Not so long ago a surfer had to be able to decipher complicated weather charts in order to know when and where to surf. Nowadays much of that raw data is processed by reasonably accurate online surf forecasts, which save him or her the trouble. Still, the more you know about how waves are formed — from their conception in the open ocean to their birth (or death) upon the shore — the better you will be at predicting the surf. Plus, using terms like "bathymetry" and "closely packed isobars" at parties is known to drive members of the opposite sex wild.

How Waves Are Formed: Where do waves come from?

Waves are created by winds blowing far out to sea in a shorewards direction, and transferring their energy from the air to the water. There are three important variables here which determine the character of the resultant swell: the wind’s strength; its duration, or the amount of time for which it blows; and its “fetch", or the distance of open water over which it blows in the same direction. The greater these variables, the larger the swell and the longer its “period" — this is the amount of time in between successive waves as they pass a given point.

And where does the wind come from? Wind is basically just air moving from an area of high pressure to one of low pressure, and by studying pressure charts, which consist of contour lines called isobars, an experienced surf forecaster will be able to form a decent idea of the short-term surf conditions at a given spot. A low pressure system out to sea, represented by closely packed isobars gathered together in a roughly circular formation, translates as strong winds blowing over the water’s surface and thereby creating waves. In the northern hemisphere, wind blows anticlockwise around a low pressure and vice versa.

A surface pressure chart provided by the Met Office. The dense grouping of isobars in the Atlantic - 928mb represents an extremely low pressure - suggest a big swell is on its way to Cornwall and Europe's west coast.

How are waves formed? A surface pressure chart forecasting good waves for Cornwall

How Waves Are Formed: Wave period, wind swells and groundswells

Wave period is an indication of the amount of energy transferred by the wind to the ocean. Thus longer period swells result in waves that are both bigger and more powerful when they make landfall; they are also better able to refract around headlands. A surf forecast predicting 3 feet of swell with a 15 second period, say, implies far bigger waves than a forecast predicting 3 feet of swell with just a 7 second period.

Swells with a period of around 10 seconds or fewer are considered “wind swells"; they originate relatively close to shore and are usually still somewhat disorderly when they reach land, where they’re often accompanied by the same onshore winds that created them. Those with a period of around 13 seconds or more are considered “groundswells"; these form a lot farther out to sea and tend to be of a better quality, but that’s not to say that good waves for surfing can’t be had from an 8 or 9 second swell, or that groundswells automatically result in better surf. Many beach breaks, for instance, prefer swells in the 9-13 second range, and transform into giant “close-outs" when faced with longer period groundswells.

As swell travels towards land, it organises itself not only into clearly defined, individual lines of swell but also into groups. Watch the waves at any surf break and you’ll notice that periods of relative flatness alternate with “sets" of larger waves, each set usually containing between two and four waves but often more.

How Waves Are Formed: Ideal surfing conditions

Offshore winds, ie. wind blowing out to sea, will groom approaching swell into “clean" and neatly ordered lines. This is the optimum wind direction, but very light, variable winds also result in clean conditions. Cross-shore or onshore winds, by contrast, result in “messy" waves, and if stronger than, say, 20mph may render conditions unsurfable.

Offshore winds at Godrevy in Cornwall, perfect for surfing. Photo: iStock

Godrevy Surf

Photo: WSL / Rowland

How are waves formed? Mick Fanning at Lower Trestles with offshore winds

A perfectly glassy right-hand wave. Its smooth, glassy texture and the absence of spray blowing off the back of the wave suggests there's little to no wind. Photo: iStock.

Tides also affect conditions, but generally only to the extent that the state of the tide will determine the shape of the seabed over which the waves break (see below). Often but not always, low tide means the waves will be steeper and will break in shallower water.

Both the tides themselves and the way they affect the waves will vary greatly depending on the coastline and the particular beach. There is usually about 6 hours 12 minutes between a low tide and high tide, though highly irregular tide patterns obtain on certain coastlines.

How Waves Are Formed: Why do waves break?

Photo: iStock.

Girl swims under the wave while surfer is sihlouetted above

As a wave nears the shore and thus enters shallower water it begins to grow, rearing up until the water’s depth is equivalent to the height of the wave. At roughly this point, it breaks: the lower part of the wave slows down due to friction with the seabed, but the upper part of the wave continues apace, causing it to spill over. The shape of the seabed — the bathymetry — will determine exactly how it spills over. The steeper the the seabed, the steeper the wave, and the more violently it will rear up and break: heavy barrelling waves are thus formed when deep water suddenly gives way to very shallow water; gentler waves ideal for beginners will break when this transition is much more gradual, over seabeds characterised by a shallow gradient.

 How Waves Are Formed: Wave characteristics

Lower Trestles, one of the world's foremost A-Frame reef breaks, groomed by a light offshore wind. Photo: WSL / Rowland

How Are Waves Formed? Lower Trestles is a famous A-Frame reef break wave

Initially beginner surfers should stick to the white water; good waves for intermediate or advanced surfers, however, will peel laterally, allowing them to traverse the “wave face", or unbroken part of the wave. A wave is either a left-hander or right-hander depending on the direction in which it peels as seen from the surfer’s perspective, ie. from out to sea. Typically surfers prefer going “forehand", their chest facing the wave, rather than backhand, but you should practise going both ways. Some waves peel in both directions starting from the same point, and these are known as “A-frames"; waves that don’t peel at all, breaking all at once, or peel too quickly to be surfed, are called “close-outs", and good surfers will stay clear of these.

How Waves Are Formed: Basic types of surf break

Jeffreys Bay in South Africa: the right-hand point break par excellence. Photo: WSL / Totee

How Are Waves Formed? Jeffrey's Bay, the famous right hand point break in South Africa

There are four main types of surf break: beach breaks, reef breaks, point breaks, and rivermouths. A beach break is simply a spot where waves break over sand; at reef breaks waves break over a rock bottom; point breaks occur when lines of swell hit a stretch of land at an angle, breaking along the shoreline rather than directly towards it; and a rivermouth wave will break along a sandbar deposited by the river at the shoreline. Most popular waves in the UK are beach breaks, and broadly speaking they are by far the best type of surf break for beginner surfers.

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