The phenomenon of the sky turning red at night can seem unusual or even alarming for those who witness it. However, there are some simple scientific explanations for why and how the sky can take on a reddish hue after sunset. In this article, we’ll explore the main reasons that can cause the night sky to turn red, looking at how particles and molecules in the atmosphere scatter and absorb light. We’ll also consider some famous examples of blood-red skies throughout history and how they came about.
Rayleigh scattering
The primary reason we see a red sky at night is due to a phenomenon called Rayleigh scattering. This effect describes how small particles in the atmosphere, those smaller than the wavelengths of visible light, scatter the shorter blue wavelengths more strongly than the longer red wavelengths. Essentially, as sunlight passes through the atmosphere at sunset and sunrise, the shorter blue light gets scattered away by particles while the longer red light travels more directly through. This leaves us with the residual red hues.
During the day, the sky appears blue because the effect of Rayleigh scattering is stronger. The blue light scattered all around the sky dominates, while the reddened sunlight is lost against the brightness. But during sunrises and sunsets, when sunlight has to pass through more atmosphere and travel farther at a slant, much more of the blue light is scattered away, leaving behind the longer red wavelengths to give the sky a red tinge.
Mie scattering
Another mechanism that can contribute to redder skies is Mie scattering. This refers to the scattering of light off of larger particles in the atmosphere around the size of the wavelengths of visible light, like smoke, dust, pollution, water droplets, and so on. Mie scattering tends to favor scattering the shorter blue light less than the longer red light. So unlike Rayleigh scattering, Mie scattering will selectively reduce the bluer daylight colors even more relative to the red end of the spectrum. The net effect enhances the reddening of sunrises and sunsets. Conditions with more aerosols, clouds, pollution, and moisture favor the contributions of Mie scattering.
Particles that absorb light
In addition to scattering, certain particles present in the atmosphere during particular conditions can also preferentially absorb more blue light than red light. For example, small solid organic particles or porous aggregated particles effectively filter out more blue light, resulting in transmitted sunlight that appears more red. These absorbing particles provide another mechanism for the sky to redden deeper at sunrise and sunset.
The importance of a clear sky
For these light scattering effects to result in a noticeably red night sky, the air must be reasonably clear and transparent. Dust, pollution, smoke, and thick cloud cover can all serve to block and reflect sunlight before it reaches our eyes, preventing us from seeing the red wavelengths. So the best night skies for observing a red hue are those with minimal obstructions along the horizon. However, as we will see, extremely dense levels of particles responsible for Mie scattering can still generate intensely red skies under the right conditions.
Famous examples of blood-red night skies
Let’s look at some notable examples of strangely red night skies recorded throughout history and what caused them:
The dust bowl droughts in 1930s America
Prolonged drought on the prairies and plains during the 1930s dried up soil that got picked up into massive dust storms. The arid conditions combined with particles suspended in the air from dust led to vivid red sunsets and sunrises with yellow, orange, and pink hues. Both Rayleigh and Mie scattering from the unusual amounts of dust amplified the red wavelengths of sunlight.
Canadian forest fire sunsets in 1950
Intense wildfires in Canada in 1950 pumped huge plumes of smoke into the atmosphere that got carried across Eastern North America. The haze from the smoke and suspended particles led to striking reddish-orange sunsets. Mie scattering of light by the smoke particles enhanced the red hues.
Krakatoa volcanic eruption sunsets in 1883
The powerful eruption of Krakatoa volcano in Indonesia in 1883 injected massive amounts of ash and aerosols into the stratosphere. These particles were then spread worldwide by winds in the upper atmosphere over the next few years. Spectacular red sunsets were recorded in paintings and writings as far as Europe and North America.
| Year | Event | Cause of red sky |
|---|---|---|
| 1930s | Dust bowl droughts in America | Dust particles suspended in air |
| 1950 | Canadian forest fires | Smoke particles in atmosphere |
| 1883 | Krakatoa volcanic eruption | Volcanic ash aerosols |
How particles generate a red sky
To summarize, red night skies are caused by:
– Rayleigh scattering – Scattering of short blue light by atmospheric gas molecules and tiny particles
– Mie scattering – Scattering of blue light by larger particles like dust, smoke, water droplets
– Absorption of blue and green light by solid and porous aerosols
This leaves the longer red light to travel through and give the sky its distinctive hue. The most dramatic red skies occur when extra particles are present in the air from events like fires, volcanic eruptions, and dust storms. But clear dry air and the right conditions are needed for the colors to stand out.
Conclusion
The sight of a blood-red sky at night can be unusual but has a logical explanation in terms of the scattering and absorption of different colors of sunlight. Understanding the physics of how particles in the atmosphere interact with light helps explain this interesting phenomenon. While some red skies are due to unusual events like volcanic eruptions, they can also occur under normal conditions when the time is right at sunset or sunrise to reveal the lingering red wavelengths of sunlight filtered through the air. So although attention-grabbing, a red sky at night is a beautiful astronomic event rather than an ominous sign.