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

by Jordanna Sheermohamed of Weather Forecast Solutions

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. Just as we observe a blue sky as a result of sunlight scattered through the atmosphere, water molecules also scatter the sunlight. Water molecules absorb of the longer wavelengths of the visible light spectrum, more specifically the reds, oranges, yellows, and greens. The remaining shorter wavelengths, 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 additionally be seen when the sun moves behind a cloud, and the ocean colors instantly shift to become slightly darker; 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 appear the 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 of and composition of organic matter within the water itself also 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; when plants absorb 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 is 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, giving 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. Just another taste of how looks can truly be deceiving.