Morse Code of Weather: explaining the vibrant colors of sunrises and sunsets

BISMARCK, N.D. (KFYR) - Here’s a fun experiment for you to do at home that does a great job of explaining why the sky is blue and why sunsets are often a display of beautiful orange and red colors.

All you need is three glasses, water, milk, and a flashlight. Fill two of the glasses with water and the other one with milk. Then pour a splash of milk into one of the glasses with water in it and stir it up. So, you should end up with one glass with just water, one with mostly water mixed in with a splash of milk, and another with just milk.

Now turn the lights off and shine a flashlight on the side of each of the glasses. First, for the glass with just water, nothing happens as we would expect. Second, for the glass with water and a splash of milk, the liquid turns a light blue color. And finally, for the glass with just milk, the milk turns an orange color.

To explain these results, first, let’s remember that white light (in this case coming from the flashlight) is made up of a full spectrum of colors from red to violet. When you shine white light through a prism, you get that full spectrum of colors.

Also, light is a form of energy that travels in waves with different wavelengths depending on the color. Red and orange light has longer wavelengths compared to violet and blue light with shorter wavelengths.

Let’s answer the “why is the sky blue?” question first. As the white light from the sun passes through the atmosphere, it is interrupted by various particles. But light travels in a straight path unless something is in its way. When light hits relatively large particles — such as water, dust, or pollen — it simply bounces off in different directions. However, when it hits oxygen and nitrogen molecules, which is what our atmosphere is mostly composed of, these molecules are smaller than the wavelength of light and the light gets absorbed by the molecule. The molecule later radiates the same color that was absorbed.

Although all colors of light can be absorbed, shorter wavelengths of light — mostly the blue color — are absorbed more frequently than longer wavelengths, such as red and orange colors, because they travel in shorter waves. As a result, blue is scattered more than any other color, which is why the sky appears blue.

This process is referred to as “Rayleigh scattering” and is illustrated in the diagram below.

A similar process is happening in our experiment with the glass of mostly water that has just a splash of milk in it. The milk particles are acting like the oxygen and nitrogen molecules in our atmosphere, and since they are not highly concentrated, the blue light is scattered and that’s the color we see.

Now let’s figure out why vibrant orange and red colors usually appear when the sun is setting or rising. Basically, it’s because the sunlight has to pass through more of Earth’s atmosphere when it’s lower on the horizon.

In the diagram below, you can see how at noon when the sun is directly overhead, the white light from the sun has to pass through less of Earth’s atmosphere compared to at sunset.

When the light passes through a thicker layer of Earth’s atmosphere at sunrise or sunset it has to interact with more oxygen and nitrogen molecules that scatter most, if not all, of the blue light out by the time it reaches the ground. This leaves predominantly the orange and red colors that we see at sunrise or sunset.

A similar process is happening in our experiment with the glass of just milk. The higher concentration of milk particles are acting like the thicker layer of the atmosphere that sunlight has to pass through at sunrise or sunset, leaving the orange color that we see in the experiment when we shine the flashlight into the glass.

Here’s a different perspective of the light from the sun when it’s low on the horizon at sunrise or sunset. The white light from the sun — which is made up of a full spectrum of colors — travels a long distance through Earth’s atmosphere before reaching the ground. This allows the blue and violet light, as well as many of the other colors with shorter wavelengths, to be scattered out by the molecules and particles in our atmosphere, which only leaves the orange and red colors for us to see.

A couple of additional facts you may be curious about:

• Why isn’t our sky purple if shorter wavelengths of light are scattered more than longer wavelengths by the oxygen and nitrogen molecules in our atmosphere? After all, violet light has a shorter wavelength than blue light. The answer is that the physiology of the human eye makes us more sensitive to blue light than to purple light. Plus, violet light is only a very small amount of the visible light spectrum compared to a much larger range of wavelengths for blue light.
• Sunrises and sunsets can appear even more orange and red when wildfire smoke fills the sky. The additional smoke particles in the atmosphere essentially help to scatter out even more of the colors of light before they reach the ground, leaving only the orange and especially red colors. If there’s too much smoke, however, this can mute the sky colors and subdue the brightness of sunrises and sunsets.

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