Every diver will have a thing or two to say about undersea visibility. They will disagree on what is a good visibility, why it changes and how fast, what will it look like tomorrow or even what it was on today's dive. One thing is sure though - visibility is a holy-grail of any enjoyable dive.
Yet there is surprisingly little information online about undersea visibility. Or is there?
In the UK - where undersea conditions can change overnight and good visibility is not a frequent guest - divers join forces online to help each other stay informed. The UK Viz Reports group on Facebook serves one simple purpose: Share on the ground visibility reports with fellow divers and get updates from them in return.
There are discrepancies, however, in how different divers measure undersea visibility. Scuba divers usually refer to horizontal visibility near the seafloor as it is where they typically spend most of their dive times. Freedivers and spearfishers in contrast, go down to the depth and back to the surface all the time. Hence they also weigh in vertical visibility. The two metrics - horizontal and vertical visibility - are often correlated yet not necessarily equal.
The vertical visibility, it turns out, has been well studied by marine scientists. Invented by an italian astronomer Angelo Secchi, secchi depth is a standard way for measuring vertical visibility in a column of sea water. It is typically measured from a boat by lowering a black-and-white disk (secchi disk) into the water. The minimal depth at which the visual pattern of the disk is no longer visible from the surface is called secchi depth.
Source: https://datastream.org/en/guide/secchi-depth
Secchi disk has been used by navies, sailors and marine scientists for over a century. Yet the need for manual measurements is a significant limitation of the method. One needs an army of enthusiastic sailors taking measurements every day to provide a good coverage of the visibility in time and space.
Luckily scientists have found a way to remove the need for human input. With high resolution optical satellites designed and launched by the European Space Agency (as well as other space agencies), secchi depth can now be measured anywhere in the world on daily basis.
Source: https://www.frontiersin.org/articles/10.3389/fmars.2021.695938/full
The animation below shows the variation in secchi depth around the British coast for several weeks of 2022 as measured from the ESA satellites. Clearly it varies a lot and many factors are at play - wind, swell, currents, plankton to name just a few. Yet there are times when a good visibility can come as a nice surprise to your favourite dive spot!
More recent visibility estimates across the top UK dive spots can be found in our app (currently in beta testing) https://marla-client.vercel.app.
The secchi depth estimates from satellites are by all means not perfect and the design of better algorithms is an active research. Yet they can often provide the only piece of information on undersea visibility when no other data is available. The quality of visibility measurements can be further improved by mixing different data sources - satellites, user reports, underwater sensors - as well as modern artificial intelligence algorithms. Our team is working on several exciting developments in this space which we will be keen to share for your feedback. Watch this space for more updates. Meanwhile, enjoy your dives!
What a great post!