Summary: ​Natural cycles of light and dark are needed for ideal wellbeing. Artificial light at night disrupts this rhythm, but does it lead to ill health? This post explores the surprisingly complicated relationship between light at night and human health, and what we still need to learn to find the optimal balance between them.

Light at night (and light pollution) are in the news these days for a reason that may surprise some readers. Consider some recent headlines:

"Bright outdoor light at night may increase stroke risk"
"Does More Outdoor Light at Night Help Cause Macular Degeneration?"
"The light you see may be as important to your health as the food you eat"
"Getting Too Much Bright Light at Night May Increase Your Cancer Risks"

The popular media are notorious for running stories about human health that seem contradictory. What's good for people one week seems harmful to them the next. Some of this results from the ways the media report news about science in general. But more often it reveals something about the often messy process by which we collect and judge evidence.
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The same is true of questions about light and health. What do we really know about this subject? While some conclusions drawn by researchers are concerning, we're still a long way from definitive answers. In this post, we'll dig into the evidence and learn whether there's a real 'dark side' to light at night.

Scientists study artificial light at night (ALAN) and its effects on human health along three lines of enquiry:

• Laboratory studies in controlled conditions
• "Real-world" cohort studies
• Correlation studies that match remote measurements of light with data on disease incidence

These are standard research approaches and none of them is especially problematic. Of course, there are always details of study design that are better or worse in different regards. But these basic methods are well understood, as are the limitations of their results. To get the "big picture" of what ALAN may do to our health and wellbeing requires using all the tools we have.

Laboratory studies are usually small, involving a few volunteers exposed to ALAN. Researchers measure their responses to light with various tests. While the controlled exposures mean that the "dose" of light is definite, the sample sizes are small. Information they yield can be compared against results from cohort studies. These look at effects in larger groups of people ("cohorts") in situations where the exposure isn't controlled as well. The idea there is that the larger sizes of cohorts make it easier to find the individuals who are not very representative of the group. If the presumed health effect in a cohort study is real and prevalent, it should stand out in the results.

Both laboratory and cohort studies of ALAN and human health tend to consider only indoor light exposures. And those studies show that ALAN affects people in ways that harm them. (For a comprehensive review of what we know about ALAN and health, see DarkSky International's "Artificial Light at Night: State of the Science 2023" report.) Effects reported in scientific papers include:

• Sleep problems
• Increased inflammation throughout the body
• Temporary changes at the genetic level
• Metabolic disorders (obesity, diabetes, etc.)
• Higher risk for cardiovascular disease and certain kinds of cancer
• Problems with mental health including mood disorders

​These observed changes largely stem from the tendency of ALAN to disrupt the body's "circadian rhythm". This is a roughly 24-hour cycle of biological activities tied to the rising and setting of the Sun. Over billions of years, living things became sensitive to the reliable sequence of light-dark-light-dark in nature. ALAN mimics certain qualities of sunlight, which confuses organisms. And because ALAN is so new in the environment, they have not yet evolved adaptations to it.

All this marks ALAN as an emerging "lifestyle risk" for people. We use light at night for many reasons associated with modernity and progress. Science is telling us that there can be too much of a good thing in that regard. But so far we have only considered evidence linked specifically to indoor light at night exposure. Should outdoor exposures also concern us?

That's where the picture gets murky.

The third kind of study mentioned above attempts to answer the question by looking down from overhead. The world's cities shine at night with an intensity that makes them visible to astronauts aboard the International Space Station. Since the 1970s, satellites orbiting the Earth have measured "nighttime lights" (NTL) on our planet.

The signals they sense represent only a fraction of total light emissions. While some light emitted upward escapes the atmosphere completely, other light scatters back down to the ground. Simple models of the situation imply some relationship between light levels on the ground and the intensity of NTL seen from orbit. Reality is more complicated, and the actual relationship can be quite complex.
Potential trouble results when researchers use NTL measurements in human health studies. The idea seems reasonable enough: match up NTL data with information about the spatial distribution of various maladies. Of course, there are many other social and environmental factors that influence human diseases. Once we account for and remove those factors in the data, only the effects of ALAN should remain. But do they?

On the surface, the answer appears to be "yes". Studies find correlations — that is, mutual relationships or connections between two or more variables. People who live in areas with higher NTL levels do seem to suffer more from various illnesses than people who live in darker places. That conclusion persists even after eliminating other possible explanations for the results.

