Frequently asked questions
Understanding a theviz forecast
What is the diving visibility in Cape Town today and tomorrow?
theviz publishes a modelled visibility forecast for 18 Cape Town dive sites, updated three times a day. The ranked answer for today, tomorrow and the following day is on the Cape Town visibility page, and each site has its own page with the median, the 80% interval and the probability of diveable conditions.
Which Cape Town dive site has the best visibility?
It depends on the wind, and it changes daily. Cape Town has two effectively separate diving coasts: the Atlantic seaboard sites clean up on a southeaster, while False Bay and the Simon’s Town sites come good on a northwester. That is why a single Cape Town visibility number would be misleading, and why the forecast ranks every site separately each day.
What is theviz?
theviz is a probabilistic water-clarity and dive-condition forecast for named sites across the Cape Peninsula, False Bay and the Overberg coast.
Why can the water be murky after the swell has dropped?
Waves stir sediment from the seabed, and those particles stay suspended after the surface settles. theviz models the recent history of bottom stirring and the site-specific time it takes to settle, which is why the forecast has memory.
What does the 80% visibility interval mean?
It is the range between the 10th and 90th percentiles across the forecast ensemble. A broad interval means the weather models or the site-response assumptions disagree; it is a cue to be more cautious.
What does the probability above a visibility threshold mean?
It is the share of model outcomes that exceed the selected threshold. It is not a promise that visibility will reach that value.
How often is the forecast updated?
Three times a day. Each run reads six days of swell and wind history and projects three days forward at hourly resolution.
Does “Diveable” mean it is safe to dive?
No. It is a modelled condition assessment, not a safety instruction. Diveability is called on the weakest safety gate — entry and exit, bottom surge, current, wind or surface chop — and is independent of clarity, so a site can be clear and undiveable at the same time. Check local conditions, regulations, marine protected areas, your buddy, equipment and personal limits before diving.
How should I use a forecast?
Use it to compare sites and windows, then verify conditions locally. Logged observations help evaluate and improve the model but do not replace direct assessment.