The stunning orange and red colors that often paint the sky during sunrise and sunset are caused by the scattering of sunlight as it passes through the atmosphere. This scattering process preferentially removes the shorter wavelengths of violet and blue light from the beam of sunlight, leaving behind the longer wavelengths of orange and red.
The key reason for this phenomenon lies in the fact that the shorter wavelengths of violet and blue light are more prone to scattering within the atmosphere compared to the longer wavelengths of orange and red. As the sun dips below the horizon during sunset or emerges above the horizon during sunrise, its light has to pass through more atmosphere compared to when it is higher in the sky. This increased depth of atmosphere accentuates the scattering of the shorter blue and violet wavelengths out of the path of sunlight, leaving the longer orange and red wavelengths to travel through relatively unimpeded.
In this article, we will explore in greater detail the physics behind why sunrises and sunsets often appear orange and red. We will look at how the composition of the atmosphere, the wavelength-dependent scattering of light, and the angle of the sun all play roles in creating these colorful scenes in the sky. Gaining a deeper understanding of the optics involved not only satisfies intellectual curiosity about this daily phenomenon, but also allows viewers to appreciate sunrises and sunsets at an even more profound level.
The Role of the Atmosphere
The key reason that sunsets and sunrises often appear orange or red lies within the earth’s atmosphere. Our atmosphere consists of various gases, water vapor, dust, and other particulates. This mixture scatters sunlight in all directions before, during, and after sunrise and sunset.
Three primary mechanisms are responsible for the scattering of light within the atmosphere:
Rayleigh scattering
This type of scattering occurs when particles in the atmosphere that are smaller than the wavelengths of visible light (such as nitrogen and oxygen molecules) interact with the incoming beam of sunlight. According to the principles of Rayleigh scattering, shorter wavelengths of light are scattered much more strongly than longer wavelengths. Violet and blue light have wavelengths around 400-500 nanometers, while orange and red light have longer wavelengths from 600-700 nanometers. This means violet and blue light are scattered about 3-4 times more than orange or red light by air molecules during their passage through the atmosphere.
Mie scattering
This type of scattering occurs when particles in the atmosphere that are around the same size or larger than visible light wavelengths (such as water droplets, dust, smoke, or pollution particles) interact with the incoming sunlight. Mie scattering shows less wavelength dependence compared to Rayleigh scattering, so it scatters all colors of visible light more equally. But it still tends to scatter shorter wavelengths slightly more than longer ones.
Absorption by atmospheric gases
Certain gases in the atmosphere such as ozone preferentially absorb specific wavelengths of ultraviolet and violet light as sunlight passes through. This provides yet another filter that removes the shorter wavelengths from the beam of sunlight.
Together, the combined effects of Rayleigh scattering, Mie scattering, and gaseous absorption remove a greater proportion of the shorter violet and blue wavelengths compared to the longer orange and red wavelengths. This filtering process accentuates the orange and red colors that manage to pass relatively unimpeded through the atmosphere during sunrises and sunsets.
The Importance of Sunlight’s Angle
Another key factor that accentuates the orange and red colors during sunrises and sunsets is the low angle that the sunlight takes through the atmosphere.
When the sun is high overhead around midday, its light only has to pass through a short chunk of atmosphere. But during sunrises and sunsets, the sunlight is traveling through the atmosphere at a very shallow angle, and therefore must pass through a greater depth of atmospheric gases and particles before reaching an observer on the ground.
| Sunlight’s angle | Depth of atmosphere traversed |
|---|---|
| High overhead at noon | Short depth |
| Low angle during sunrise/sunset | Long depth |
This exaggerated passage through greater amounts of atmospheric scattering molecules and particles filters out even more of the shorter violet and blue wavelengths at sunrise and sunset compared to midday. This leaves a higher proportion of orange and red light to pass through relatively unscattered, enhancing the warm colors observed during these times.
In fact, the greatest intensity of orange and red colors is often seen when the sun is between 5-6 degrees below the horizon. At this angle, the sunlight streaming sideways through the atmosphere has the longest path length through the gases and particles to maximize the preferential scattering of short wavelength blue light.
The Role of Particle Size
The composition of the atmosphere also plays a key role in enhancing the orange and red colors of sunrise and sunset. Compared to nitrogen and oxygen gases, water vapor condenses into much larger liquid droplets and particulate matter in the air can take the form of dust, smoke, or pollution particles up to a micron (one millionth of a meter) in diameter.
According to Mie scattering principles, these larger particles show increased scattering of all colors more equally. But they still tend to scatter violet and blue light slightly more than the orange or red.
So when the atmosphere has higher amounts of water vapor, dust, smoke, or pollution particles, the enhanced Mie scattering removes even more short wavelength violet and blue light compared to clear dry air. This filtering effect further exaggerates the orange and red hues seen during sunrises and sunsets. Locations with higher humidity and particulate matter like cities or tropical climates will often showcase deeper orange and red colors for this reason.
The Impact of Altitude
An observer’s altitude above sea level also significantly impacts the coloring seen at sunrise and sunset. At higher altitudes, there are fewer scattering particles in the less dense atmosphere compared to sea level. This means less short wavelength blue and violet light is removed from the beam of sunlight.
Accordingly, sunrises and sunsets viewed from mountain tops or airplanes will tend to appear less intensely orange and red compared to those seen at sea level or on land below. At very high altitudes, the effect is diminished enough that sometimes the scattered white light dominates instead.
Human Perception Factors
Not only do the principles of atmospheric optics explain the orange and red colors of sunrise and sunset, but the mechanics of human vision also play a key role.
Firstly, the human eye-brain visual system is much more sensitive to faint illumination at lower light levels. So while the orange and red hues during sunrise and sunset are less intense compared to midday light, our eyes perceive them to be more intense and vibrant against the darker skies.
Secondly, color contrast effects enhance the appearance of warm colors against cool backgrounds. So the presence of orange and red colors against the dark blue sky during twilight provides a high color contrast that makes them stand out even more to an observer.
Lastly, the area of the retina where rod photoreceptor density peaks (responsible for night vision) also overlaps with the area most sensitive to wavelengths of red light. So in the low light of sunrise and sunset, the rods boost our perception of the dominant red wavelengths.
Conclusion
In summary, sunrise and sunset colors stem from an intricate dance between sunlight and Earth’s atmosphere. The atmosphere’s gases and particles preferentially scatter shorter wavelengths of violet and blue light away from the beam of sunlight, allowing the longer wavelengths of orange and red to pass through more easily in the direction of an observer. The low angle of sunlight during these times maximizes its path through the atmosphere, enhancing this scattering effect. Higher moisture and particulate matter further filter the short wavelengths. Human visual perception factors also boost the vibrancy of orange and red against the darker twilight skies. Gaining a deeper understanding of all these interacting factors allows viewers to better appreciate the stunning bursts of color that brighten early mornings and evenings.
