The Science of ScubaWeather - Visibility

The key feature that sets ScubaWeather apart from other weather forecasts for scuba diving is the scientific calculation of the underwater visibility. I thought that knowing how far you could see before you got in the water could be very useful. So I developed an algorithm that combines daily satellite spectral data of the water clarity downloaded from the NASA MODIS Aqua and MODIS Terra satelites (Kd at 490 nm and [chla]), the physics of underwater optics (developed by R.W. Priesendorfer in the 1950’s for the US Navy on how far an underwater swimmer could see), and the biology of how the human eye adapts to different light levels to estimate how far you should be able to clearly see underwater. More specifically, how your eyes can detect contrast, think shark, from the background. We take it for granted, but being able to visually identify an object from its surroundings is complicated, wrapping around physics, biology and technology.

How far you can see underwater depends primarily on how much light is attenuated by the water and what is in suspended in it, readily measured by the quantity Kd490, and by the concentration of chlorophyll. These quantities can be estimated from satellite spectral sensors, which is why I use the products from the two MODIS satellites.

Validating the visibility algorithm is challenging for two reasons. First there is no standard universally accepted definition of how to objectively define visibility. And secondly there is very little data. Rainbow Reef Dive Center of Key Largo, Florida is an exception; they have consistently recorded the visibility for its dives since 2013. Using the subjective estimates of visibility made by the divemasters one can compare their observations against the algorithm. Except there is another twist. Each satellite is slighlty different and the estimates of Kd490 and [chla] are different, which leads to different visibility estimates. The following figure shows the visibility at the Spiegel Grove during July 2025.

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