Why Is the Ocean Blue?

Water View from under the Ocean blue

When observed from space, the Earth presents a striking azure appearance. From the planet’s outer perspective, as we gaze downward, it reveals its continents and oceans. The predominant vastness consists of oceans, which, along with seas, lakes, and rivers, collectively cover at least 75% of the entire planetary surface.

While Asia, claiming the title of the largest continent, extends across approximately 45 million square kilometers (about 17.4 million square miles), the Pacific Ocean alone covers more than 165,250,000 square kilometers (63,800,000 square miles). This measurement is equivalent to the entire landmass of Earth.

Blue Oceans and Light

The mesmerizing blue color of the ocean is not solely due to sunlight being refracted, as one might assume. Instead, it arises from the behavior of sunlight as it encounters water molecules. One of the key factors contributing to this phenomenon is the selective absorption and scattering of sunlight by water. When sunlight reaches the ocean, water molecules absorb colors from the red part of the spectrum, while blue and green colors are scattered in various directions. This scattering of shorter wavelengths, particularly blue and green, leads to the characteristic blue appearance of the ocean.

It is the red and yellow wavelengths that disappear first, between 33 and 98 feet of depth (10 to 30 meters). Green also vanishes at a distance of about 197 feet (60 meters), leaving only blue at a distance of 295 feet (90 meters), which absorbs it entirely. Therefore, the color that is most refracted by the sea is blue, which explains its color.

When there is a lot of photosynthetic phytoplankton in the water, chlorophyll can sometimes absorb blue wavelengths more quickly, giving the sea a greenish hue because it is the only visible wavelength that is still present.

However, sunlight’s interaction with the atmosphere also plays a pivotal role in bestowing the ocean with its azure hue. As sunlight enters our atmosphere, another physical process comes into play: the scattering of white sunlight. The atmosphere acts as a scattering medium, diffusing sunlight in all directions. The shorter wavelengths of light, such as blue and green, scatter more than longer wavelengths, contributing to the blue color of both the sky and the ocean.

The reflection of scattered light on the ocean’s surface further enriches the ocean’s color palette. As sunlight interacts with the water’s surface, it reflects the scattered blue and green light from the atmosphere. This reflection accentuates the ocean’s blue appearance and adds to the captivating visual spectacle that we observe from the shorelines.

The combination of these physical processes—selective absorption and scattering by water molecules, atmospheric scattering of sunlight, and the reflection of scattered light by the ocean’s surface—collectively contributes to the ocean’s stunning blue color. This coloration is not merely an aesthetic feature; it is a manifestation of the complex ways in which sunlight interacts with Earth’s elements.