3.2 Effects of weather on the sea

3.2.1 Wind against tide

When wind, or waves are going the opposite direction to a tidal stream, they become shorter, taller and often break. This can make for challenging (scary / fun) conditions off headlands and in tide races.

Photograph of the tide race at Penrhyn Mawr. Contrast the smooth eddy in the foreground with the rough water in the tidal stream (going left to right).

3.2.2 Offshore winds

If the wind is strong, it often makes sense to seek coastlines that are sheltered from the wind. However, the wind on these coastlines will blow offshore, creating a potential hazard. What seems like a light wind close to land and cliffs may be a strong wind further out. If the group gets blown away from land, getting back to shore may be difficult.

3.2.3 Cross seas / Confused seas

If two sets of waves are coming from different directions across the same stretch of water, the resulting interference pattern can create a confused sea with seemingly random large ‘haystack’ shaped waves. This can occur if the wind direction changes through a large angle, superimposing a wind driven chop on an existing swell. The image below shows a confused sea created by changing wind direction in a cyclonic weather system.

Waves formed by a cyclone in the Indian Ocean. Image taken by POA(PHOT)Owen King RN. Public sector information licensed under the Open Government Licence v3.0

3.2.4 Reflected waves, clapotis

Waves hitting vertical cliffs will reflect back and interfere with the incident waves. This creates a pattern of larger and sharper waves that can be challenging to paddle in. When the waves don’t hit the cliff square on, the interference pattern will be more complex, creating a confused sea.

Extreme example of clapotis caused by waves reflecting off a sea wall and interfering with incident waves. Photo: YuriyVZ/iStock

3.2.5 Wave diffraction and focusing

Waves tend to diffract (bend) around obstructions and as they enter shallow water. One result of this is that a headland will provide shelter from waves coming from the far side, but waves will bend around the headland to hit the shoreline behind it to some degree.

As waves enter bays, diffraction causes them to spread. As a result, waves within bays are typically smaller than those on straight coastlines or the open sea. By contrast, waves tend to focus onto headlands, creating larger waves at exposed points.

Diffraction of waves aroud bays and headlands

Diffraction of waves around both sides of an island can form a confused cross-sea behind the island.

3.2.6 Boomers

Waves break when they enter shallow water. Normally, it’s obvious where this will happen - on a beach or against cliffs. Sometimes, waves will break on an underwater reef. This is especially problematic if the depth of the reef is such that only the largest waves will break over it. A paddler might paddle over the reef, not having seen any problems, only to have the biggest wave of the day break over them.