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Sea Science: Why is the ocean blue, green and everything in between?

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Sea Science: by Jordanna Sheermohamed

A lot of what we see in life has to do with what happens on the microphysical scale. Sunlight is scattered as it passes through solids, liquids and gasses. We observe a blue sky as the result of sunlight scattered through the atmosphere; in the same vein, sunlight also is scattered through water molecules.

Water molecules absorb the longer wavelengths of the visible light spectrum – more specifically the reds, oranges, yellows, and greens. The remaining shorter wavelengths, the blues and purples, are reflected back and therefore  seen by the observer. A small amount of pure water is observed as clear, since there is minimal depth and particles to scatter or absorb the light.

Sunlight’s influence on ocean color can also be seen when the sun moves behind a cloud and the ocean colors instantly become slightly darker. This is because the cloud has reflected a lot of the incoming light upwards, with minimal light being transmitted downwards.

This helps to explain one of the most obvious factors of ocean color: depth. The deeper the water, the more amount of light is absorbed, hence, the darker the observed color.

Aerial images of waters surrounding islands or shallow waters will show lighter and whiter blues, much of which is a result of the minimal depth. Deep ocean drop-offs are often marked by the changeover from lighter blues to a dark navy color.

Alongside the microphysical factors, the amount and composition of organic matter within the water itself plays a vital role in the observed water color. Phytoplankton – microscopic plants that inhabit oceans, seas, and freshwaters around the globe – contain chlorophyll, which is a key player in the photosynthesis process, which is when a plant absorbs energy from light to produce the chemical energy needed to sustain its life.

Chlorophyll absorbs much of the red and blues of the light spectrum, leaving the green colors we commonly observe in plant life. While the human eye isn’t strong enough to see most phytoplankton, large aggregations of the plant life can be more easily observed as a result of the accumulated amount of chlorophyll in a given area. This will often give water a more green to blue-green appearance.

For example, the blue and turquoise waters often found in the Mediterranean are not only a result of the light-reflecting white sands and rocks, but also the minimal amount of algae that would normally produce a more green tint.

While not necessarily the most beautiful to look at, murky or brown waters are merely a function of elevated amounts of sediment within the water. This color isn’t a commentary on its cleanliness, but an indication of particle concentration. These particles scatter the remaining blue and purple wavelengths of visible light, resulting in its mostly unappealing appearance.  

The next time you see the calming blues of the Caribbean waters, the intimidating black-blues of the Pacific, or the turquoise of the Mediterranean, remember – the details are in the science, not the eye. It’s just another case  of how looks can truly be deceiving.

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm (WeatherForecastSolutions.com). Comments are welcome below.

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About Jordanna Sheermohamed

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a private weather-forecasting company (www.WeatherForecastSolutions.com). Comments are welcome at editor@the-triton.com.

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