How Light Pollution Impacts Cloudy Night Sky Views

Light pollution is the skyglow from artificial lights, which contaminates the natural night sky. Before artificial lights, clouds hid the light coming from the stars. Now, because of light pollution, the sky grows brighter when clouds roll in. The impact of clouds on the brightness of the night sky depends on the level of cloudiness, the altitude of the clouds, and the population density of the area. Clouds are a kind of atmospheric factor that scatters artificial light from the ground, and they have a significant impact on the brightness of the night sky. Research has shown that cloud coverage dramatically amplifies sky luminance. On overcast nights, the sky can be up to 10 times brighter than on clear moonless nights. This has important implications for chronobiological and chronoecological studies in urban areas, as the amplification effect of clouds on light pollution has previously not been considered.

Light pollution and cloudy skies: a complete flip

Light pollution is the skyglow from artificial lights, which contaminate the natural sky at night. Before artificial lights, clouds hid the light coming from the stars. Now, because of light pollution, the sky grows brighter when clouds roll in. This is a complete flip from how things were.

The night sky glows from light pollution, and clouds are a kind of atmospheric factor that scatter artificial light from the ground most effectively. Therefore, they have a significant impact on the brightness of the night sky.

There is a clear linear dependence of night sky brightness and cloudiness. The brightness of the night sky depends on the altitude of the clouds' base. Most of the night sky brightness comes from the light scattered on the lowest altitude clouds, while the least contribution comes from the light scattered on high-level clouds.

Light pollution gets worse the higher the humidity in the atmosphere. Humidity forms aerosols, ice crystals, fogs, and instrument dew, which get in the way between the observer and the star.

The scientists tested night-sky brightness with sky-quality meters at 44 locations around the world, including a mix of rural, suburban, and city sites. Of the 22 cloudy sites examined, only two grew darker when clouds obscured the sky. The remaining locations brightened as the clouds reflected light pollution.

The researchers also discovered that night sky brightness varied dramatically around the world. The brightest nighttime measurement was recorded in the Dutch town of Schipluiden, outside of Delft. Light pollution in Schipluiden was 10,000 times brighter than the darkest night sky detected in the study, which was reported from the Kitt Peak National Observatory in Arizona.

Light pollution and the impact on animal behaviour

Light pollution has a significant impact on animal behaviour. Animals and plants have evolved to rely on the natural light of the sun, moon, and stars to govern their behaviour and life processes. These include patterns of rest and activity, growth, reproduction, migration, and navigation. Light pollution disrupts these natural cycles, affecting the health and wellbeing of wildlife.

Attracting and Trapping Organisms

Artificial light attracts some organisms, such as moths, frogs, and sea turtles, leading them away from their natural habitats and disrupting their normal behaviours. For example, moths are drawn to artificial lights, which can act as a trap, exhausting and killing them. Similarly, the glow from coastal cities can disorientate female sea turtles as they look for suitable beaches to nest, leading them towards roads where they are at risk of being hit by cars.

Repelling Organisms and Habitat Loss

On the other hand, artificial light repels some organisms, excluding them from habitats where they could otherwise survive. This effectively results in habitat loss. For example, bats, an important part of the local insect population, may be drawn to lit areas for easy foraging, while others avoid these areas, leading to competition and disruption of the local ecosystem.

Altering Day/Night Patterns

Artificial light alters the natural day/night patterns of both diurnal and nocturnal creatures. Nocturnal animals may not get enough sleep or downtime for their bodies to repair themselves, and their reproductive cycles may be affected. For diurnal species, such as songbirds, artificial light may cause them to breed too early or make them more susceptible to predators as they reveal their location by singing.

Impact on Food Availability

Light pollution can also affect the availability of food for some species. Nocturnal animals may be deterred from feeding in lit areas due to the risk of predation. For example, bats, which are important pollinators and predators of insects, may avoid lit areas, impacting the local ecosystem.

Disrupting Migration

Migratory birds that navigate by moonlight and starlight can be disoriented by artificial light, causing them to wander off course and towards dangerous urban landscapes. Additionally, artificial light can bleach their visual pigments, causing them to lose sight of the horizon and circle endlessly, leading to exhaustion or collision with light sources.

Impact on Reproduction

Artificial light has been found to alter the nesting and breeding behaviours of some species. For example, in frogs, it can affect nest hiding behaviour and calling, reducing their breeding success.

Benefiting Invasive Species

Light pollution can also benefit invasive species, such as cats, foxes, and cane toads, which take advantage of artificial lights to feed, further disrupting ecosystems.

Overall, light pollution has far-reaching consequences for animal behaviour, health, and ecosystems, and it is important to take steps to reduce its impact on the natural world.

