Orange night sky

Why does the sky appear orangish in colour when it is cloudy at night or when it is raining at night? I have been asking this question since childhood, but no one has been able to answer me. Bidisha, Kolkata, West Bengal, India

Seattle skyline and its light pollution.  Photo courtesy of Commander John Bortniak, NOAA Corps.

You ask about a night orange sky. Since the sky has color (orange), we know that light must shine from somewhere to reflect the orange color to our eyes. Where does the light come from? Not the Sun (since it’s night and cloudy). Nor can it come from the cloud-obscured Moon or stars. So, it must come from the city lights. Kolkata (Calcutta), with a population of 4.4 million people — India’s second largest city — has many lights.

Light pollution is the answer. Kolkata has sodium-vapor lights. Such lights reflect off low clouds and color the sky reddish orange, says astrophotographer Jerry Lodriguss.

This pollution comes from city lights that (unnecessarily) direct light upward and brighten the sky. Even on clear nights, the bright sky above big cities overpowers all but the brightest stars.

Further Surfing:

Astropix: The brightness of the night sky

BBC: Light pollution

(Answered Sep. 26, 2003; updated Oct. 20, 2007)

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Readers' Comments

• And, don't forget refineries and other such industrial complexes. About twice a year I get a panic-stricken phone call from my mom, who takes one look to the north and thinks something huge and combustible is on fire. It's only the petro-chemical industries on the Houston Ship Channel, in Pasadena, etc., but under the right conditions, the light from the flares combined with all of the lights on the cracking towers and such can produce a glow that truly does look like a conflagration!  ShoreAcres, League City, Texas
   
• On the border between ElPaso and Juarez, Mexico at night you can tell the difference between the cities by the color of the street lights and "sky glow". Juarez is green and El Paso is more white/yellow.  GardenGrrl, Lewisville, Texas
   
• Living in larger city, the light "pollution" makes dimly lit neighborhoods a bit safer. When power was out, I had to sleep in front yard (armed) watching out for looters after hurricanes. I loved seeing the stars, but I don't like total darkness at night after hurricane. That is a different perspective on nighttime lighting. If I'm camping then I want to see stars.  Ivansvrirv, West Palm Beach, Florida

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