It is said, famously, that "correlation is not causation". While correlation is the appearance of a relationship between variables, it says nothing about why they are related. Causation is the idea that cause precedes effect, and that for every effect there is a cause. Nature and society are rife with spurious correlations, in which there is no plausible cause-and-effect relationship.

It's fair to ask whether there's an underlying cause for the observed correlation between NTL intensity and health problems. Do the laboratory studies sufficiently establish a causal link? We don't know. They reveal a clear connection between ALAN exposure and ill health in both individuals and groups. But they don't (yet) tell us what the threshold of harm is.

Researchers refer to the "dose-response" relationship between cause and effect. So far, we don't have a strong handle on that even under controlled conditions. In the case of outdoor lighting, we know even less about individual exposures. And there's reason to believe that the majority of light exposure most people get is from distinctly indoor sources. (Think about watching television or staring at the screen of your phone late at night, long past your usual bedtime.)
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The bottom line is that the effect of outdoor lighting isn't zero, but the amount of indoor light exposure most people get probably dwarfs it. Most studies claiming one health effect or another from ALAN rely on NTL data as an estimate of actual exposures. But therein lies the problem: we don't know what those exposures really are. So we can't take extreme positions like "streetlights give people cancer". The evidence just doesn't support that.

So what can we conclude from all this? Is it hopelessly confused?

For the benefit of preserving health and wellbeing we should try to limit all ALAN exposure, whether from indoor or outdoor sources. There is little scientific doubt that the observed connection between light and health is real. The hypothesis about why, with its strong biological underpinnings, is sound.
Some of it has to do with timing. During the daytime, we need exposure to bright light, especially that rich in short wavelengths (i.e., blue). This helps calibrate the internal 'clock' that maintains the circadian rhythm. Inadequate daytime light exposure is almost as harmful as too much ALAN exposure. While humans have some resilience to seasonal changes to the length of the day, we have no natural defense against the influence of ALAN.

What can we do with the information we have in hand, knowing it is incomplete? There is a case for taking steps to limit ALAN exposure, especially in sleeping rooms at night. Those spaces should be completely dark to avoid disrupting the circadian rhythm. Sleeping with lights on at night is not advised. Good "sleep hygiene" includes avoiding bright light exposure in the hours before going to bed at night.

Heavy window coverings may be needed to prevent the intrusion of outdoor lighting into sleeping rooms. While we know that exposure to bright indoor sources of light at night is bad for people, we know much less about how very dim outdoor lighting coming in through windows affects us. As one prominent light/health researcher once put it, "Photons are photons, and the body and brain don't care where they come from."
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The owners of outdoor lighting can take care to ensure light is not shining into people's homes. Public policies governing lighting can assist by making this "light trespass" against the law. They can also assist by limiting the intensity, color, and hours of operation of outdoor lighting. That approach also benefits nocturnal wildlife, because plants and animals can't go indoors and draw the blinds to avoid ALAN exposure.

When practiced well, science is a remarkable self-correcting process. New information suggests new explanations that replace old ones. That is clearly true for health research in general, and more so for efforts to understand how ALAN affects our wellbeing. Although we have learned much, there are still significant unanswered questions. These include:

• Does measured exposure to ALAN in outdoor space affected human health?
• Does outdoor light at night entering indoor spaces affect sleep and health?
• Are the observed correlations between outdoor ALAN and health problems the result of cause and effect?

These questions suggest directions for future research. We need more compact and reliable light sensors small enough that study participants can wear them. Such "wearable dosimeters", especially those that provide color information in addition to light intensity, are a potential game-changer. They may be able to better discriminate the indoor and outdoor components of ALAN exposure to disentangle their effects. That would, in turn, make light and health research less dependent on NTL measurements as proxies for real exposure data.

Of course, that is not to say that we should stop relying on satellite measurements altogether. But the available nighttime satellite data isn't optimized to address health research questions. It often doesn't allow for detailed identification of outdoor light sources in relation to where people may be seeing them. Satellites may pass over a given location at night at irregular intervals or at times of night when little human light exposure is expected. And some don't detect all the lights we know exist on the ground.

