Skyglow is the diffuse brightening of the night sky over populated areas, caused by artificial light scattering off particles and molecules in the atmosphere. It’s the reason you can see a faint dome of light hovering above a city from miles away, and why most urban residents have never seen the Milky Way. Skyglow is the most visible and widespread form of light pollution, and by citizen science estimates, it’s getting roughly 9.6% brighter each year.
Natural vs. Artificial Skyglow
The night sky has never been truly dark. Natural sources of sky brightness include moonlight, starlight, a faint upper-atmosphere phenomenon called airglow (where gas molecules release photons after absorbing solar radiation during the day), and the combined glow of the zodiacal dust band. These natural sources follow predictable cycles. Moonlight varies on a monthly rhythm, peaking at full moon and dropping to near zero at new moon. Seasonally, winter full moons sit higher in the sky and produce brighter nights than summer ones. Over longer timescales, the moon’s 18.6-year orbital cycle shifts its brightness contribution as well. Together, these patterns create a reliable “lunar clock” that organisms have evolved to use for timing migration, reproduction, and foraging.
Artificial skyglow overwhelms all of this. In cities, sky brightness levels reach six times those recorded in rural areas just nine to twenty kilometers away. That artificial glow masks the natural monthly and seasonal rhythms of lunar light, replacing a dynamic cycle with a constant wash of brightness. For any organism that depends on darkness cues or celestial navigation, the signal is drowned out.
How Light Creates a Dome in the Sky
Skyglow isn’t light you see directly from a lamp. It’s light that escaped upward or sideways from outdoor fixtures, then bounced off tiny particles and gas molecules suspended in the atmosphere before reaching your eyes. Two types of scattering drive this process.
Air molecules themselves scatter light, with shorter (bluer) wavelengths scattering far more efficiently than longer (redder) ones. This is the same physics that makes the daytime sky blue, and it’s why bluish-white LED streetlights contribute disproportionately to skyglow compared to warmer-colored alternatives. Larger particles like dust, soot, and water droplets scatter light differently. Rather than favoring specific wavelengths, these particles redirect photons based on their size and composition, often pushing light forward or downward in concentrated beams.
Air pollution makes skyglow worse. The more aerosol particles in the atmosphere, the more photons scatter back toward the ground, increasing the brightness seen from below. Fog and low clouds amplify the effect dramatically, acting like reflectors that bounce city light across a wide area. On clear, dry nights the glow is somewhat contained; on hazy or overcast nights it can spread far beyond city limits.
How Fast Skyglow Is Increasing
A global citizen science project called Globe at Night, published in the journal Science, found that the number of stars visible to the naked eye decreased steadily between 2011 and 2022. The rate of change corresponds to a 7 to 10% annual increase in sky brightness as perceived by the human eye, with an average of 9.6% per year. At that pace, the sky’s brightness doubles roughly every eight years.
Notably, this increase is faster than what satellites have detected. That discrepancy likely comes from the shift toward LED lighting, which emits more light in wavelengths the human eye is sensitive to but that older satellite sensors don’t capture well. In other words, the sky may be getting brighter even in places where satellite data suggests stable or declining light output.
Effects on Wildlife
Skyglow’s ecological reach extends well beyond city borders. Many species navigate, forage, and reproduce based on natural darkness cues, and a persistent artificial glow disrupts all of these behaviors.
Migratory birds are especially vulnerable. Artificial lighting attracts night-migrating birds from as far as five kilometers away. Once drawn in, they circle illuminated areas, burning through energy reserves they need for long-distance flight and increasing their risk of colliding with buildings. On foggy or low-cloud nights, the problem intensifies: light reflecting off the cloud base disorients birds flying at lower altitudes, and multiple mass-mortality events involving hundreds of birds in a single night have been documented under these conditions. Year-round, illuminated habitats can cause birds to avoid areas they’d otherwise depend on for survival, reshaping local ecosystems. Offshore species face similar problems from coastal lighting and lit vessels.
Insects and birds that navigate using polarized light from the sky, a subtle celestial pattern invisible to humans, lose that compass when artificial skyglow interferes with the signal. For pollinators, predators, and prey species active at night, the consequences ripple through food webs.
Effects on Human Health
Your body uses darkness as a signal to produce melatonin, the hormone that regulates your sleep-wake cycle. Even ordinary indoor room light (under 200 lux, far dimmer than a typical office) is enough to suppress that signal. In a study published in The Journal of Clinical Endocrinology and Metabolism, exposure to room light in the late evening delayed the onset of melatonin production in 99% of participants and shortened the duration of melatonin release by about 90 minutes. When light exposure continued through the entire night, total melatonin output dropped by a median of 73.7%.
Half of the maximum suppression effect occurred at just 100 lux, substantially dimmer than the 350 to 500 lux recommended for office environments. This means the light levels most people encounter at home in the evening are biologically significant. While skyglow itself rarely reaches indoor-light intensities, it contributes to a broader environment where true darkness is increasingly rare. People who sleep in rooms with uncovered windows in brightly lit urban areas receive more ambient light exposure than those in darker settings, compounding the indoor sources.
The Energy Cost of Wasted Light
Not all outdoor lighting produces skyglow. Light aimed downward, where it’s needed, stays mostly out of the atmosphere. The problem is light that escapes upward or sideways from poorly designed or unshielded fixtures. The International Dark Sky Association estimates that 30% of all outdoor lighting is wasted, either left on when not needed or aimed directly at the sky. In 2017, that waste translated to roughly 60 billion kilowatt-hours of electricity in the United States alone, costing more than $6.3 billion and producing over 23 billion pounds of CO2 emissions.
How Skyglow Is Measured
Two main tools track sky brightness. Ground-based devices called sky quality meters (SQMs) measure how bright the sky appears from a specific location, reporting values in magnitudes per square arcsecond (a unit where higher numbers mean darker skies). These devices are sensitive enough to detect changes from cloud cover, correlating with satellite cloud observations at rates above 94%. Satellites, meanwhile, map light emissions from above, showing where light escapes the ground. The two approaches complement each other: satellites reveal which areas emit the most light, while ground-based readings capture what the sky actually looks like to someone standing beneath it, including the scattered and reflected light that satellites can miss.
Reducing Skyglow
The most effective way to reduce skyglow is to keep light pointed downward. Fully shielded fixtures, where the light source is recessed within the housing so no light escapes above the horizontal plane, prevent the upward emissions that fuel atmospheric scattering. Many dark-sky ordinances require full shielding for any outdoor fixture producing more than 450 lumens, roughly the output of a 40-watt incandescent bulb.
Color temperature matters nearly as much as direction. Cooler, bluish-white light (above 3,000 Kelvin) scatters more aggressively in the atmosphere than warmer light. Dark-sky standards typically cap outdoor lighting at 3,000 Kelvin for general use and 2,700 Kelvin for more sensitive areas. Fixtures should also confine their light to the property where they’re installed, avoiding spillover onto adjacent land or into the sky.
Dimming and scheduling round out the approach. Lights that activate only when needed, at the minimum brightness required for safety, eliminate the waste that accounts for a large share of skyglow. Cities that have adopted these combined strategies report measurable reductions in sky brightness without compromising public safety. For the estimated 80% of the world’s population living under light-polluted skies, these are straightforward engineering choices, not sacrifices.