Unusual colors in the sky often captivate observers, and purple light can be particularly striking. This phenomenon, while seemingly mysterious, arises from various causes, including natural atmospheric processes and human-made light emissions. Understanding these different sources helps decipher the captivating displays overhead.
Natural Occurrences
The natural world offers several explanations for purple light in the sky, primarily linked to sunlight interacting with Earth’s atmosphere. During twilight hours, at sunrise and sunset, the sun’s low angle causes its light to travel through a greater thickness of the atmosphere. This extended path leads to increased scattering of shorter wavelengths, such as blue and violet light, by atmospheric particles. As blue and violet light scatters, the remaining longer wavelengths, predominantly red and orange, combine with some scattered blue light, creating a purplish hue visible near the horizon.
This atmospheric scattering creates “afterglow” or “foreglow,” where a broad arch of sunlight appears in the twilight sky, often containing purple light. Moisture in the atmosphere, from impending storms or cloud cover, can further enhance these colors by reflecting and refracting sunlight. Auroral displays, like the Aurora Borealis, can also exhibit purplish or pinkish colors. These occur when charged particles from the sun collide with nitrogen molecules in Earth’s upper atmosphere.
Another distinct atmospheric phenomenon is “STEVE” (Strong Thermal Emission Velocity Enhancement). This event appears as a purple-pink arch across the sky. STEVE results from interactions between charged particles in solar storms and Earth’s magnetosphere, with hot rivers of gas flowing rapidly through the magnetosphere.
Human-Made Contributions
Human activities also contribute to instances of purple light observed in the sky. A prominent cause involves certain light-emitting diode (LED) streetlights that have developed a peculiar purple glow. This issue stems from a manufacturing defect.
White LED streetlights are made with a blue LED chip coated in a yellow phosphor layer. When this phosphor coating degrades, cracks, or delaminates, the underlying blue light becomes more visible. This exposure of the blue diode, sometimes combined with remnants of the yellow phosphor, results in the light appearing purple instead of its intended white. These defective streetlights can reduce visibility and be disorienting.
Other artificial sources can cast a purple light into the night sky. High-powered lasers, used for entertainment or astronomical observations, can project visible beams that appear purple. The specialized chemicals used in fireworks can also produce vibrant purple hues. Achieving this color typically requires a precise combination of strontium compounds (red light) and copper compounds (blue light).
The Physics of Purple Light
Understanding why light appears purple involves the fundamental principles of light and its interaction with matter. Visible light is a segment of the electromagnetic spectrum, with each color corresponding to a specific wavelength. Violet light has the shortest wavelengths, and red light the longest.
Violet light has wavelengths ranging approximately from 380 to 450 nanometers, while blue light typically falls between 400 and 500 nanometers. Atmospheric scattering, particularly Rayleigh scattering, plays a significant role in how we perceive sky colors. This scattering occurs when light encounters particles much smaller than its wavelength, such as the nitrogen and oxygen molecules that make up Earth’s atmosphere. Shorter wavelengths, like blue and violet light, are scattered much more efficiently than longer wavelengths, like red light.
This preferential scattering of blue and violet light is why the sky appears blue during the day. Although violet light scatters more than blue, the human eye is more sensitive to blue light, making the sky appear blue rather than a deep violet. When conditions allow for a combination of scattered blue light and less scattered red light to reach the eye, such as during sunrise or sunset, the brain interprets this mixture as purple. This optical mixing, whether from natural scattering or artificial light sources, is how the color purple becomes visible.
Interpreting Your Observation
When observing purple light in the sky, several factors can help determine its origin. Consider the time of day: if the purple light appears during sunrise or sunset, it is most likely a natural atmospheric phenomenon. Purple light seen outside of these twilight hours points more strongly towards an artificial source.
The location of your observation also provides important clues. In urban environments, particularly near roads or industrial areas, defective LED streetlights are a common source of purple light. These lights often appear static and can affect individual fixtures or entire sections of a street. In contrast, dynamic, dancing purple lights, especially in higher latitudes, might indicate an auroral display.
The duration and movement of the light are also indicative. A persistent, stationary purple glow suggests a fixed artificial source like a streetlight or industrial lighting. If the light is intermittent or moves, such as flashes and bursts, it could be fireworks or high-powered lasers used for events. By considering these contextual details, you can better interpret the source of the purple light you are seeing.