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 (Priesendorfer), 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.

I describe in the About Us page how the origins of ScubaWeather began with a search to estimate underwater visibility. Briefly, I was working offshore as the Chief Scientist for an environmental sampling program. We were using a small remotely operated vehicle (ROV) to take pictures of a pipeline on the bottom and you usually couldn't see anything because of the suspended matter in the water. This got me to thinking that if I could forecast the water clarity/visibility a day or two ahead, then I could pick the ideal time to do the ROV ops.

What began as a simple quest, became an extended adventure down the rabbit hole. My initial search uncovered many posts from divers asking if visibility could be provided for a dive site, but no good answers. Diving deep (yup, pun intended) into the literature on the subject I found the pioneering work conducted by R.W. Priesendorfer in the 1950’s for the US Navy on how far an underwater swimmer could see. It depended primarily on how much the light was attenuated, measured by the quantity Kd490, and by the concentration of chlorophyll. These quantities could be estimated from satellite spectral sensors, which is why I use the products from the two MODIS satellites.

There may well be those who can find good reason to criticize certain aspects of how I developed the visibility algorithm. To them I quote from Priesendorfer's Hydrologic Optics Volume II.

Nature Seen Through Half-Shut Eyes

"A man who takes a magnifying glass into a picture gallery and examines the canvases at a distance of 3 inches may acquire much interesting information about the texture of paint, but he does not see the picture. It is better to stand away. If trivial details still intrude, it is better to half-shut the eyes. As a final step, it is well to shut the eyes completely and think about what has been seen."
J.L. Synge Science, 5 October 1962

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