Researchers are supplementing existing NTL data sources with new ones. Some have experimented with platforms closer to the ground, such as high-altitude balloons and unmanned aerial vehicle (also known as 'drones'). These get much closer to light sources and can measure them in more detail. They are coming up with new ways to ground-truth the remote measurements by collecting information about lighting on the ground. As scientists pair these new data about outdoor lighting with better measurements of the 'dose' it yields, the quality of health research results will improve.

​When electric lighting was introduced to the world in the late nineteenth century, it was celebrated for its myriad benefits to society. Almost 150 years on, we now understand it to represent both promise and peril. As our knowledge increases, we may yet find ways to balance risk and reward that cater to social needs for ALAN while avoiding poor health outcomes.

Summary: Although subject to widely held folk beliefs, the truth about how outdoor lighting and crime interact is very unsettled scientifically. This post examines the evidence, identifies shortcomings in current research and practice, and suggests the factors and considerations that really matter.

Summary: Light pollution represents wasted electricity, which in turn means millions of tons of greenhouse gases added to the atmosphere every year. Even when we use energy-efficient outdoor lighting, the world's never-ending tendency to consume more resources causes increasing emissions of climate-warming carbon. Reducing our use of outdoor artificial light at night can help solve both the problems of light pollution and global climate change.

In January 2024 we wrote about a big idea that follows from U.S. environmental policy history: the idea of a "Clean Night Skies Act". In a similar vein, many people are looking toward policy models to address other big environmental problems like global climate change. That, too, intersects with light pollution, energy production and consumption, and environmental policy. In this month's post we focus on the ways in which light pollution is an influence on climate change — and what we can do about it.

We light the world at night in remarkably inefficient ways. Think about the typical street light, for example. From source to destination, where does the light go? More importantly, how much of the light reaches the intended target?

Some loss occurs within the light fitting or fixture itself. Much of the light may not reach the target, instead illuminating things and places where it is neither wanted nor needed. Often the light travels upward into the night sky, either directly from the source or after reflecting from a surface. And much of the world's outdoor lighting is operated at times when no people are around to make use of it.

The result of all these effects is that very little light actually makes it to the viewer's eye. By one estimate, less than one photon, or particle of light, out of move than 22 million reflected from a surface is actually useful for human vision at night. [1] That means that we generate considerably more light at night than ever reaches viewers' eyes. The resulting sense of waste is staggering.

In turn, it means that we use a lot of energy to power all that lighting. DarkSky International estimates that the world spends at least $50 billion each year to produce light at night that escapes directly to space. [2] In the U.S. alone, outdoor lighting consumes some 380 terawatt-hours of electricity per year. That's enough to power 35 million homes for one year. And well under 1% of that energy produces light that achieves its intended effect.
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All that wasted energy comes at more than an economic cost. The world produces over 60% of its electricity by burning fossil fuels. [3] If even 1% of that production takes the form of wasted light at night, that's still 146 million tons of carbon dioxide emitted into the atmosphere every year. [4] Although renewable energy sources make up an increasing share of total world electricity production, they're not yet enough to offset these emissions. Human-caused carbon emissions continue putting more heat energy into our atmosphere with increasingly dire consequences. There's no escaping the conclusion: light pollution is making the problem of climate change worse.

When confronted with this news, many people think that it can't be as bad as it sounds. After all, we live in the era of very energy-efficient light sources like light-emitting diodes (LEDs). And at a certain level, that's true: LED and other kinds of 'solid-state lighting' are very efficient. In fact, many LED lighting products routinely achieve efficiencies about ten times higher than earlier technologies like incandescent filament lamps.

If LED replaced those old lamps on a one-for-one basis, we would expect global energy use from lighting to start coming down. In 2017 the United Nations Environmental Programme estimated that a transition to energy-efficient lighting would reduce global electricity demand for lighting by 30–40% by 2030. [5]

But that doesn't seem to have happened yet. In fact, it may be that there has been no energy benefit from LED lighting at all. Along with the rapid adoption of LED for its perceived energy savings came a vast decrease in the cost of LED lighting equipment. The price of that equipment fell by a factor of 15 during the decade of the 2010s, [6] making LED suddenly very affordable. As the up-front cost of replacing earlier technology with LED declined, the "payback time" through energy savings decreased. Many governments felt they couldn't turn down the opportunity LED presented.

This is an example of what economists refer to as "elastic demand": as the price of something drops, demand for it tends to goes up. We experience this in everyday situations. For instance, when the price of gasoline/petrol goes down, do you drive less or more? In this case, plummeting LED prices seems to have led to strong elastic demand for lighting products.

