wave is a pulse of energy through water
WHAT IS A WAVE
So far we've talked about tidal waves, but the waves we're most interested in as surfers are officially known as surface waves because they are formed by wind blowing over the surface of the sea. For simplicity, we'll just call them waves.
Out at sea windy storms occur, which cause friction between the air and water molecules. This friction creates a wave that travels away from its source (like dropping a pebble in a bowl of water). The winds out at sea are very strong and blow across huge areas of water, creating waves that can travel across entire oceans. As the waves travel out from the eye of the storm, they organise themselves into well defined groups. These groups are known as sets, and this organised structure gives the waves even more power - making them perfect for surfing.
The common misconception about these waves is that the water is flowing across the ocean. In reality, the water molecules flow in a spiralling orbiting motion within one wavelength, and this motion transports energy to the next wavelength and it's column of orbital motions. It is this energy that flows across the ocean - not the water itself. When the energy reaches the shallows, the lower part of the orbital columns are compressed; forcing them to slow down. This braking action makes the top of the wave pitch up, and when the sea depth is 1.3 times the height of the wave, it breaks - and this is where you want to be.
WHAT IS SWELL
If there is enough time between the storm creating the wave and the wave reaching land, the waves will organise themselves into sets. For a wave to join a set, it must travel at the same speed and have the same energy as the other waves in that set. The energy and speed of waves are directly linked to the period of the swell; the time in seconds between two consecutive waves. Generally speaking, the longer and stronger the wind blows over the sea, the longer period that swell will have.
The period of a swell can give us a lot of information. Short period swells (below 10 seconds) typically denote low energy and messy conditions. These swells cannot travel as far, as they have not had time to organise themselves into sets. Short period swells are often found in areas where the wind is still blowing strongly. This is often the situation in Aberdeen; and as you have probably seen windswell is held in pretty low regard by surfers because of these conditions. When swells travel further, the period grows longer and the wind has more time to transfer energy into the water - this means the waves will be taller when they reach shallow water.
Doubling the period will increase the height of swell by 50% - to explain this we'll use an example. If we compare two 1.2 metre swells one with a 10 second period, and one with a 10 second period. The 10 second swell will grow to almost 2 metres waves on the beach. In comparison the 20 second wave will grow to a good 3 metres on the beach. Long period swells, also known as groundswells, contain more energy, and so can travel further from the storm that created them - which increases the chance of nice offshore winds and better surfing conditions.
To calculate the speed of most waves, multiply the period by 1.5. Using this formula, longer period waves travel faster - this also explains why big wave surfers use jet skis to catch waves.
HOW DO SEABED & TIDE AFFECT WAVES
So, just like us, every wave is unique and breaks in a variety of different shapes and sizes. When a wave approaches shallow water, the shape of the seabed underneath has a huge impact on how the wave breaks. This is important for surfers because it can make the difference between a gentle roll and a hollow barrel. As a rule of thumb, the shallower the seabed the longer and slower the breaking form will be. On the other hand, if the seabed rises steeply, the sudden change in depth makes the wave break over a short distance with a steep face.
This seems like a fairly easy concept to get to grips with, but in reality the seabed is full of architectural irregularities. This is where tide comes in - the depth of water often has a profound affect on how waves break in each seabed landscape. Take a flat beach with steep banks near the shore (high tide line), long and slow waves will form at low tides. As the tides approach high water the water level will rise over the steep banks, and the waves will be fast and steep. And just to confuse matters even more, I'll add in a curve ball. All waves are accompanied by strong currents that shift the sand banks of a beach break. This means that the tide:wave combination changes year in and year out - unless the break is impermeable to the changes of the sea.
There is one type of break that is robust to the changing sea, this is a reef break. Reef breaks can be found close to shore and deep at sea. They often cause abrupt changes in the bathymetric landscape, resulting in fast, steep waves. These breaks are extremely tide dependent. If the tide is too high, the wave may simply flow past without touching bottom. If the tide is too low, the top of the reef may be completely out of the water - making it unsurfable. This means that many reefs can only be surfed at mid-tide.
As we have discovered in stream - the seas of the Pentland Firth are notorious for their power and their danger. The infamous strait is also home some world-class waves. This is because when powerful Atlantic swell storms in from the North West, a reef at Thurso East moulds the swell into picture perfect right hand* barrels.
Thurso East forms waves on all tides, but it is best surfed at midtide when the barrel provides Europe's longest ride* (*with the exception of tidal river bores) - our own North Shore.
*Just to refresh - a right hand break is a wave that breaks from left to right when you're surfing it.
HOW DOES WIND AFFECT WAVES
So what makes waves actually surfable? The main factor is wind - i.e. the movement of air from areas of high to low pressure. When hot air rises, it creates low pressure at ground level and and wind blows in from colder areas to replace the lost air. This is seen on huge global low pressure weather systems, or closer to home in the sea breeze.
As the sun warms up throughout the day, the land and sea heat at different rates. The sea has a greater ability to absorb the sun's warmth and it heats up and cools down at a greater rate than land. At the beginning of the day, the land is cooler than the sea, so wind blows away from the coast to replace the rising sea air. This is an offshore wind - which is ideal for clean surfing conditions. The wind blowing against the incoming waves helps prop up the face, allowing it to travel into shallower water and creates a steeper form i.e. the only reason to wake up early.
The early morning offshore doesn't last forever, because as the sun rises it begins to heat up the land. By midafternoon the land is much hotter than the sea and this creates a low pressure over land, and a high pressure over sea. The colder sea air moves towards the coast and its lower pressure. This is an onshore wind - which often brings messy surfing conditions. Onshore winds create choppy windswell along with the original groundswell, causing waves to break sooner without letting them peel.
As with everything, there are exceptions. If offshore winds exceed 40 kilometres per hour they deteriorate the groundswell waves. High offshore winds also mean you are paddling into the wave - which is very tiring (and usually just as cold). The one silver lining is that the lineup will probably be empty.