SCUBA diving in and around False Bay and the Cape Peninsular can range from absolutely stunning to downright dangerous depending on the conditions, which can change from one extreme to the other in a matter of hours. In order to plan when and where to dive it is important to be able to understand the confusing array of numbers, colours and arrows that make up the wind and swell forecasts displayed on most websites. This article will provide you with a very basic understanding of what it all means and how to use it to plan your next Cape Town diving excursion (or aquarium visit should conditions be bad).
Before going any further it must be said that nothing is guaranteed. Forecasts can be wrong and even if they aren’t, the ocean doesn’t always care and may throw you bad viz while conditions look perfect from above. Furthermore, anyone who claims to have “the gift” when it comes to reading the weather (including myself,) is not to be trusted.
This piece will focus specifically on using Windguru (possibly the most widely used site by Cape Town divers) although the basic concepts carry through to any conditions forecast.
What does it all mean?
As shown in figure 1, the forecast provides us with a whole host of variables. Of which, we as divers are interested in: wind speed, wind direction, wave, wave period and wave direction.
Most of the variables above are self-explanatory. Wind is always described by its origin and so a SE wind blows from the South East and a sea breeze from the sea towards land. “Wave” describes wave height in meters and wave period tells us how many seconds will pass between waves.
How does it affect the ocean?
Wind moves the surface water and creates waves. The higher the wind speed, the faster it will move surface water and the larger the waves it will create. Once a wave has been created, it can travel a vast distance and last a long time after the wind that created it has died down resulting in many different waves created in different parts of the world with different speeds, heights and periods hitting the shore simultaneously. Although Windguru does give you some information on different waves and swell it can all get very complicated, therefore most divers use the main wave forecast in figure 1. Big waves mean bad diving, but waves forecast for the False Bay side of the peninsular usually take a day to arrive, making it important to check the wave forecast for the day before any planned dives in this area. Although False Bay is in the Atlantic, we refer to them separately for ease of explanation.
Waves also create surge and whilst the wave height is the biggest factor, wave period can also tell you a lot about what the surge will be like. A longer wave period will mean better surface conditions, but stronger surge which will be felt much deeper than on a short wave period. Surge stirs up the substrate, reducing visibility, so don’t be deceived by rough surface conditions if the wave period is really short, you may get very decent bottom conditions at deeper sites.
Wind direction plays a huge role in diving conditions as it not only creates waves, but moves surface water. The surface water of the ocean carries a huge amount of floating debris and particulate matter, which reduce visibility, and it is exposed to the most sunlight, which can cause algae to grow in massive blooms. A sea breeze will blow this dirty surface water into shore where it will collect. The shape of the peninsular means that the prevailing summer wind (SE) hammers the False Bay side, whilst blowing surface water away from the Atlantic and Rooi Els coasts. This can result in excellent diving on the Atlantic side after a strong SE wind, which has a similar effect on the Rooi Els side.
Wind direction also affects the water temperature quite considerably. Surface waters are generally slightly warmer (thanks to the sun) and not far South of False Bay even warmer water, washed down from Mozambique by the Agulhas current, can be found. This warm water is very clean and can be blown into the bay by a few days of SE wind occasionally pushing the temperature in the bay over 20°C with good visibility. A shore breeze will have the opposite effect, blowing dirty surface water away and causing cold clean water from the deep to be brought up to take its place (a phenomenon known as an upwelling). An upwelling can often result in the most spectacular diving conditions, with visibility on the Atlantic side sometimes reaching 30m, although usually at the cost of it being a mere 7 or 8°C. The effect of wind direction can be so great that water temperatures may fluctuate by nearly 10°C in just a few hours with a change in wind.
Finally, what happens when the wind stops completely? Algae multiply and grow at staggering rates in these conditions and causes blooms which can turn the ocean green or red in a matter of hours. These “red tides” not only mean that you should stay away from local shellfish, but that visibility will often be reduced to just 1m. This only happens in the top layer of water and happens so quickly that the algae blocks out the light preventing any algae growth deeper than just a few meters from the surface. While shallow and inshore dive sites would be out of the question, pack a torch and head out to a deeper reef where there may not be much light, but you could be in for some very clean water.
Although there is no better way to know what you’re in for than asking the diver exiting the water, “What’s the viz?”, it is possible to get a pretty good idea of where and when it’s worth diving based on what some meteorologists (or more likely computers) say the weather will be doing over the coming days. I have gone into some detail on different individual factors, but it is important to remember that it is the combination of factors that will ultimately result in good or bad conditions. Don’t be disappointed when you get it wrong and if you’re unsure then contact your local dive centre and ask them (then don’t be disappointed when they get it wrong too). Happy diving.