The sky can take on a variety of colors throughout the day and evening. While blue skies are the most common, skies can also appear pink, orange, red, and even purple under the right conditions. A purple sky is often seen at sunrise or sunset when there are high clouds in the atmosphere. The reason for these colorful skies all comes down to how different wavelengths of light interact with the atmosphere.
How Light Creates Color in the Sky
Visible light from the sun contains all the colors of the rainbow. Each color has a different wavelength, with violet and blue light having shorter wavelengths and red and orange light having longer wavelengths. As sunlight enters Earth’s atmosphere, tiny air molecules and particles in the air absorb and scatter sunlight in all directions.
Scatter occurs when light bounces off particles in the air and gets redirected. The amount of scatter depends on the wavelength of light and the size of the particle it hits. Shorter wavelength violet and blue light gets scattered much more than longer wavelength red and orange light as it interacts with molecules in the air. Under normal conditions during midday, the sky appears blue because more blue light is scattered. At sunrise and sunset when sunlight travels a longer path through the atmosphere to reach your eyes, even more blue light gets scattered away, allowing more red and orange light to pass through, creating pink and orange hues.
How Purple Skies Occur
For a sky to take on a purple hue, two things are needed:
- Particles in the air that will scatter more red light than usual
- Enough sunlight reaching your eyes with blue and red wavelengths to mix and create purple
Here are some common reasons why skies may turn purple:
High Cloud Cover
High clouds like cirrus, cirrostratus, and cirrocumulus clouds contain ice crystals. These ice crystals are much larger than air molecules, enabling more scattering of longer wavelength red light. When the Sun is low on the horizon, its light travels through more atmosphere and is scattered by the cloud crystals. This scatters away much of the blue light, while allowing more red light to pass through. The remaining blue light mixes with the red light and creates the illusion of a purple sky to an observer on the ground.
Volcanic Eruptions
Large volcanic eruptions can eject tons of gases and ash high into the stratosphere. Sulfur dioxide gas gets converted into tiny sulfate particles that linger for months after an eruption. These aerosol particles are efficient at scattering red light coming from the rising or setting Sun. When combined with some remaining blue light that also passes through, vibrant lavender and purple sunrises and sunsets can occur.
Forest Fires
Smoke and ash plumes from large forest fires also provide lots of fine particles that scatter red light. If the smoke gets carried high in the atmosphere by winds, brilliant purple daytime skies may be observed downwind from the fires. This occurred in New England and New York after major fires in western Canada in 2022.
Air Pollution
Air pollution from human activities also introduces lots of fine particulates into the air that are great at scattering red light. Cities with high levels of air pollution, like factories releasing smoke or vehicle exhaust, see more red sunsets. As pollution levels have decreased in many urban areas due to emission regulations, extremely red sunsets are less common than in the past.
Storm Clouds
Thunderstorm clouds contain large amounts of water droplets and ice crystals that effectively scatter light. As the setting or rising Sun illuminates these thick storm clouds, vibrant purple and pink hues often develop. This occurs because most blue light is scattered away while more red light passes through the clouds to reach your eyes. The red mixes with the remaining blue light to provide the purple color.
When Purples Skies Are Most Visible
For purple skies to be most noticeable, optimum conditions are needed. Here are the key factors that increase chances of seeing brilliant purple sunrises or sunsets:
- Particles like clouds, smoke, or air pollution to scatter light
- Particles are located high up in the stratosphere or upper troposphere
- The Sun is close to the horizon so sunlight travels a longer path through more atmosphere
- There are minimal low clouds or fog near the surface blocking the view
- There is sufficient sunlight reaching the observer’s eyes
This combination produces the vivid purple light displays that photographers love. The most brilliant purples occur minutes after sunrise or before sunset when the Sun nears the horizon. High thin clouds or high level smoke are often present. The short path the sunlight takes through the lower atmosphere keeps the view open for observers. And enough sunlight reaches their eyes to reveal the full color display.
Absorption of Light Also Plays a Role
In addition to scattering, absorption of some colors by particles also enhances purple sky formation. Ozone gas in the upper atmosphere absorbs ultraviolet and blue light from the Sun. This prevents those colors from reaching Earth’s surface. Meanwhile, tiny air molecules absorb orange and yellow light. These absorption effects result in less blue and orange light available, causing enhanced scattering of red light and allowing purples to dominate sky color.
Other Factors Influencing Purple Sky Color
A few other more subtle factors can affect the purple coloration seen in certain skies:
- Size of particles – Smaller particles scatter light more, resulting in deeper purple hues
- Viewing angle – More dramatic purple is visible when looking directly opposite the Sun
- Surface reflectance – Light reflected off ground, water or snow can add to purple color intensity
- Altitude – More purple color is visible at higher elevations with less atmosphere
Being aware of these various conditions allows keen weather observers to anticipate when vivid purple skies may occur in their area.
Conclusion
In summary, purple skies require the right combination of atmospheric conditions. Particles like clouds, smoke or pollution must be present to effectively scatter sunlight. These particles are most effective when located high in the stratosphere. In addition, the Sun must be low on the horizon so its light travels through more atmosphere. This results in most blue light being scattered away allowing more red light to pass through. When the remaining blue light mixes with the red, the magical purple colors become visible. So next time you witness a brilliant purple sunrise or sunset, you’ll understand the special atmospheric circumstances that combined to create it.
| Type of Particles | Effect on Light Scattering |
|---|---|
| Small molecules like nitrogen and oxygen | Scatter blue and violet light most |
| Larger particles like smoke or water droplets | Scatter red, orange and yellow light |
| Ice crystals in high clouds | Efficient at scattering red light |
| Light Absorption Effects | Resulting Color |
|---|---|
| Ozone absorbs UV and blue light | Less blue light reaches surface |
| Air molecules absorb orange/yellow light | Less orange light reaches surface |
| Optimum Conditions for Purple Skies |
|---|
| High altitude clouds or particles present |
| Sun low on the horizon |
| Open view of the horizon |
| Enough sunlight reaching observer’s eyes |
