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Why is the ocean greenish blue?

Why is the ocean greenish blue?

The ocean often appears greenish blue in color. This is due to the way sunlight interacts with the water molecules.

Quick Answer

The ocean is greenish blue because water molecules absorb the red and violet wavelengths of visible light. The remaining blue and green wavelengths are reflected back, giving the appearance of a greenish-blue color.

Absorption of Light in Ocean Water

When sunlight enters the ocean, the different wavelengths (colors) that make up white light interact with the water molecules in different ways:

  • Red light is quickly absorbed in the first few meters.
  • Violet and ultraviolet light are absorbed in the first 10-20 meters.
  • Blue light penetrates deeper, up to 100 meters.
  • Green light penetrates the deepest, up to 200 meters.

The red and violet wavelengths are absorbed rapidly, leaving mostly blue and green light to be reflected back to our eyes. This gives the ocean its distinctive greenish-blue color.

Absorption Depends on Wavelength

The amount of absorption for each wavelength depends on the energy of the light. Higher energy violet and ultraviolet light is absorbed first. Lower energy red light is absorbed next. Blue and green light have the lowest energy and penetrate furthest before being absorbed.

Wavelength Color Penetration Depth
400-450 nm Violet 10-20 meters
450-495 nm Blue 100 meters
495-570 nm Green 200 meters
570-750 nm Red Few meters

This selective absorption process leaves mostly blue and green light to be reflected back out of the water, creating the ocean’s greenish-blue appearance.

Other Factors That Influence Ocean Color

While selective absorption is the primary reason for the ocean’s color, several other factors can affect the exact hue:

1. Water Composition

The chemical composition of the water itself can alter absorption. Dissolved salts and organic compounds absorb different wavelengths of light. For example, more dissolved organic matter will selectively absorb more blue light, making the water appear greener.

2. Suspended Particles

Small particles suspended in the water can scatter and reflect light. These include both organic particles like plankton and inorganic particles like sediment. More scattering will make the water appear lighter and greener.

3. Viewing Angle

The angle at which sunlight enters and reflects out of the water affects color. At high sun angles around noon, more blue light is reflected, making the water bluer. In early morning or late afternoon, more red light is reflected, making the water greener.

4. Weather and Sea Conditions

Rough seas and cloudy weather can diffuse and scatter light through the water column. This adds more green hue to the reflected color. Calm, sunny conditions allow for deeper blue light penetration and reflection.

Geographic Variations in Ocean Color

While the ocean generally appears greenish blue, the exact hue varies in different parts of the world due to factors like:

  • Water temperature – Colder polar waters appear deeper blue.
  • Salinity – Higher salinity absorbs more red and green light.
  • Turbidity – Higher levels of sediment make the water greener.
  • Nutrients – More nutrients increase plankton growth and green color.
  • Habitat – Coral reef waters tend to be bluer.

Some examples of geographic color differences:

Location Typical Color
Caribbean Blue-green
Arctic Ocean Deep blue
Baltic Sea Greenish
Great Barrier Reef Blue

Measuring and Classifying Ocean Color

Oceanographers use remote sensing tools like satellites, planes, and underwater sensors to measure and analyze ocean color:

  • Spectrophotometers – Measure the intensity of different wavelengths of light reflecting out of the water, providing a spectrographic signature of the color.
  • Forel-Ule scale – Compares the observed water color to a standard color chart with 21 shades of blue to green.
  • CUBE protocol – Samples water in clear containers and classifies the color into categories like blue, green, olive, etc.
  • Ocean Color Algorithms – Mathematical models that translate raw satellite spectral data into measurements like chlorophyll-a concentrations, depth, and diffuse attenuation which indicate color.

Continuously monitoring ocean color allows researchers to detect changes in phytoplankton, sediments, and dissolved organic matter over time. This informs studies of climate change, pollution, and other impacts on ocean ecosystems.


The greenish blue color of the ocean is primarily caused by the selective absorption of longer red and violet wavelengths of sunlight by the water. The remaining blue and green wavelengths reflect back to our eyes, creating the familiar ocean shades of blue to green. Variations in water composition, particles, viewing angle, and geographic location all contribute to the exact greenish-blue hue seen at different places and times around the world. Advanced tools allow oceanographers to precisely measure and monitor even minute shifts in ocean color.