The sky is blue. Even kindergartners color the sky in blue crayon. And many amateur astronomers know why the sky is blue, and if not, they know exactly where to look on the web for an explanation. Short, blue wavelengths of light scatter strongly off the molecules of nitrogen and oxygen that make up our atmosphere. Like all visible wavelengths, blue light is transmitted, but some of it is bounced sideways multiple times. Upon reaching our eyes, this light turns the black of space into the beautiful shade of clear sky blue. Dust, aerosols and water droplets alter the sky’s appearance, and sunsets are red precisely because blue’s been scattered or absorbed leaving only the warmer colors. It’s not gone though: look up and away from the setting sun: that sky is colored by what remains of the blue rays.

Another simple question is “Why is the ocean blue?” In my job, I need to have a ready answer, one that is correct and easy to understand. But an online search will return a surprising variety of answers. Does the ocean reflect the blue sky? The water is less colorful on cloudy days, but then why isn’t a bucket of water blue on any clear day? Why is the water dark around sunrise and sunset, instead of matching the reds and oranges along the horizon?

You have to look deeper for an answer. Water, even the clearest ocean water is, in fact, faintly blue. A glass of water appears colorless, but a tall enough column of water is perceptibly blue because the reds and oranges are absorbed by the chemical bonds of the Mickey-mouse shaped molecules of water. Blue and green light penetrates most deeply, explaining why free-diving snorkelers and SCUBA divers see a change to blues and greens as they descend. Unless taken with flash equipment, underwater photos have a tell-tale aqua tint to them. Even in the clearest oceanic water, 99% of the visible light is absorbed and to a lesser extent scattered in the top 100 m (330 feet). The deep sea and most of the ocean floor is in perpetual midnight, and the only light there is produced by bioluminescence from deep-living fishes and invertebrates.

The beautiful, almost electric turquoise color of shallow tropical seas is a combination of absorption by the shallow water and reflection off the sandy bottom. Coral sands are subtly yellow and the combination of aqua blue and a bit of enhanced yellow reflection produce the lovely turquoise shades we associate with the Caribbean and most tropical islands. Darker areas are likely to be sea grasses or corals.

The ocean water isn’t always blue everywhere of course. Around Delmarva, waters lack post-card shades of blue, and our turbid, brownish waters afford poor visibility for divers. Dissolved organic compounds and suspended mineral particles as well as living cells color the sea especially near coastlines. Fortunately, air is relatively transparent to all visible light, so ocean color as seen from airplanes or satellites can be used to assess the health and productivity of ocean waters. 

Put simply: the ocean is blue because the water is blue and the ocean is deep. Ocean optics is a (pardon the pun) deep subject, but better enjoyed in photos or underwater webcams if not in person. So even if you can’t take a tropical getaway, you can see this for yourself on Google Maps or Google Earth and via several NASA sites. As you explore the shades and tints of the Bahama Bank and imagine yourself lounging on the beach with a boat drink in hand …