YozoraAtlas

How dark is the sky in Porto?

Sky darkness for 5 towns and parks in Porto, worked out from NASA satellite images of light at night (2025). None of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Baião (20.37, Bortle 5), the brightest Porto (18.49, Bortle 7). Milky Way season in Porto runs April to August: on moonless nights the bright core stands clear of the horizon, climbing to about 20° at its highest.

19 Bortle class, 1 pristine to 9 city center park or dark-sky place city

Numbers like 21.95 or 16.97 are the brightness of the sky overhead on a clear, moonless night, in mag/arcsec². The scale runs backwards: a lower number means a brighter sky, from about 22 for a natural sky to about 17 in a city center.
How the readings are made

NASA satellite night-lights map of Porto with every place marked

Darkest skies in Porto

  1. BaiãoBortle 520.37
  2. FelgueirasBortle 519.74
  3. BougadoBortle 619.28

Among the places on this site. Plenty of land between them is just as dark, with no town or park to name it by.

Brightest city skies in Porto

  1. PortoBortle 718.49
  2. Póvoa de VarzimBortle 618.95

All 5 places in Porto

Amber names are parks and certified dark-sky places. The figure is the Bortle class: 1 darkest, 9 city center.
Baião5Bougado6Felgueiras5Porto7Póvoa de Varzim6