A red sunset is often referred to as a “blood red” sunset. This phenomenon occurs when the light from the setting sun passes through more of the Earth’s atmosphere than usual, scattering away shorter wavelengths of light and leaving mostly long wavelengths at the red end of the spectrum.
What Causes a Red Sunset?
There are a few different factors that can contribute to a red sunset:
- Pollution – Dust, smoke, and pollution particles in the atmosphere can scatter away blue and green light from the sun, enhancing the red tones.
- High altitude clouds – Cirrus, cirrostratus, and other high clouds have ice crystals that efficiently scatter light, producing vibrant red sunsets.
- High humidity – Moisture in the air can scatter more red wavelengths.
- Particles from volcanic eruptions – Sulfuric gases and particulates released by volcanic eruptions increase scattering.
The most common cause is when the sunlight passes through more atmosphere than normal on its way to the observer. This occurs when the sun is near the horizon, at either sunrise or sunset, as the sunlight travels through more air mass. The blue and green wavelengths from the sun get scattered away by the molecules in the atmosphere, leaving mostly red hues that reach the viewer.
Where are Red Sunsets Most Common?
Red sunsets tend to occur more frequently in certain geographic locations:
- Tropical regions – Places near the equator often have very moist, humid air that promotes scattering of red wavelengths.
- Polluted cities – Urban areas with higher air pollution see more vivid red sunsets.
- Higher elevations – Locations at high altitude have less atmosphere above them, accentuating the scattering effect near the horizon.
- After volcanic eruptions – Particles spewed into the stratosphere can create blood red sunsets globally for months or years after a major volcanic event.
In general, stable, clean air conditions will lead to less dramatic sunsets while unstable, dusty, humid, or polluted air tends to enhance red sunsets. However, the time of day and location of the setting sun on the horizon ultimately determines how much reddening occurs.
| Location | Red Sunset Frequency |
|---|---|
| Urban cities | High |
| Tropical regions | High |
| Mountain regions | Moderate |
| Rural countryside | Low |
Optimal Conditions for Red Sunsets
The most brilliant red sunsets tend to occur under the following conditions:
- Pollution – More particles in the air increase the scattering effect.
- High humidity – Water vapor enhances the scattering of red light.
- Partially cloudy – Some clouds can reflect the sunset colors back down to the ground.
- Clean air above – Clear air above allows sunlight to pass through before scattering at the horizon.
- Sun near horizon – The most dramatic effect occurs closer to the horizon with sunlight passing through the maximum air mass.
These factors all contribute to the vivid red hues seen at sunset. The clean air above with moisture and pollution lower in the atmosphere creates ideal scattering conditions to remove all but the red light. When combined with the sun’s angle at the horizon, this can maximize the blood red colors.
What is Rayleigh Scattering?
The reddening of sunsets is caused by an optical phenomenon called Rayleigh scattering. Here’s how it works:
- Light is a form of electromagnetic radiation that travels in waves.
- Shorter wavelengths of visible light (violet, blue, green) have a higher frequency than longer wavelengths (yellow, orange, red).
- Gas molecules in the atmosphere interact more strongly with the shorter waves, scattering the higher frequencies away.
- By sunset, the dominant remaining visible light is the longer, redder wavelengths since the blues and greens have been scattered away.
Lord Rayleigh first described this elastic scattering process in 1871. The amount of scattering is inversely proportional to the fourth power of wavelength, so blue light scatters over 10 times more than red light from the sun.
Other Colorful Sunset Phenomena
While red is the most common sunset color, other vivid displays can also occur:
- Orange sunsets – Caused by some scattering and intermediate sunlight angles.
- Purple sunsets – When blue and violet light is refracted towards the observer.
- Green flashes – Green wavelengths focused at the top of the sun during setting.
- Crepuscular rays – Sun rays shining between gaps in clouds.
Location, weather conditions, time of year, and other factors can influence the possible colors seen at sunset. But the scattering of sunlight through the atmosphere explains most reddened sunsets.
Famous Red Sunsets in History
Some noteworthy red sunsets in history include:
| Year | Red Sunset Event |
|---|---|
| 1883 | Krakatoa volcanic eruption afterglow |
| 1912 | Red sunset observed after Katmai eruption |
| 1947 | New York City red sunset from air pollution |
| 2020 | San Francisco red skies from wildfire smoke |
These vivid red sunsets were caused by volcanic ash or excess pollution in the atmosphere. The dramatic colors spawned cultural references in art, literature, and photography of the time.
Red Sunsets in Art and Culture
Red sunsets have inspired artists and photographers for centuries. Famous red sunset depictions include:
- Haystacks painting series by Claude Monet
- Red Sunset over Water by J.M.W. Turner
- Sunset Boulevard film poster with red sunset
- Red Sky at Morning sailor’s rhyme
- Red Sunset photography of the American West
The allure of blood red skies has captivated people’s imaginations as both beautiful and ominous. Vibrant red sunsets continue to provide striking natural canvas for artistic expression.
Conclusion
In summary, red sunsets occur due to the scattering of short wavelength light by particles in the atmosphere near the horizon at sunset. Rayleigh scattering selectively removes the blues and greens from white sunlight, leaving mostly long wavelength red hues. Pollution, humidity, and volcanic ash can exaggerate the effect. Red sunsets have inspired artists for centuries and remind us of nature’s ephemeral beauty.