In the first half of the 2010s, satellite measurements of light at night revealed steady increases over much of the world. Those increases mirrored changes in global economic output during the same period. The authors of a 2017 study on this concluded that "the results presented here are inconsistent with the hypothesis of large reductions in global energy consumption for outdoor lighting because of the introduction of solid-state lighting." [7]
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It is possible that there has been no net environmental benefit due to the introduction of highly energy-efficient lighting. If this is true, then then situation becomes circular. The very factor that influenced the rush to adopt this technology was not realized in the end. If anything, the available evidence suggests that LED encouraged the use of more light at night. It seems to have resulted in installing lighting in places it where didn't previously exist and adding more to places where it did. One way to look at the outcome it is that the world traded one environmental problem for another.

Maybe we have the notion of "efficiency" all wrong. Lighting scientists and engineers, and policy makers following their advice, focused on raw energy consumption. But their obsession with "lumens per watt" seems to have led us down the wrong path. Worse, given certain realities in terms of human behavior, LED may have not achieved its stated goals at all. It made light cheap, and people consumed more of it without attention to its negative consequences.

Real energy savings, and real carbon emissions reductions, come from cutting consumption of light at night. The Five Principles for Responsible Outdoor Lighting embody this idea:

1. All light should have a clear purpose
2. Direct light only toward where it’s needed
3. Illumination should be no higher than necessary
4. Apply lighting only at times when it is useful
5. Use warmer-colored lights where possible

Adherence to these principles has the immediate result of eliminating pure waste. More savings come from lowering light levels to safe thresholds and not exceeding them. In addition, the use of adaptive lighting controls tailors availability of light to meet needs, extinguishing it otherwise.

The way to turn the corner on the problem of light pollution fueling climate change is to encourage mindful application of outdoor lighting. This model has the effect of reducing both energy consumption and light pollution. In turn, reducing electricity use lowers costs and encourages resilience in the face of climate change.
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The good news is that there is no down side to this approach. It does not entail giving up anything to achieve the stated benefits. Illumination levels remain adequate, preserving public safety. Governments and end users realize the benefits of energy and cost savings. The nighttime environment suffers less from light pollution. And in world that desperately needs a "win" on environmental issues, reducing the impact of light pollution may be the easiest win of all to secure.

[1] Bará, S. (2022). A note on the overall efficiency of outdoor lighting systems. Zenodo. https://doi.org/10.5281/ZENODO.6588229
[2] DarkSky International. (2022 August 2). "Light is Energy: Estimating the Impact of Light Pollution on Climate Change". https://darksky.org/news/light-is-energy-estimating-the-impact-of-light-pollution-on-climate-change/
[3] International Energy Agency. (n.d.) "World Energy Balances: Overview" https://www.iea.org/reports/world-energy-balances-overview/world
[4] International Energy Agency. (March 2023) "CO2 Emissions in 2022" https://www.iea.org/reports/co2-emissions-in-2022
[5] United Nations Environmental Programme. (2017 October 2). Accelerating the global adoption of energy efficient lighting. https://www.unep.org/resources/publication/accelerating-global-adoption-energy-efficient-lighting
[6] Freeing Energy. (n.d.) "Energy Fact – Cost of LED lighting dropped 15-times as volumes increased" https://www.freeingenergy.com/facts/led-bulb-light-cost-price-historical-decline-g213/
[7] Kyba, C., et al. (2017). Artificially lit surface of Earth at night increasing in radiance and extent. Science Advances (Vol. 3, Issue 11). https://doi.org/10.1126/sciadv.1701528

Summary: Recent years have seen the 50th anniversary of landmark U.S. environmental policies that achieved tremendous success in pollution reduction and natural conservation. As the awareness of artificial light at night as a significant source of environmental pollution rises, we examine whether a similar federal regulatory approach could meaningfully reduce light pollution in the United States.

​Light pollution is a serious environmental problem, but we have effective technical solutions in hand. Where the political will exists to confront the problem, public policy enshrines the decision and brings consequences if the law is not followed. In the United States, policies governing the use of outdoor lighting are usually made at the local level. With tens of thousands of such local governments, achieving change in the U.S. has been slow. But environmental history offers an alternative approach that could produce meaningful results. We call this effort the "Clean Night Skies Act". Is it a realistic option?

