Can We Measure Air Pollution by Looking at the Color of the Sky?

Is it possible to see air pollution just by looking at the color of sunlight in the sky? That sounds tough, sometimes I hear people say air pollution is invisible:

But after years of living in Beijing and Delhi, I’ve been amazed at how well we can detect air pollution with our eyes.

But it’s a bit complicated. If you’re not thinking about it, it’s easy to overlook. I discovered that when I spent two years living in heavily polluted Chinese cities like Nanjing and Guangzhou without really noticing the pollution.

How did I play basketball outdoors in Beijing, get dizzy afterwards, and just think, “huh, that’s weird!”

How did I bike through Guangzhou’s hot and humid pollution in 2007 to my English tutoring job and not realize my difficulty breathing and constricted throat was from the pollution?

These days I wear masks and run purifiers and no longer suffer any symptoms. Now my eyes are finely tuned AQI detectors. Here’s what I’ve noticed about how polluted skies look:

1. The night sky turns a red color on polluted days.

Here’s a picture I took in Beijing of the night sky on a polluted day. Not how the air pollution has turned the sky red:

Beijing night sky red color air pollution effects

And here’s a nighttime Beijing sky on a good day. Without the effects from air pollution, the sky is a much more normal black color.

Beijing night sky black color when not affected by air pollution

2. Sunlight on polluted days turns a yellow or orange color. Clean sunlight is clear and white, almost colorless.

Here are two pictures I took at the same place in my apartment, same season, at roughly same time of day. But the day on the left was a polluted day. The day on the right was a clear day.

air pollution effects on color of light and sky. Yellow/orange color when polluted, clear when not polluted

See how the sunlight on a polluted day (left) looks more orange or yellow? Now in Beijing, I can tell if it’s a bad day just by looking at the color of the sunlight coming through my window.

Beijing Pollution Appearance Contrast Light Circled

3. Air pollution makes clouds look fuzzy

When pollution is bad, all those tiny particles scatter sunlight, making things appear fuzzier, less distinct. To see how air pollution affects clouds, the Smart Air team used a timer to take pictures of Beijing’s air for several weeks. See how the edges of the clouds are pretty distinct in the sky on an unpolluted day in Beijing:

small effect of air pollution on clouds on unpolluted day

Now here’s a worse day. The edges of the clouds against the sky are starting to look fuzzier, less distinct.

edges of clouds fuzzier in sky due to air pollution

Of course, then there are the really polluted days days. Note how fuzzy the clouds become on a day when the PM2.5 is 147 micrograms (US AQI: 198).

Beijing clouds indistinct sky hazy on polluted days

When I fly back to the US, one of the things I marvel at is how distinct the clouds are. “Look at those crisp edges on that cloud!” I never thought that would be something I’d appreciate.

Bottom line: How air pollution affects the color of the sky

If you train your eyes, you’ll be able to use the sky color to start seeing even moderate levels of pollution in the sky. Red sky at night, orange/yellow sky during the day or fuzzy clouds could all be indications of high levels of air pollution.

Smart Air

Note: I’m not an expert in air pollution. I’m just a data nerd who got into making DIY purifiers and testing whether purifiers could clean the air in my home after coughing through a Beijing winter. If you’re living in a city with dangerous air, check out my open data tests of air purifiers and the specs on no-nonsense purifiers before big companies convince you clean air needs to cost $1,000.

Breathe safe!