Why is Sunrise So Colorful?
2017-03-05 15:00:29.000 – Taylor Regan, Weather Observer
Have you ever wondered why we see such color in the sky at sunrise or sunset? Or perhaps why we perceive the sky as blue through the majority of the day? The answers to these questions have to do with how visible light reacts in and with our atmosphere.
First, let’s take a look at visible light. What is it made of? And why is it white? Visible light, just like radio waves or microwaves, is a form of electromagnetic radiation, and electromagnetic radiation is a form of energy. The sun emits energy, partly in the form of light waves that reach the earth as sunlight. These light waves are emitted in a range of frequencies, which together combine to form the “visible light” portion of the electromagnetic spectrum. Visible light is really a blend of several different frequencies, each of which is perceived as a different color by the human eye, and when combined, appear as white light.
Isaac Newton, in 1666, was the first person to realize that white light was actually a combination of the colors of the rainbow. He demonstrated this by passing a beam of sunlight through a prism to break up the light into its color components. Each of the components of visible light travels along at different wavelengths; as such they are bent differently when passing through a prism, accounting for the spreading out of visible light into a rainbow of color. The image below shows the colors that make up the visible light spectrum, along with the wavelengths of each color. Notice how the colors violet and indigo (at the top) have much shorter wavelengths than red and orange (at the bottom). This is a key factor in why we are able to perceive different colors in the sky at different times of the day.
Visible light travels in a straight line unless it is reflected, bent or scattered; this is where the atmosphere comes into play. The air around us is made up of many small molecules such as oxygen and nitrogen. These molecules are very small compared to the wavelength of the colors which combine to form visible light. As sunlight reaches the atmosphere, the smaller wavelengths are more effectively scattered over the atmosphere. For example, blue and violet light travels along shorter wavelengths, and are more effectively scattered than red or orange. However, if scattering was the pure driver in color, then humans would actually see the sky as violet. Interestingly, not only is there less violet emitted than blue, but human sight is less sensitive to violet than it is blue. This is because humans have three main color receptors, blue, green, and red, named for the colors they most strongly respond to. As the color receptors are stimulated to different extents, they indicate the variety of colors we see. The image below is an example of a clear sky during the day, in which the deep blue sky is clearly visible above the summit. This is the result of the blue component of sunlight being scattered across the atmosphere.
You may have noticed that on occasion, the blue of the daytime sky doesn’t seem quite so blue, or that a sunset seems relatively muted. Purity of color can be impacted due to an excess of particles suspended in the atmosphere. When pollutants and other large particles are trapped near the surface, within the boundary layer, the result is that the colors we see are often “washed out.” The particles are too large to effectively scatter the light into its components. The image below depicts a sunset in which a good deal of haze was present, resulting in the pastel peaches and baby blue tones that are present.
Because large particulates often impede the ability to see pure tones and vibrant colors of sunrises and sunsets, clear air is helpful in witnessing the most “beautiful” sunrises or sunsets. For this reason, winters are especially noteworthy, particularly in the northeast, with the cold, dry, and clear air that is typically sitting overhead. Often times however, simply rising above the boundary layer allows a clearer line of sight between the observer and the setting sun. As seen from a plane, a not so spectacular sunrise/set can become spectacular as you exit the boundary layer because the air is much clearer.
All of this leads us back to our main question, why are sunrises and sunsets often so colorful? As the blue light is scattered out into the atmosphere, what remains are the colors with longer wavelengths, such as orange and red. And as the sun gets lower in the sky, the sunlight must past through an increasing amount of the atmosphere, filtering out more of the blues. The diagram below helps to demonstrate the scattering effect through the atmosphere.
Diagram by Stephen F. Corfidi “The Colors of Sunset and Twilight” September 2014
While we naturally see more reds and oranges during sunrises or sunsets, there is another element that helps contribute to the most spectacular events. Clouds, composed of water droplets and/or ice crystals help to further add color to the sky by reflecting the light of the setting sun. The compilation below highlights some of the spectacular colors that can be evoked by the atmostphere and the setting sun, with or without the presence of clouds.
Taylor Regan, Weather Observer