The sky can take on a red or reddish hue due to several natural phenomena. This is most noticeable during sunrises and sunsets when the sun is low on the horizon. But even during the day, the sky may appear more red than usual under certain conditions. Understanding why the sky sometimes looks red requires an examination of the properties of light and how it interacts with the atmosphere.
Rayleigh Scattering
The main reason sunrises and sunsets often appear red is due to a phenomenon called Rayleigh scattering. This refers to the scattering of sunlight off of particles in the atmosphere. Shorter wavelengths of light, like blue and violet, are more prone to scattering than longer wavelengths like red and orange.
During sunrise and sunset, the sun’s light has to pass through more of the atmosphere to reach our eyes. Much of the blue light gets scattered away, leaving more of the longer red wavelengths to come through directly. The result is the predominance of reddish hues.
Mie Scattering
Another process that can lead to red skies is known as Mie scattering. This involves the scattering of light off of larger particles like dust, smoke, pollution, or water droplets.
Mie scattering affects all wavelengths of visible light more equally. But when combined with the wavelength filtering effects of Rayleigh scattering, it can also create vivid red colors in the sky. This is especially true at sunrise/sunset when the light travels farther through haze or polluted air.
Effects of Clouds and Moisture
The presence of clouds and moisture in the air can enhance red skies. Water droplets in clouds provide surfaces for additional Mie scattering of sunlight. Low hanging clouds near the horizon can reflect long red wavelengths downward, intensifying the colors.
High levels of moisture may also help red light pass through while scattering out shorter wavelengths. This can lead to vivid red colors even when the sun is higher in the midday sky if thick, humid air is present.
Atmospheric Conditions for Red Skies
Certain atmospheric conditions make red skies more likely:
| Low sun angle | Sunrise/sunset when light passes through more atmosphere |
| Clouds/haze | Reflect and scatter red light |
| High humidity | Moisture scatters blue light |
| Dust/pollution | Particles for Mie scattering of all colors |
| Fires/volcanoes | Smoke particles enhance scattering |
So when the sun is low and one or more of these conditions are present, longer red wavelengths dominate after scattering, causing the vivid red hues.
Rayleigh Scattering Elsewhere
The same principles that create red sunrises and sunsets on Earth are responsible for some other red skies in our solar system:
– On Mars, the iron oxide-rich dust gives the sky a reddish appearance much of the time due to particle scattering. Sunrises/sunsets are also more red.
– Jupiter’s moon Io has extensive volcanism that spews sulfur dioxide high into the atmosphere. This leads to strong Rayleigh and Mie scattering and a reddish-yellow sky.
– The lower density atmospheres on Mars and Io allow more pronounced scattering effects since light can travel farther before encountering particles.
Unusual Red Sky Causes
Less common causes of unusually red skies on Earth include:
– Severe pollution events like intense wildfire smoke or volcanic ash. The high density of particles enhances scattering.
– Blood rain, caused by microalgae or dust from Sahara sand storms mixing with rain to stain it red. This can tint wet sidewalks and surfaces.
– Noctilucent clouds at high altitudes can reflect sunlight at twilight after the sun has set below the horizon, creating an eerie red glow.
– In crepuscular rays, the contrast with bluer parts of the sky is exaggerated, making the angled sunbeams appear reddish.
– Rarely, reddish sky glows called airglow may come from chemical interactions in the upper atmosphere. These are usually brief and faint.
Conclusion
In summary, red skies occur due to the wavelength selective scattering of sunlight by particles in the atmosphere. This effect is most pronounced at sunrise and sunset when light travels farther through air, but can happen under the right conditions any time of day. Understanding the key principles of Rayleigh and Mie scattering allows us to explain why the sky sometimes takes on such vivid reddish hues.
