Magic Seaweed explained for Sea Swimmers

Brilliant Blog by Freyja Hunt – how to read magic seaweed to aid sea swimming choices

This is a brilliant blog by Seabird Freyja. Everyone has a different favourite forecasting app that they use to see if it is safe to swim. The most commonly used app is Magic Seaweed that was designed for surfers see what swell was approaching but it can be used to look at sea swimming conditions too!


Magic Seaweed (MSW) surf report provides a seven day forecast of sea conditions. Here’s a guide to understanding the data so you can get a better idea of what to expect before heading down to the beach.




This is the first column in blue. In Brighton and Hove, this is essentially the height of the shore break (or the white bits that can knock you over). This will give you an idea of how difficult it will be to enter and exit the water. MSW is designed for surfers so the measurement used is that of the surfable wave rather than the total wave height. For us sea swimmers it might be worth adding a little extra on to this measurement.

It is worth noting that the value given is the average height. 1 wave in 23 is likely to be twice the average height and one in 1,175 is three times the average height. Therefore, it is worth taking this as a rough guideline and always be on the lookout for larger waves when getting in and out.

In terms of height of the shore break, my rule of thumb is anything above waist height is capable of knocking me over.



Swell – listed in the second column – is the height of the waves once you are past the shore break. A big swell can be a lot of fun as you bounce around above and below your swimming buddies.

The next column gives an indication of the wavelength, or the time between the crest of each wave in seconds. The longer the time, the gentler and more undulating the waves will feel. Conversely, shorter times between each crest means the waves will come more frequently and you may be more likely to get a mouthful of sea water.


The black arrow to the right is an indication of the direction the swell is travelling. If you are doing a point to point swim, this is worth bearing in mind – if the swell is travelling in the same direction as you, it will feel like it is pushing you along. If you are swimming into the swell you will again, be more likely to get lung-fulls of sea water.



Wind is the main factor influencing how rough the sea is going to be. The stronger the wind is and the longer it has blown for, the larger the swell is likely to be.

The right-hand number column denotes wind speed. The larger number being the steady wind speed, and the small number being the gust speed. The arrow shows the direction the wind is travelling in. In Brighton and Hove the prevailing wind is South Westerly.



It is worth considering that MSW doesn’t factor in local sea breezes. Sea breezes are caused as the land changes temperature faster than the sea. For example, in the morning the sun heats up the land quicker than the sea. This triggers the air on the land to rise up and and cooler air is drawn in from the sea to replace it. Sea breezes are generally onshore in the afternoon (as the land heats up and air rushes in from the sea) and offshore in the morning (where the land falls below sea temperature overnight and air moves from land to sea).  You might therefore expect the wind to be slightly stronger in the afternoon than denoted on MSW.


Tidal Information

Magic Seaweed also shows the times and heights of the high and low tides. In Brighton and Hove, low tides generally vary between 1 and 2.5 meters and high tides between 5 and 6.5 meters above chart datum. The difference between the two is the tidal range. The tidal range has an effect on currents – the larger the tidal range, generally the stronger the currents will be. The tidal range during spring tide in Brighton is around 6 metres.



In a nutshell

The first column is the height of the shore break and gives you an idea of how difficult it is to enter and exit.

The second column is the height of the swell and tells you how bouncy it will be once you are in and past the shore break.

The third section tells you wind speed and direction – or the best direction to swim in to avoid getting a mouth full of sea water.

The box below informs you of the times of high and low tides and the tidal range. From this, you can have a go at working out the direction and strength of the current.


See, didn’t we tell you, a brilliant blog. Thank you Freyja for allowing us to host it on our site. I use Wind Guru, Nautide and Imray too!