The bright night sky above populated areas, known as sky glow, is a form of light pollution caused by human activity. Sky glow is the diffuse illumination of the night sky, visibly brighter than the natural darkness of a remote location. This glow is a direct result of artificial lighting escaping upward from cities. The collective effect creates a visible dome of light that can extend for hundreds of miles, obscuring the view of stars and the natural night sky.
The Origin of Artificial Sky Glow
The sources of artificial sky glow originate entirely on the ground from a multitude of light fixtures. This pollution is not caused simply by the presence of outdoor lights, but by their poor design and excessive use. A large amount of the light contributing to sky glow is either intentionally directed upward or is reflected off surfaces like pavement and buildings.
Major contributors include unshielded streetlights, commercial advertising billboards, and brightly lit sports stadiums. Many older or improperly installed fixtures allow light to spill horizontally and vertically, a problem known as light trespass. Even interior lights escaping through windows add to the glow over urban centers. The issue is compounded by the sheer number of light sources, where the combined effect of thousands of individual lights creates the widespread light.
How Light Brightens the Upper Atmosphere
The light emitted from ground sources encounters the Earth’s atmosphere and is scattered in all directions, rather than traveling simply into space. This scattering process transforms focused beams of light into a diffuse, widespread glow. The atmosphere contains gas molecules, like nitrogen and oxygen, and aerosols, which are tiny particles of dust, soot, and water droplets.
Light scattering from gas molecules is primarily governed by Rayleigh scattering, which explains why the sky is blue during the day. This principle dictates that shorter-wavelength light, such as blue light, is scattered much more effectively than longer-wavelength red light. Light interacting with larger particles like dust and water vapor is described by Mie scattering.
The scattered light is redirected, with some of it traveling back down toward the ground, creating the visible dome of sky glow over cities. This effect is exacerbated by atmospheric conditions such as humidity, fog, and air pollution, which provide more scattering agents. Clouds over a city can reflect the light back down, increasing the brightness of the night sky by a factor of up to 1,000 compared to a clear night.
Ecological and Health Consequences
The constant presence of sky glow disrupts the natural 24-hour cycle of light and dark, impacting human and ecological health. For humans, exposure to artificial light at night, particularly blue-rich light, suppresses the production of the hormone melatonin. This hormonal disruption interferes with the body’s circadian rhythm, leading to sleep disorders and linking to increased risks of obesity, diabetes, and cardiovascular diseases.
Ecologically, the bright night sky disorients numerous species that rely on natural darkness for survival. Nocturnal insects, such as moths, are attracted to and trapped by artificial lights, which disrupts their mating and feeding behaviors and leads to population decline. Migratory birds use celestial cues for navigation, and sky glow can cause them to become confused and veer off course, often with fatal results.
The natural process of sea turtle hatching is also severely impacted. Hatchlings use the light of the moon and stars reflecting off the ocean to find their way to the water, but artificial light sources along coastlines often draw them inland instead. This leaves them vulnerable to predators and exhaustion. Furthermore, light pollution has rendered the Milky Way invisible to more than one-third of the world’s population, representing a cultural and scientific loss.
Strategies for Reducing Sky Brightness
Mitigating sky glow is achievable by focusing on three main principles of responsible outdoor lighting design.
Full Shielding
Full shielding requires light fixtures to be fully covered on the sides and top so that all light is directed downward onto the intended area. This prevents light from escaping horizontally or upward into the atmosphere, immediately reducing the source of the glow.
Appropriate Light Levels
The second principle involves using appropriate light levels through dimmers, timers, and motion sensors. By reducing the intensity of light or ensuring it is only active when necessary, communities can lower the overall amount of light available to be scattered.
Warmer Color Temperatures
Finally, selecting warmer color temperatures for lighting, specifically 2700 Kelvin or lower, is a highly effective strategy. These warmer lights emit less of the short-wavelength blue light that scatters most easily, thereby reducing the intensity and reach of the resulting sky glow.