The widespread brightness that hangs over populated areas at night, often called the “light dome,” is a phenomenon known as skyglow. This pervasive glow is the most noticeable form of light pollution, resulting from the excessive or misdirected use of artificial outdoor lighting. Skyglow represents a global alteration of the natural night environment, obscuring the view of stars and planets. This fundamentally changes the natural light-dark cycles that have governed life on Earth.
Primary Sources of Artificial Nighttime Brightness
The origins of the light contributing to skyglow stem from nearly every type of outdoor and indoor illumination in urban and suburban settings. A major contributor is public street lighting, particularly older or unshielded fixtures that emit light horizontally or directly upward. Commercial properties also add significantly through brightly illuminated billboards, advertising signs, and exterior facade lighting.
Industrial facilities, parking lots, and residential areas contribute through light spill from security and decorative fixtures. Even interior building light escaping through windows adds to the ambient brightness. The underlying issue is “uplight,” which is light intentionally pointed into the sky or reflected upward from surfaces like pavement. Unlike localized light trespass, skyglow is the cumulative effect of countless misdirected photons across a large region.
How Skyglow Illuminates the Night Atmosphere
Localized light sources create a massive, glowing dome over cities due to the physics of atmospheric scattering. When light is emitted upward, it collides with particles and molecules within the Earth’s atmosphere. This collision redirects the light in various directions, including back down toward the ground, creating the diffuse glow we observe.
Two primary mechanisms drive this effect: Rayleigh scattering and Mie scattering. Rayleigh scattering occurs when light hits tiny air molecules, causing shorter, bluer wavelengths to be scattered more effectively. Mie scattering involves larger particles, such as dust, aerosols, and water droplets, which scatter all wavelengths of light more uniformly.
The composition of the local atmosphere significantly influences skyglow intensity. Areas with higher levels of air pollution, dust, or humidity experience a more severe brightening effect because these particles increase the amount of light scattered. The presence of low clouds or snow cover can dramatically amplify the effect, as reflective surfaces scatter and reflect the light more efficiently back toward the ground.
Ecological and Health Consequences of Excess Light
The disruption of the natural light-dark cycle by constant nighttime brightness has profound consequences for both human health and natural ecosystems. For humans, artificial light at night interferes with the circadian rhythm, which is the body’s internal 24-hour clock that regulates sleep-wake cycles and other biological processes. Exposure to light after dark suppresses the production of the hormone melatonin, which is essential for regulating sleep and is a natural antioxidant.
The blue light wavelengths, commonly found in modern, high-Correlated Color Temperature (CCT) LED lights, are particularly effective at suppressing melatonin. Chronic disruption of the circadian rhythm has been linked to potential health issues beyond sleep disorders, including increased risk factors for mood disorders, obesity, and cardiovascular diseases.
The ecological impacts are similarly widespread, affecting nocturnal wildlife that rely on darkness for survival. Insects, such as moths, are drawn to artificial lights, disrupting their feeding, mating rituals, and navigation, leading to significant population declines. This decline then affects other species, like birds and bats, that depend on insects as a food source. Migratory birds and sea turtle hatchlings use natural light cues for navigation. Artificial skyglow can disorient these animals, causing birds to stray from their migratory paths and hatchlings to move inland toward city lights instead of out to sea. Even plants are affected, as excess light can alter their photoperiodic responses, which govern seasonal cycles like leaf shedding and flowering.
Effective Strategies for Reducing Light Pollution
Reducing light pollution involves a three-pronged approach focusing on direction, color, and timing. The principle of full shielding is primary, requiring that all outdoor fixtures direct light only downward, ensuring no light is emitted above the horizontal plane. These “full cutoff” fixtures prevent light from escaping into the atmosphere and contributing to skyglow.
Choosing the correct light color is also important, particularly by using warmer Correlated Color Temperature (CCT) lighting. Lighting with a CCT of 3000 Kelvin or lower significantly reduces blue light, which is the wavelength most easily scattered and disruptive to biological systems. For highly sensitive areas, amber or monochromatic red lights are often recommended.
Finally, using adaptive controls manages light intensity and duration. Installing motion sensors ensures lights are only fully illuminated when an area is occupied, while timers can turn off or dim lights during late-night hours.