The oceans cover over 70% of the Earth’s surface and contain about 96.5% of all the water on Earth. The vast bodies of water that make up the oceans exhibit a wide variety of colors, ranging from crystal clear turquoise to dark midnight blue. In some areas, the ocean waters appear a brilliant, almost neon blue. So why do some parts of the ocean look so blue while others appear more green or gray? The main factors that determine ocean color are the water molecules themselves, any particles or organisms suspended in the water, and the way the water reflects and absorbs sunlight.
The Water Molecules
Water molecules alone absorb almost all sunlight except blue light. The oxygen and hydrogen atoms that make up water molecules vibrate at frequencies that match red, green, and yellow wavelengths of sunlight. This vibration allows the water molecules to absorb these colors. However, water molecules do not absorb wavelengths in the blue portion of the light spectrum. So when white sunlight enters the ocean, mostly blue light gets transmitted back out. We see this reflected blue light as the characteristic blue color of the ocean where no other particles or organisms are present.
Suspended Particles
In many areas of the ocean, there are suspended particles and plankton that can affect the color. Sediment from rivers and material churned up from the seafloor creates cloudy conditions, blocking sunlight penetration. The reduced sunlight shifts the color toward greener shades of blue-gray. Bits of dead plants and algae can also tint the water. Large populations of phytoplankton floating near the surface turn the water a greenish-blue hue. In general, anything suspended in the water that absorbs or scatters blue wavelengths will make the waters appear less blue.
Absorption of Other Colors
In very clear open ocean waters, there is not much suspended material, allowing sunlight to penetrate deeper. The water molecules absorb longer red, orange, and yellow wavelengths first. This absorption continues progressively for the green wavelengths. What gets reflected back out is the shorter blue light. These clear, deep waters will appear especially blue due to the cumulative absorption of the other colors.
Light Reflection
The sea surface also plays a role in how blue the water looks. Rougher surfaces from wind and waves reflect sunlight in all directions, creating a deep blue appearance. Smoother sea surfaces act more like mirrors that reflect the sky. So on calmer days, the blue color is not as vibrant. Multiple reflections between the surface and particles in the water can also accentuate blue light.
Rayleigh Scattering
Another factor that contributes to the deep ocean blue is a process called Rayleigh scattering. This is the same phenomenon that makes the sky look blue. Light travels as electromagnetic waves, with each color having a different wavelength. Blue wavelengths are shorter than other visible colors. As sunlight enters the water, these short blue light waves bounce off water molecules in every direction. The bouncing effect results in more blue light being reflected back out. Thus, Rayleigh scattering enhances the blue color of clear ocean waters.
Geographic Differences
| Location | Color | Reason |
|---|---|---|
| Tropical waters | Clear turquoise blue | Few particles, more light penetration |
| Coastal regions | Greenish-gray | Sediment from rivers reduces clarity |
| Polar seas | Deep blue | Very few particles, more blue light reflection |
The oceans display an amazing diversity of blues, from pale turquoise to indigo. Tropical waters tend to be the clearest, exhibiting vivid turquoise hues. Coastal areas appear greener due to sediments from rivers. The Southern Ocean surrounding Antarctica is a deep blue from having minimal suspended particles. The table above summarizes the major geographic differences.
Unique Blue Water Phenomena
There are some special ocean waters that are unusually blue due to unique circumstances:
- The Blue Lagoon in Malta has a bright turquoise color from white limestone reflecting sunlight.
- Moraine Lake in Canada is a deep blue from light refracting off the rock flour in the water.
- Blue holes contain very deep clear water that looks intensely blue when viewed from above.
- Some glacial lakes also appear a distinctive blue hue from rock flour ground by glaciers.
These waters illustrate how the right combination of factors can produce exceptionally vibrant shades of blue.
Conclusion
The blueness of the ocean results from a complex interplay of light absorption, reflection, scattering, and particles in the water. Water molecules themselves absorb most colors except blue, transmitting back out the blue wavelengths. Additional absorption and scattering by suspended material shifts the hue toward green or gray. In very clear, particle-free water, cumulative absorption of longer wavelengths makes the blue color more pronounced. Rayleigh scattering of blue light also adds to the effect. The result is mesmerizing waters spanning the spectrum from pale aquamarine to deepest ultramarine.
