As spring blooms, and the sun begins to warm our skin, there’s nothing that says “tis the season” like a deep blue sky. But have you ever stopped to ask yourself why exactly the sky is so blue?

As it turns out, the color of the sky starts with the sun.

Incoming sunlight, although white in appearance, is really a combination of all colors of the rainbow. Each color is produced by a wave of energy with a unique length (for length of a wave, think distance between the highest points of two ocean waves). The first few colors of the rainbow (starting with red) have longer wavelengths or a greater distance between peaks, where the last few (the blues) have shorter wavelengths or a smaller distance between peaks.

As sunlight reaches Earth’s atmosphere, these waves begin to interact with the oxygen and nitrogen molecules that make up our atmosphere. This atmosphere is the air that you are breathing as I type and you read. The interaction of light and air molecules (the oxygen and nitrogen) causes the sunlight to scatter in all directions, similar to a water balloon dropped onto the floor. The interaction between the balloon and the floor breaks the water that filled the balloon down into individual bits of water, sending droplets in every direction, just as the white light strikes the air molecules, sending the different colors in every direction.

The blue light, or the shorter wavelength light, has higher energy than the red light, or the longer wavelength light. Thanks to the interaction with the air molecules, the more energetic blue light scatters much more than the red light. If you’re still thinking about the water balloon, remember that the larger, heavier water droplets barely jump up after impact (the red light or longer wavelengths), but the smaller, lighter water droplets fly far and are more likely to strike another target (the blue light or shorter wavelengths). The blue light’s other target is your eyes. When the blue light reaches your eyes, you see blue.

You may be saying to yourself that’s a great explanation, but then why during sunsets and sunrises is the sky not blue?

When setting and rising, the sun is no longer directly overhead. The sun appears lower in the sky, meaning the sunlight travels a greater distance through the atmosphere to reach you. This extra distance means there are more gas molecules for the light to bounce off before reaching your eyes, so the blue light scatters away without reaching your eyes. This extra scattering of blue light leaves the longer wavelengths of red, yellow and orange to reach your eyes.

Where the daytime light was a water balloon dropped from above, the light during sunset and sunrise is a balloon thrown at a friend. The smaller drops (the blue light) now scatter away, leaving the larger drops (the red light) to strike the floor below your friend or the red light to reach your eyes.

As you venture out into the warmth of the sun this spring, hopefully you’ll have a greater understanding of the deep blue that greets your eyes.

Curious about something in the world that surrounds you? Email me your questions (or comments) at guido.26@osu.edu.