The unnatural dome of light hanging over a city at night is a common result of a widespread environmental issue known as light pollution. This pervasive glow is a direct consequence of artificial light at night (ALAN) escaping into the atmosphere. Although light pollution does not leave a physical substance, it profoundly alters the natural cycle of darkness that has governed life on Earth for millennia. Understanding this glow requires examining the mechanism by which light is scattered, the sources of the light itself, and the resulting impacts on human and ecological health.
The Physics of Sky Glow
The specific brightening of the night sky above populated areas is termed sky glow, and it occurs due to the interaction between upward-directed artificial light and atmospheric particles. Light that is emitted from ground sources does not simply disappear but instead encounters molecules of gas, dust, and aerosols in the air. This process, known as atmospheric scattering, redirects the light back toward the ground from above, creating the diffuse, luminous haze visible for many miles outside a city.
Scattering is governed by two primary mechanisms depending on particle size. Rayleigh scattering occurs when light interacts with tiny air molecules, such as nitrogen and oxygen, scattering shorter, bluer wavelengths more effectively. Larger particles, including water vapor, dust, and pollutants (aerosols), engage in Mie scattering. Mie scattering is less dependent on the light’s wavelength but significantly increases the overall brightness of the glow. Higher concentrations of airborne pollutants and humidity intensify this effect, making the sky glow more pronounced.
The cumulative effect of this scattered light reduces the contrast between celestial objects and the background sky, making star observation difficult. The process turns the atmosphere above an urban center into a large, inefficient secondary light source. This physical mechanism is distinct from light trespass, which is light falling directly onto a property where it is not wanted, as sky glow is the illumination of the air itself.
Key Anthropogenic Sources of Nighttime Light
The source of the artificial light driving sky glow is overwhelmingly anthropogenic, originating from poorly designed or excessive outdoor illumination in urban and industrial centers. A significant contributor is the improper shielding of fixtures, such as streetlights and commercial lamps, which allows a substantial portion of their light to project sideways or directly into the sky. Studies estimate that roughly one-third of all light in developed countries is wasted in this manner due to unshielded or poorly directed lamps.
Modern lighting technology, particularly the widespread adoption of Light Emitting Diodes (LEDs), has compounded the issue. While LEDs are energy-efficient, early generations often emitted a blue-rich white light with a high color temperature, sometimes exceeding 4000 Kelvin. This specific blue wavelength scatters much more efficiently in the atmosphere due to Rayleigh scattering, resulting in a greater geographical spread of sky glow compared to older, warmer-toned lighting.
Beyond streetlights, a concentrated network of light sources from various urban functions contributes to the overall problem. These include over-illuminated commercial properties, brightly lit electronic billboards, and extensive lighting for sports fields and parking lots. The sheer density of these sources in areas of urban sprawl creates a massive light dome that can be detected from space. Even interior light leakage from empty office buildings and homes contributes to the upward flux of photons that eventually scatters to create the pervasive nighttime brightness.
Biological Consequences of Disrupted Darkness
The loss of natural nighttime darkness has profound implications for the biology of living organisms, extending from human health to the function of entire ecosystems. For humans, exposure to artificial light at night suppresses the production of melatonin, the hormone responsible for regulating the sleep-wake cycle. This suppression disrupts the body’s internal clock, known as the circadian rhythm, which has been linked to an increased risk of sleep disorders and various chronic health conditions.
The continuous presence of light alters metabolic processes regulated by the circadian system. Long-term exposure is associated with increased incidence of obesity, diabetes, and cardiovascular diseases. Furthermore, the disruption of the natural rest cycle can contribute to psychological issues, including mood disorders such as depression and anxiety. This physiological impact stems from the fact that human biology evolved under a predictable cycle of bright day and dark night.
In the natural world, nocturnal wildlife relies on darkness for navigation, foraging, and reproduction, and this behavior is easily disrupted by ALAN. Migratory birds, which use celestial cues to navigate their long journeys, can become disoriented by city lights, leading to fatal collisions with illuminated structures. Similarly, sea turtle hatchlings that emerge on beaches rely on the brightest light source—the natural horizon over the sea—to find the ocean, but artificial lights can lead them inland to dehydration and predation. Even insect populations, many of which are nocturnal, are drawn to light sources, where they become exhausted or easy prey, leading to declines that cascade through the food web and affect species that rely on them for sustenance.