​2019-2023 saw the 50th anniversary of  several landmark U.S. environmental policies. These include the Clean Air Act, Clean Water Act, Endangered Species Act, and the National Environmental Policy Act. The period from the mid-1960's to the early 1970's marked a major shift in the way the U.S. manages pollution control and natural conservation. Before that time, the few environmental policies existed. There was not yet a broad understanding of the potential for human activities to harm the environment. But it was becoming clear that those activities were often harmful to both people and wildlife.

The pace of environmental change accelerated after the Second World War. Pollution from heavy industry fouled the air over U.S. cities and poisoned its waterways. In 1962, the American conservationist Rachel Carson published Silent Spring, a book now considered a classic of environmental literature. Focusing on the effects of the indiscriminate use of chemical pesticides, Silent Spring was a clarion call. Many regard its publication as the start of the modern environmental movement.

In considering the problems that Carson described, it became evident that many of the underlying problems are cross-boundary. That is, they don't respect jurisdictional boundaries. Air and water pollution drift from the places they are emitted into other regions. Endangered species can roam across large territories. Earlier attempts to control pollution and promote conservation at more local levels thus failed, and environmental quality in many places in the U.S. was poor by the mid-1960s. This was the result of rapid economic expansion, population growth, industrialization and urbanization in the century since the Civil War.

​Public attention to the problem rose during the second half of the 1960s. Two environmental disasters in 1969 were turning points in the story: the Santa Barbara oil spill in January-February and the fire on the Cuyahoga River in June 1969. Life magazine documented the latter with a photo spread in its August 1 issue, and Americans were horrified by what they saw.

Faced with rising public alarm, Congress took action. Investigations showed decades of federal mismanagement of America's environment and natural resources. The problem and its proximate causes were clear, and the public demanded significant change. In the span of only four years, Congress created all the major pieces of modern federal environmental policy.

Those laws were spectacularly successful. Emissions of air pollutants in the U.S. have dropped almost 80% since 1970. About 99% of all creatures listed under the Endangered Species Act have avoided extinction. If efforts to address these problems were undertaken only at the local level, it's unlikely these incredible results could have been obtained.  And some states have enacted their own stronger or more restrictive policies governing environmental issues within their boundaries. 

The U.S. federal government makes and implements environmental policy through a distinct process. Congress investigates issues and sets high-level goals and requirements in legislation. These acts establish new agencies to write regulations flowing from the goals of the legislation, or task an existing agency with the job. Agencies then write regulations through the federal rulemaking process, which includes public input. Enforcement actions are brought either within the agencies or through the federal courts. Courts may overturn regulations or acts of Congress in part or in whole. Congress may revisit environmental legislation and amend it if things aren't working well. And even when environmental policies become law, it usually takes the efforts of outside intervenors to ensure that the government enforces the laws as it should.

How might this approach work if applied to reduce light pollution?

First, Congress would take up the matter by putting its on its legislative agenda. Interested members of Congress may direct their staff to collect information on the topic. It may hold hearings on the subject, and some Members may state their willingness to sponsor legislation. Either chamber, or both, could consider simple resolutions to the effect that it is the sense of the chamber that the issue is important.

Up to this point, the political risk associated with participation is low. If Congress moves toward writing new legislation, the stakes suddenly become higher. Depending on the exact content of a bill, legislators may be more or less willing to support the effort. It can take many years or even decades to achieve the consensus in Congress required to move major legislation. Once the consensus emerges, the rest of the process can move with lightning speed.

The Clean Night Skies Act is patterned on the environmental bills of the 60's and 70's. The text of the bill itself need not be lengthy or complex. It finds that, under some circumstances, artificial light at night (ALAN) is a harmful environmental pollutant and makes light pollution reduction a priority for the country. It then tasks the Environmental Protection Agency with writing regulations aimed at reducing its influence. As with other environmental pollutants, the EPA would determine "safe exposure" thresholds to ALAN. It would direct states and municipalities to come up with their own plans and policies to achieve light emissions standards. And just like in the Clean Air and Water Acts, it would fine jurisdictions that failed to hit the targets.

