Why Is the Ocean Blue If Water Is Clear?

A glass of water looks transparent, yet an open ocean can appear intensely blue. The difference is not simply that the sea reflects the sky.

Sunlight contains all the visible colors. When it enters water, different wavelengths do not travel equally well. Water absorbs longer red, orange, and yellow wavelengths more readily than shorter blue wavelengths. Across the short distance inside a drinking glass, the effect is too weak for most people to notice. Across many meters of ocean, it becomes powerful.

As sunlight travels through the water, much of its warmer colored light disappears into the water molecules. More blue light remains available to be scattered and returned toward the surface, where it reaches the observer.

The sky can influence the appearance of calm water through reflection, but it is not the fundamental source of the ocean's blue color. Deep water would retain a blue appearance even beneath a sky that was not blue. Pure water itself has a faint blue color that becomes visible when there is enough of it.

Real seas are rarely pure. Microscopic algae can make water appear green because their chlorophyll changes which wavelengths are absorbed and reflected. Sediment, dissolved material, depth, weather, and the angle of the Sun can produce turquoise, brown, gray, or even reddish water.

Most sunlight eventually disappears with depth. Below roughly one thousand meters, the ocean receives essentially no sunlight at all.

The ocean is blue because a vast body of apparently clear water becomes a selective filter, quietly removing colors from sunlight as the light travels through it.