could easily move Ireland out of the way, it wouldn’t necessarily mean that Blackpool would have world-class waves. Firstly, you might end up with a wide continental shelf which could interfere with the waves before they reached the coast, and, secondly, there is no reason to assume that the coastal geology around Blackpool would create better surf spots than the ones already on the west coast of Ireland. More info HERE, and in my continuing series of articles on coastal geology.
Anything we've missed? Sound off in the comments below.
Cover shot by Ellis Collins