A lot of factors favor taking this kind of approach. There is a scientific consensus on the significance of the problem. We're not waiting for a technical solution to that problem — we know what works.  The solution involves merely reducing wasted outdoor light at night, which has no obvious downside. And better outdoor lighting that reduces light pollution has the extra effect of improving nighttime visibility, which in turn enhances public safety.
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But there is a lot that must happen first before we can get there. Light pollution really isn't on the federal policy agenda right now. Only some states have confronted the issue with legislation, but more seem interested every year. A Clean Night Skies Act starts with building a coalition of supporters in Congress. We must articulate a rationale for policy change (we're working on that). We have to educate Congressional staff about light pollution and ensure continuity across election cycles to keep the issue on their radar.

It's true that the Clean Night Skies Act faces significant headwinds right now. The American electorate is bitterly divided over almost every issue. Light pollution is often framed as an  "environmental" issue, making it an instant turn-off for some voters. We have to overcome complacency and political malaise that prevents the country from dealing with its problems. The American public perceives that outdoor ALAN is cheap to consume and has no downsides to its use; plus, people are afraid of the dark. Congress is in a state of near-total paralysis, enacting fewer than one percent of introduced bills this term. And there is some concern that light pollution as an environmental concern could be swept aside by attention to climate change.

More importantly, light pollution hasn't yet had a "river on fire" moment that tips the balance toward a definitive solution. It is still acceptable in our society, given that its manifestations are often unseen by most people. Of those who know about light pollution, many associate it with astronomy and conclude that it's not a problem for them. As the world continues to brighten, we risk this "window of acceptability" both shifting and widening. In the meantime, there are alternatives. We could invoke existing environmental laws and argue before courts that they do (or at least should) apply to light pollution.

But there are difficulties with this. In 2010 a group of light pollution activists sued the EPA, which had resisted calls to regulate light pollution under the terms of the Clean Air Act. The court sided with the government, finding that the Act's definition of "air pollutant" did not include ALAN. The message to the petitioners was, in short: if you want Congress to regulate light pollution, you must ask it to do so in the form of either new legislation or an amendment to the Clean Air Act. Although some other argument might carry the day, it seems that courts are unlikely to rule differently in the future.

Another stopgap option is to convince a presidential administration to take action by executive order. As the Biden Administration took office, DarkSky International urged the President to issue an executive order regulating outdoor lighting on federal property. So far the Administration has not responded to the request. Even if it chose issue such an order, its effect would be limited in scope and fall far short of what the Clean Night Skies Act aims to achieve. Still, doing so would send a strong message to Congress that could help the Act's chances of becoming law in time.

Lastly, we should continue to watch the ongoing experiment in state legislatures across the country. Several states have enacted pieces of light pollution legislation in the past 25 years. Others are considering doing so in the current or coming legislative terms. There are also opportunities to strengthen building and energy codes in the states, which sometimes govern how outdoor lighting is used. 

So where do we go from here? It pays to pursue an approach with many prongs. We shouldn't wait around for Congress to take action. For now, we can continue working from the bottom up from municipal councils to state legislatures. Advocates can raise the issue with members of Congress and their staffers, informing and educating them about both the problem and its solutions. We should state what it is we want from the federal government in the way of policy, and use that agenda to draft prospective legislation.

If all this sounds like a major undertaking, that's because it is. Big problems often become inter-generational efforts as one group works on the project for some years and then hands it off to their successors. The Clean Night Skies Act may take many years to become the law of the land. Its success requires sustained attention, which in turn means keeping eyes on the prize. The future of our nighttime environment may depend critically on its success.

Summary: ​The extent and brightness of skyglow depends significantly on what happens near the ground. Rain, snow and ice can all change how artificial light moves through the nighttime environment, and the way we measure and model light pollution must adapt to ever-changing weather conditions. 

Winter is underway in the northern hemisphere. For many people who live at northerly latitudes, this means cold weather. There are many factors that effect how bright the night sky is, which we wrote about here and here. (Note that we also previously explored whether holiday lighting is a concern this time of year. ) Winter weather patterns can become of those influences. How do these patterns affect the ways artificial light at night (ALAN) moves through outdoor spaces? And how does this in turn change the brightness of the night sky?

​From source to target, outdoor light interacts with the components of the atmosphere. That can change the color of the light, its distribution in space, and other qualities. In the cartoon picture below, light rays emitted from a street light can take various paths before reaching a surface or detector. 

Some of the light escapes to space where satellites may detect it. Sometimes it reflects off the ground before proceeding up into the night sky. And some light rays end up in our eyes (or, in this case, a telescope lens). But most importantly for our purposes here, the Earth's atmosphere can absorb or scatter light.

