One dataset, sampled per city

Every figure on this site comes from NOAA and the Colorado School of Mines Earth Observation Group's VIIRS Day/Night Band annual composite for 2025, a cloud-free average of upward radiance measured from orbit, at roughly 500m resolution. Each city's figure is the median of a 3x3 pixel window (about 1.4km) centred on its coordinates, the median rather than a single centre pixel, so a city centroid that happens to land on a lit intersection or car park doesn't skew the whole town's number.

Six bands, built from this dataset's own shape

The six brightness bands, Dark sky, Rural, Suburban edge, Suburban, Urban, Inner-city, are cutoffs drawn from how this specific dataset of 3,206 US cities actually distributes, not borrowed from a third party's Bortle crosswalk table. Truckee, CA sits at the darkest end measured, Oklahoma City, OK at the brightest, and Roswell, NM sits right in the middle of the full range.

Why this is not a Bortle class

A satellite measures light radiating upward from a city, a Bortle class is a 9-point scale defined from a human eye looking up on the ground. The two correlate closely but are not the same quantity, covered in full in the Bortle scale guide. This site reports a brightness band, deliberately never a Bortle number.

What the dataset does not cover

Every place in this dataset is a named incorporated US city of 10,000 people or more, so "dark sky" here means dark for a city, not the true zero-light end of the scale that only unincorporated wilderness reaches. See the data quality guide for the exact scope and its edge cases.

Where to go next

See the range this plays out across: Truckee, CA, Roswell, NM, Oklahoma City, OK.

The Bortle Scale, and Why This Site Doesn't Report One · Your Brightness Band Isn't the Whole Answer: When to Actually Look Up