Sightings of purple light appearing in the sky have become a topic of increasing public interest globally. The origins of this unusual glow are diverse, ranging from simple technological malfunctions to complex atmospheric events. Understanding the cause requires separating light sources that are close to the ground from those that originate high above the Earth’s surface. This distinction determines whether the light is a localized, artificial occurrence or a rare, large-scale natural event.
Atmospheric Physics Behind the Color
The natural color of the sky is governed by how sunlight interacts with atmospheric molecules through scattering. The daytime sky appears blue due to Rayleigh scattering, where air molecules preferentially scatter shorter, high-energy wavelengths like blue and violet. Although violet light is scattered most effectively, the sky appears blue because the human eye is more sensitive to blue wavelengths.
Larger particles, such as water droplets or dust, cause Mie scattering, which is less wavelength-dependent and typically makes clouds appear white. Purple is not a monochromatic color; it is a mix of the shortest (blue/violet) and longest (red) visible wavelengths. A purple sky is produced when light from these two different parts of the spectrum—such as scattered blue light and direct red light from a low-lying sun—are perceived together.
This mixing often occurs during twilight or sunset, when sunlight travels a greater distance through the atmosphere, scattering away most short-wavelength colors. The remaining red and orange light from the sun then combines with the ambient blue light scattered high in the sky. Particulate matter, such as aerosols or specific humidity levels, can enhance this effect by influencing the scattering path and intensity of the light wavelengths.
Purple Light from Artificial Sources
The most common localized source of purple light is a defect in modern lighting infrastructure. Many white light-emitting diode (LED) streetlights use a blue LED chip coated with a yellow phosphor layer. This coating converts the intense blue light into the longer wavelengths required to produce balanced white light.
The purple glow occurs when this protective phosphor layer degrades, delaminates, or wears away prematurely due to a manufacturing flaw. When the yellow component of the conversion process is lost, the underlying blue light from the LED chip is no longer fully converted. This exposed blue light then mixes with any remaining yellow or ambient red light, resulting in a distinct purplish hue.
Another artificial source is specialized horticultural lighting used in commercial greenhouses. Plants primarily utilize blue and red wavelengths for photosynthesis. To maximize efficiency, many indoor growing operations use LED grow lights that combine red and blue chips, creating a strong magenta or purple light visible from outside. This light is noticeable when it reflects off low-lying clouds, fog, or mist, casting an unusual glow over the local area.
High-Altitude Natural Events
Rarely, purple light is a naturally occurring phenomenon high in the Earth’s upper atmosphere. The most notable example is the Strong Thermal Emission Velocity Enhancement, or STEVE, which appears as a narrow, purplish ribbon of light stretching across the sky. Unlike the traditional aurora, which is caused by charged particles from the solar wind, STEVE is a distinct process.
STEVE is caused by a high-speed flow of extremely hot, charged gas, or plasma, moving through the ionosphere at altitudes of about 280 miles. This plasma stream, which can reach temperatures of over 5,000 degrees Fahrenheit, creates friction with surrounding atmospheric particles. This friction heats the particles, causing them to emit light through thermal emission rather than particle excitation.
The purple or mauve color of STEVE results from specific emissions from atoms and molecules in the upper atmosphere, likely oxygen and nitrogen excited by this thermal energy. While traditional auroras are typically green, they sometimes show a subtle purple or pink fringe caused by excited nitrogen molecules. STEVE is a sharper, more distinct band of purple that often appears closer to the equator than typical polar auroras.