The sky can take on a variety of colors throughout the day, but it often appears red or orange when the sun is setting or rising. This phenomenon has long fascinated people, but there is actually a scientific explanation for why the sky looks red at dusk. Understanding the reason behind this colorful display requires an examination of how sunlight interacts with the atmosphere.
Rayleigh Scattering
The red and orange hues seen at sunset are caused by a phenomenon called Rayleigh scattering. This effect describes how light waves interact with particles that are smaller than the light’s wavelength. Such particles, like gas molecules in the atmosphere, scatter shorter wavelengths of light (violet and blue) more than longer wavelengths (yellow, orange and red).
During the day, sunlight has to pass through more atmosphere before reaching our eyes. The blue and violet wavelengths are scattered in all directions by the gases, causing the sky to appear blue. As the sun gets lower in the sky near sunset, its light passes through increasing thicker layers of atmosphere. Much of the blue light has been scattered away, allowing more of the longer wavelengths like red and orange to reach our eyes. This gives the sky its brilliant red and orange glow.
Particles in the Atmosphere
The specific composition of particles in the atmosphere impacts the type of scattering that occurs. Nitrogen and oxygen molecules make up over 99% of the dry atmosphere. These small particles interact more strongly with short wavelength light, producing the effect known as Rayleigh scattering.
However, larger particles like soot, ash, smog and dust also contribute to the unique colors at sunset. These more effectively scatter longer orange and red wavelengths. Places with more of these larger particles in the air will often display more vivid sunsets. Things like pollution, wildfire smoke and volcanic ash can greatly intensify the vivid reds and oranges seen.
Angle of the Sun’s Rays
Another factor impacting the color of sunsets is the angle at which sunlight passes through the atmosphere. When the sun is directly overhead at noon, its light only travels through a short horizontal distance of atmosphere. But near sunset, sunlight has to pass through a greater thickness of atmosphere at a longer angle. This extended passage means much more of the blue light gets scattered away, causing the sunlight we see to appear more red.
The effect is strengthened by the fact that sunlight has to pass through even more atmosphere near the horizon, as it travels the length of Earth’s curved surface. This allows the longest wavelengths like red and orange to dominate, leading to dramatic sunsets.
Clouds and Particulate Matter
Clouds and small particles in the atmosphere also influence the colors seen at dusk. Low clouds near the horizon can reflect the longer red and orange wavelengths, adding brightness and color to sunsets. High altitude clouds like cirrus clouds tend to block shorter wavelength blue light, while allowing longer wavelengths to pass through. This filters the sunlight and enhances warm colors.
| Particle Type | Impact on Sunset Colors |
|---|---|
| Nitrogen and oxygen molecules | Cause Rayleigh scattering which scatters blue light |
| Larger particles like dust, ash, pollution | Scatter red and orange light more effectively |
| Water droplets in clouds | Reflect red and orange light towards viewer |
Dust particles, pollutants, volcanic ash and other small particulates in the atmosphere also aid in scattering the warm long wavelength light. Locations with more of these particles present will typically display more brilliant red sunsets.
Location on Earth
The latitude of the observer also impacts the colors seen at sunset. At the equator, the sun sets rapidly, passing through a narrow range of angles. But moving toward the poles, the sun follows a longer, shallow path near the horizon before dipping below. This means sunlight streams through the atmosphere at a lower angle for a longer period of time. More blue and violet light gets removed along this extended path, intensifying the warm sunset colors.
Conclusion
In summary, the vivid red, orange and yellow hues seen during sunsets result from a combination of factors. Rayleigh scattering preferentially scatters shorter wavelengths of blue and violet light. The angle of the sun’s rays and the increased atmosphere they pass through near the horizon allow more warm colors to get through. Larger particles like pollution and clouds also help brighten sunsets by scattering or reflecting light. So while sunsets may appear purely artistic, they are really a beautiful display of fundamental physics principles at work. The next time you see a dazzling sunset, you can appreciate the complex optical phenomena producing its vivid color palette.