Light pollution and human health

Light pollution is the human-made alteration of outdoor light levels from those occurring naturally. It has been shown to have harmful effects on human health.

Circadian Rhythm and Melatonin

Humans, like most life on Earth, adhere to a circadian rhythm — a sleep-wake pattern that is governed by the natural day/night cycle. This cycle is largely influenced by photoreceptors in the eye that are more sensitive to blue wavelengths of light. Exposure to artificial light at night can disrupt this cycle.

Melatonin Suppression

Nighttime exposure to artificial light also suppresses melatonin production, which is needed to keep us healthy. Melatonin has antioxidant properties, induces sleep, boosts the immune system, lowers cholesterol, and helps the functioning of the thyroid, pancreas, ovaries, testes, and adrenal glands.

Blue Light

Exposure to any light at night can suppress melatonin, but the photoreceptors in our eyes that drive this suppression are more sensitive to blue light. Unfortunately, most LEDs used for outdoor lighting, as well as computer screens, TVs, and other electronic displays, create abundant blue light.

Glare

Glare from poorly shielded outdoor lighting also harms human health because it decreases vision by reducing contrast. This limits our ability to see potential dangers at night, with aging eyes being especially affected.

Health Risks

According to the American Medical Association, “It is estimated that white LED lamps have five times greater impact on circadian sleep rhythms than conventional street lamps. " Exposure to artificial light at night has been linked to an increased risk of obesity, depression, sleep disorders, diabetes, breast cancer, and more.

Light pollution and the weather

Light pollution is the skyglow from artificial lights, which contaminate the natural night sky. The night sky in cities is almost always as bright as that naturally experienced during a high summer moon.

Clouds are a type of atmospheric factor that scatter artificial light from the ground most effectively. They have the most significant impact on the brightness of the night sky. The paper by Tomasz Ściężor analyses the influence of both the level of cloudiness, as well as the type and altitude of clouds, on the amplification of light pollution.

A linear correlation between cloudiness and the brightness of the night sky was found. The impact of cloudiness on the brightness of the night sky was researched in places with different levels of light pollution. Measurements of meteorological elements were used together with cloud genus assessments.

It was found that among the nine genera of the identified night clouds, the Altocumulus, Cirrocumulus, and Cumulonimbus clouds are responsible for this correlation. The brightness of the night sky depends on the altitude of the clouds' base. Most of the night sky brightness comes from the light scattered on the lowest altitude clouds, while the least contribution comes from the light scattered on high-level clouds.

The ambient light level is one of the strongest factors driving animal behaviour and chronobiology. Light pollution, which causes the "light dome" over urban areas, is an unintended result of electric lighting. Approximately 10% of the world's population and more than 40% of the US population no longer view the night sky with dark-adapted vision.

The presence of clouds significantly brightens urban skies and amplifies the degree of ecological light pollution. The luminance of the night sky in Berlin is 10.1 times brighter on overcast nights than on clear moonless nights, and 4.1 times brighter than that observed at rural locations on the brightest clear nights with a high moon.

Haze depends on the density of aerosols in the atmosphere and also on the kinds of aerosols they are. Aerosols are suspensions of microscopic particles that scatter and/or absorb light. In theory, an aerosol can be composed entirely of microscopic water droplets. But in practice, pure water is very reluctant to form into droplets, even when the humidity is over 100%. Droplets typically form around some kind of pollutant.

Light pollution gets worse the higher the humidity in the atmosphere. More precisely, skyglow increases dramatically when the sky is hazy. The prevalence of haze depends on the humidity and the quantity and type of artificial and natural pollutants.

Light pollution and the moon

Light pollution is the skyglow from artificial lights, which contaminate the natural sky at night. Before artificial lights, clouds hid the light coming from the stars. Now, because of light pollution, the sky grows brighter when clouds roll in.

Clouds are a kind of atmospheric factor that scatters artificial light from the ground most effectively. They have the most significant impact on the brightness of the night sky. There is a clear linear dependence of the night sky's brightness and cloudiness.

The brightness of the night sky depends on the altitude of the clouds' base. Most of the night sky's brightness comes from the light scattered on the lowest altitude clouds, while the least contribution comes from the light scattered on the high-level clouds.

The night sky's brightness also depends on the genus of the clouds. Altocumulus, Cirrocumulus, and Cumulonimbus clouds are responsible for the linear correlation between cloudiness and the brightness of the night sky.

Light pollution is worse when the humidity is higher. Humidity forms aerosols, ice crystals, and fogs, which are impediments between the observer and the stars.

The Moon's natural cycles are almost extinct in cities because of light pollution. When the Moon rose in cities on overcast nights, the sky's brightness barely changed. Yet in rural settings, the sky brightened considerably on cloudy nights after moonrise.

Frequently asked questions