You can learn a lot about cities by how they light up at night

Artificial lighting at night is one of the clearest signs of human activity from space. Most large cities are now so bright that they effectively blot out the nighttime sky. And scientists have found this "light pollution" can have lots of unexpected side effects — from disrupting our sleep to messing with wildlife.

US cities are much brighter at night than their German counterparts

But as it turns out, there's a ton of variation in how different cities light up at night. A recent paper in the journal Remote Sensing makes use of new space imagery to study nighttime lighting in a variety of cities across the United States and Europe.

They found a few surprises: US cities, for instance, are many times brighter than German cities of similar size. And, intriguingly, East Berlin appears to emit more light per capita than its wealthier counterpart in the West. Technology, culture, lighting codes, and even the way cities array their streets all seem to have a big impact.

For the paper, the researchers gathered and analyzed images from NASA's Suomi NPP satellite and photographs taken by the International Space Station. (You can check out a great library of the latter images here.) A few findings:

1) European cities vary a lot in how they light up

Upwelling radiance of six European cities in the VIIRS two-month composite dataset, with a logarithmic color scale. The data are reprojected on a 15-arcsecond grid, which slightly distorts each city’s shape. Data courtesy of the Earth Observation Group, NOAA National Geophysical Data Center.

On the whole, capital cities in Western Europe are brighter at night than cities in Central Europe, with the exception of Warsaw (which is unusually bright) and Amsterdam (which is unusually dim). That's true even though capitals in Central Europe are often more populous.

Some of this no doubt has to do with economic development — the richer a city is, the more lighting it usually has — but the researchers hint that there may be "possible difference[s] in lighting culture," too, particularly in downtown areas. One good place to see that is in Germany...

2) East Berlin is more brightly lit than West Berlin

Cropped portion of another image of Berlin taken from the ISS (iss035e17210). Berlin’s Tegel airport is labeled with "TXL". Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center.

The researchers also found a striking difference in lighting patterns between East and West Berlin — a legacy of the old Cold War split. The nighttime lights in East Berlin give off an orange hue, largely from older low-pressure and high-pressure sodium lamps. West Berlin, by contrast, emits more white light, likely because there are more modern LEDs and fluorescents.

What's striking is that East Berlin emits more intense light per capita at night, even though the area is poorer. Indeed, that's true throughout Germany — East German cities that used to be behind the Iron Curtain are brighter than West German cities of similar size.

It's not entirely clear what accounts for the difference. In Berlin, at least, one possible factor here is that the city still uses some 40,000 gas lamps for lighting, which are difficult to detect from space. (Many of those lamps are in the West.) Another factor, the researchers note, is that Berlin's lighting policy advises using outdoor lighting only "when sensible and necessary." That came after a study between 2006 and 2008, which found that levels of lighting didn't actually make the streets any safer.

As a useful side note, the researchers also were able to determine where all this nighttime light was coming from. In Berlin, at least, about 32 percent comes from street lamps, 16 percent from industrial areas, 6 percent from buildings lit up in the city center, and 4 percent from airfields. The rest comes from housing, commercial areas, and "public service areas."

3) US cities are much brighter than comparable German cities

The VIIRS Day-Night Band (DNB) sum of lights (SOL) for 28,804 American and 4492 German communities plotted against population. Trend lines for the two datasets are plotted. If upward lumens/capita were a constant, the trend lines would have equal slopes of one, and the intercept would specify the proportionality constant. The large spread of the data shows that while population is important, it is not the only variable that determines SOL.

This last finding is striking. Germany and the US are at comparable levels of economic development. So you'd think that similar-sized cities in each country would look roughly similar at night. But that's not actually true. US cities are many times brighter than their German counterparts.

The authors caution that the difference is so striking that it might well be due to some sort of systematic bias or error in the data (although they rule out several possible errors that might have crept in).

If it's real, however, the difference could be due to lots of different factors. In Germany, much of the public lighting is decades old. Differences in tree cover may also account for some of the disparity (many American cities and subdivisions are younger than their German counterparts, and may have less abundant tree cover). Differences in street width, building height, advertising, and architectural lighting could also affect things.

In any case, the authors note, it appears you can tell a lot about a city — its history, its culture — from studying its lighting. And future research could potentially be useful in figuring out the effectiveness of light-pollution laws or devising ways to curb energy waste.

Further reading

You can find more photographs taken by astronauts aboard the International Space Station here.

Light pollution is erasing the night sky. Can we bring it back?

NASA can see your Christmas lights from space.

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