The sight of a red glow in the night sky is an unusual phenomenon that suggests a significant interaction between light and the Earth’s atmosphere. This observation indicates the presence of either a powerful light source or unique atmospheric conditions that favor the visibility of longer wavelengths. Understanding the causes requires examining the specific physics of how light is filtered and scattered across vast distances, revealing how both nature and human activity play a role in painting the sky red.
How Light Travels Through the Night Sky
The physics governing the color of the sky is primarily determined by a process called Rayleigh scattering, which explains why blue light dominates during the day. This scattering occurs when light encounters particles, such as nitrogen and oxygen molecules, that are much smaller than the light’s wavelength. Shorter wavelengths, like blue and violet light, are scattered in all directions far more effectively than the longer wavelengths of red and orange light.
This principle is reversed when a light source is viewed across a long distance, such as when the Sun is near the horizon during sunset. For light to reach the observer, it must travel through a much thicker section of the atmosphere, causing nearly all of the shorter-wavelength light to be scattered away. What remains to travel the full distance to the eye is the less-scattered, longer-wavelength red and orange light.
At night, this mechanism of atmospheric extinction ensures that any distant light source—whether natural or artificial—has its blue and green components scattered out. The only light that persists and is visible across large distances is the red end of the spectrum. This is why a widespread glow at night, even from an unseen source, often appears with a reddish hue. Larger airborne particles, such as dust or water droplets, can also contribute to this effect through Mie scattering, which further filters the light and allows red wavelengths to pass through more easily.
Natural Phenomena Causing Red Glow
One of the most dramatic natural sources of a red night sky is an aurora, known as the Northern or Southern Lights. While auroras are commonly known for their vibrant green color, a pervasive red glow signals a specific interaction with the highest reaches of the atmosphere. This red light is emitted by atomic oxygen located at altitudes typically above 200 kilometers (about 120 miles).
The red emission occurs because oxygen atoms at these extreme heights are excited by incoming solar particles, but the air density is so low that collisions are infrequent. This lack of collisions allows the excited oxygen atom to take a relatively long time—up to two minutes—to return to its normal energy state, emitting a distinct red photon at a wavelength of 630 nanometers. In contrast, the more common green light is emitted by oxygen at lower altitudes, where higher collision rates quickly quench the red emission process.
Large-scale atmospheric events can also create a widespread red glow by altering the composition of the air. Massive volcanic eruptions or intense, widespread wildfires inject fine dust, smoke, and aerosols high into the atmosphere. This layer of particulates increases the overall atmospheric extinction, intensifying the reddening effect far beyond a normal sunset. The increased concentration of these microscopic particles acts as a powerful filter, scattering away shorter wavelengths and ensuring that only the long-wavelength red light is able to penetrate the atmosphere and linger, sometimes remaining faintly visible well after dark.
Artificial Lighting and Atmospheric Contaminants
In and around urban areas, the red glow is frequently a localized consequence of light pollution, primarily from specific types of street lighting. Older High-Pressure Sodium (HPS) lamps were historically popular for municipal lighting and emit a characteristic yellowish-orange light with a strong component in the red spectrum. This orange-red light is scattered back downward by moisture, fog, and atmospheric contaminants like smog, creating a noticeable dome of light over cities known as sky glow.
A more recent source of intense, localized red light comes from specialized agricultural and industrial facilities. Large-scale greenhouses, especially those involved in vertical farming, use Light Emitting Diode (LED) systems precisely tuned to the light spectrum plants need for growth. Because red light, particularly around 660 nanometers, is highly effective for photosynthesis, these facilities often bathe their crops in a powerful red or magenta light.
The sheer intensity of light escaping from the translucent walls and roofs of these massive structures can create a vivid red or pink glow visible for miles. This artificial light is a concentrated source that, when scattered by local humidity and low clouds, directly contributes to the reddish appearance of the night sky in the immediate vicinity of these operations.