Why Is the Sea Blue?

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Have you ever wondered why the sea is blue? Have you noticed that the ocean appears a different color in different regions? Here you can learn more about the color of the ocean.

Depending on where you are, the sea may look very blue, green, or even gray or brown. Yet if you gather a bucket of sea water, it will look clear. So why does the ocean have color when you look into, or across it?

When we look at the ocean, we see the colors that are reflected back to our eyes. The colors that we see in the ocean are determined by what is in the water, and what colors it absorbs and reflects.

Sometimes, the Ocean Is Green

Water with lots of phytoplankton (tiny plants) in it will have low visibility and look greenish- or grayish-blue. That is because the phytoplankton contains chlorophyll. The chlorophyll absorbs blue and red light, but reflects yellow-green light. So this is why plankton-rich water will look green to us.

Sometimes, the Ocean Is Red

Ocean waters may even be red, or a reddish color during a "red tide." Not all red tides show up as red water, but the ones that do are because of the presence of dinoflagellate organisms that are reddish in color.

Usually, We Think of the Ocean As Blue

Visit a tropical ocean, like in southern Florida or the Caribbean, and the water is likely to be a beautiful turquoise color. This is because of the absence of phytoplankton and particles in the water. When sunlight goes through the water, water molecules absorb red light but reflect blue light, making the water appear a brilliant blue.

Closer to Shore, the Ocean May Be Brown

In areas closer to shore, the ocean may appear a muddy brown. This is due to sediments being stirred-up from the ocean bottom, or entering the ocean through streams and rivers.

In the deep sea, the ocean is dark. That is because there is a limit to the depth of ocean that light can enter. At about 656 feet (200 meters), there isn't much light, and the sea is totally dark at about 3,280 feet (2,000 meters).

The Ocean Also Reflects the Sky Color

To some extent, the ocean also reflects the color of the sky. That is why when you look across the ocean, it may look gray if it is cloudy, orange if it is during sunrise or sunset, or brilliant blue if it is a cloudless, sunny day.

Resources and Further Information

  • Helmenstine, A.M. Why Is the Ocean Blue?. ThoughtCo. Accessed March 25, 2013.
  • Mitchell, G. Voyager: Why Is the Ocean Blue?. Scripps Institution of Oceanography. Accessed March 25, 2013.
  • NOAA Ocean Facts. The Ocean Acts As a Sunlight Filter. Accessed March 25, 2013.
  • Rice, T. 2009. "Why Is the Sea Blue?" In Do Whales Get the Bends?. Sheridan House: New York.
  • Library of Congress. Why Is the Ocean Blue?. Accessed March 25, 2013.
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Your Citation
Kennedy, Jennifer. "Why Is the Sea Blue?" ThoughtCo, Aug. 26, 2020, thoughtco.com/why-is-the-sea-blue-2291878. Kennedy, Jennifer. (2020, August 26). Why Is the Sea Blue? Retrieved from https://www.thoughtco.com/why-is-the-sea-blue-2291878 Kennedy, Jennifer. "Why Is the Sea Blue?" ThoughtCo. https://www.thoughtco.com/why-is-the-sea-blue-2291878 (accessed March 22, 2023).