US cities brighter than German cities

German cities emit several times less light per capita as compared to similar-sized cities in the US, research shows.

The size of the gap grows with city size, as light per capita increases with city size in the US but decreases with city size in Germany, the findings showed.

"This work will allow us to identify comparable cities in order to uncover the reasons behind the differences," said lead author Christopher Kyba from German Research Center for Geosciences (GFZ).

These could include differences in the type of lamps, but also architectural factors like the width of the streets and the amount of trees.

Since maps of nighttime light emission highlight the areas where light pollution is especially prevalent, they provide information about which areas can best be targeted for energy savings.

"Artificial light is responsible for a sizable portion of all nighttime electricity consumption," said co-author Franz Hölker from Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) in Germany.

The researchers used data from the Suomi National Polar-Orbiting Program weather satellite to identify and measure the output of individual bright sources of light pollution for the first time.

The study found that in megacities in developing countries, the brightest light sources were typically airports or harbours.

In contrast, the brightest areas in the capital cities of Europe are often associated with leisure, for example stadiums and city centers.

While artificial light at night is a problem for astronomers and nocturnal animals, it has the potential to be an important tool in understanding human activity.

The study appeared in the journal Remote Sensing.