This happens because constituents of the atmosphere like water droplets, dust particles and molecules are small. That is, they are comparable to the size of the wavelength of visible light. As their size approaches that of the wavelength of light, they interact more light in more pronounced ways.

Since the interaction depends on the wavelength of light, the results can vary. For example, very small particles may scatter light of one color in a preferred way. In this way, the atmosphere can direct light to places far from where a lamp emitted it. This can extend the "reach" of city lights into remote locations that are very sensitive to light pollution. Air pollution can make this even worse.

Clouds are regions of the atmosphere where the "phase" of water has changed from a vapor to a liquid or solid. As individual water molecules clump together, their optical properties change. So we expect that the way light moves near and through clouds is different than when it moves through clear air.

Clouds tend to be gray from a color perspective. That is, as light passes through clouds, the water scatters or absorbs light in ways that don't depend on the light's color. Often when we see clouds in the daytime their tops are white but their bottoms are gray. Both white and gray are the same color, differing only in their intensity. A ray of sunlight reflected from the top of a cloud looks white. But rays emerging from the bottom of a cloud, dimmed from absorption by water droplets, looks much less intense.

In light-polluted places, cloud bottoms are bright. They 'amplify' skyglow by reflecting more light back to the ground. Changes to lighting on the ground can change the colors of clouds. In naturally dark places, they're black. They absorb airglow and starlight, both of which are natural sources of light at night. In short, clouds make bright places brighter at night, and dark places darker.

This matters as the world's climate continues to change due to human activities. A warmer climate in much of the world means more clouds. More clouds in turn mean brighter nights both in and beyond cities. We don't yet know what this means for the ecology, but it could be a problem for wildlife conservation in otherwise dark places.

The weather also changes the optical properties of surfaces. Imagine materials like dirt, concrete and asphalt. After a rain shower, they all look darker to the eye. The materials absorb water, which in turn makes its refractive index more like that of air. It is then less likely to reflect light rather than absorb it.

Surfaces covered by ice and snow tend to look white. The multitude of hexagon-shaped ice crystals that make up ice are great at reflecting (and refracting) light. That means ice reflects much of the light incident on surfaces it covers, making it more and more mirror-like. This effect also amplifies skyglow. Even when the sky is clear, snow and ice on the ground in and near cities makes the night sky brighter.

The two effects can combine together, too. One study found that, in an environment with a lot of ALAN, cloudy nights with snow on the ground can be as much as 3500 times brighter than a clear night in a remote location. Such bright conditions have profound effects on the nighttime environment.

The effect of weather on our efforts to understand light pollution is significant. One way is by changing the amount of light at night measured by satellites and other remote sensing platforms. A rain-dampened surface can look darker than it is when dry, while an icy surface can increase its apparent brightness as seen from orbit.

​These effects can add in with others, like changes in leaf cover on deciduous trees, to produce changes in ground brightness that vary according to the season. This effect is evident in satellite data. In winter, with ice on the ground and trees bare of leaves, cities look a little brighter. In summer, the reverse is true. But we can account for this with careful study and correction of our measurements.

Weather also impacts our ability to make accurate models of light pollution and skyglow. We now have mathematical models that are very good at predicting skyglow under clear conditions. The physics of light transmission are well-understood. Computing power has increased by leaps and bounds in recent years, and models now run quickly on desktop computers.

But it starts to get complicated once the properties of the atmosphere change. The momentary state of the atmosphere then becomes important. We can simulate simple situations, like a uniform layer of clouds over a city. But more realistic weather becomes progressively more difficult to model. We are still far from making reliable skyglow models that deal with weather in believable ways.

With all this talk about weather and skyglow, it's easy to think things are hopeless. Of course we can't do anything about the weather. But even in very cloudy places, where seeing the night sky is rare, it's important to prevent light pollution. Such conditions are "darker than dark" and provide refuge for threatened species. In and near cities, the problem is even more acute due to the amplification of skyglow by clouds. These areas need even more consideration to limit the effects on urban wildlife.

No matter what the weather is like, protecting darkness has distinct social and environmental benefits. Solutions are simple and cost effective, but navigating the complexity of options can be challenging. Every day, we help clients understand the sources of light pollution and their best options for reducing it. Contact us today to find out how we can help you achieve your dark-sky goals.