Image of Texas and Louisiana, showing areas where lights this month are brighter than long-term averages. Dark green pixels show the location of lights that are at least 50% brighter. Image by Jesse Allen, NASA’s Earth Observatory.

It is true, and you have scientists’ word on it: The holiday season is up to 50 percent brighter than the rest of the year. You may have seen NOAA’s photos of Earth at night, with major cities glowing brightly, while rural areas are scarcely visible. Those images, dating back to 2012, are composites, painstakingly compiled from 312 orbits, showing each portion of the globe when it was both cloud and moonlight free.

Now, just in time for the holiday season, NASA and NOAA have released a new wave of images, each taken on one specific night, around 1:30 a.m. local time, and demonstrating that Americans, Europeans, and some others light up December nights with bright lights. Thanks to the Visible Infrared Imaging Radiometer Suite (VIIRS), an instrument aboard the Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite, clouds and moonlight are irrelevant. VIIRS sees right through the clouds, and a new algorithm eliminates the impact of moonlight.

Some of the new images were first displayed at a press conference during AGU’s Fall Meeting in San Francisco on 16 December. Unlike the earlier ones, these were in color. The color indicates how much brighter or dimmer a city is on one night than over a long-term average. Green indicates brighter than average; red means dimmer.

It is apparent that U.S. cities are much greener, that is, brighter, in December, a trend that actually begins the day after Thanksgiving and continues until New Year’s Eve. For example, Dallas, Tx., glows bright green, as does Houston. In Puerto Rico, where Christmas celebrations last until January 20, the increased brightness does as well.

It is not just a gimmick, said Miguel Román of the NASA Goddard Space Flight Center in Greenbelt, Maryland, at the press conference. Light intensity can track patterns of human activities, he said. There is one important limitation, though. VIIRS cannot evaluate the bright lights of New York or much of anything north of St. Louis during the holiday season where the reflectivity of snow affects the baseline brightness levels.

Image of a portion of the Middle East, showing areas where lights during Ramadan are brighter than long-term averages. Dark green pixels show the location of lights that are at least 50% brighter. Image by Jesse Allen, NASA’s Earth Observatory.

As a control for the December brightness in America, the researchers studied several Middle Eastern countries with Muslim majorities, as well as Israel, to see if Ramadan—a month when devout Muslims fast during the day, but eat, conduct commerce, and socialize at night—produced brighter than average nights. It did. For example, Cairo’s spikes in brightness over three years, from 2012 to 2014, matched Ramadan’s movement, which was earlier each year according to the civil calendar.

Not all Muslim-majority cities increased their brightness equally during Ramadan. Román attributed that to such factors as availability of electric power, economic conditions, degrees of religiosity, and even war.

Eleanor Stokes of Yale University, who analyzed the Middle East data, told reporters that Tel Aviv, Israel, did not show any change from normal nighttime brightness during Ramadan, further evidence that the phenomenon seen elsewhere in the region was linked to the Muslim holiday.

Both Román and Stokes say that the techniques developed for this study can be applied to observing and tracking various human cultural activities in the future, particularly as the demand for electrical energy increases with growing populations.

—Harvey Leifert

Citation: Leifert, H. (2014), Holidays really do light up the night, Eos, 95, doi:10.1029/2014EO020911.

Text © 2014. The authors. CC BY-NC 3.0
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