{"id":6749,"date":"2025-11-26T16:23:27","date_gmt":"2025-11-26T08:23:27","guid":{"rendered":"https:\/\/smartairfilters.com\/?p=6749"},"modified":"2025-11-26T16:23:29","modified_gmt":"2025-11-26T08:23:29","slug":"measure-air-pollution-look-at-night-sky","status":"publish","type":"post","link":"https:\/\/smartairfilters.com\/en\/blog\/measure-air-pollution-look-at-night-sky\/","title":{"rendered":"Can We Measure Air Pollution by Looking at the Color of the Sky?"},"content":{"rendered":"

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:<\/p>\n\n\n

\n
\"air<\/figure><\/div>\n\n\n

<\/p>\n\n\n\n

But after years of living in Beijing and Delhi, I\u2019ve been amazed at how well we can detect air pollution with our eyes.<\/p>\n\n\n\n

But it\u2019s a bit complicated. If you\u2019re not thinking about it, it\u2019s 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.<\/p>\n\n\n\n

How did I play basketball outdoors in Beijing, get dizzy afterwards, and just think, \u201chuh, that\u2019s weird!\u201d<\/p>\n\n\n

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\"playing<\/figure><\/div>\n\n\n

How did I bike through Guangzhou\u2019s hot and humid pollution in 2007 to my English tutoring job and not realize my difficulty breathing and constricted throat was from the pollution?<\/p>\n\n\n

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\"Guangzhou<\/figure><\/div>\n\n\n

These days I wear masks and run purifiers and no longer suffer any symptoms. Now my eyes are finely tuned AQI detectors. Here\u2019s what I\u2019ve noticed about how polluted skies look:<\/p>\n\n\n\n

1. The night sky turns a red color on polluted days.<\/h3>\n\n\n\n

Here\u2019s a picture I took in Beijing of the night sky on a polluted day. Note how the air pollution has turned the sky red:<\/p>\n\n\n

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\"Beijing<\/figure><\/div>\n\n\n

And here\u2019s a nighttime Beijing sky on a good day. Without the effects from air pollution, the sky is a much more normal black color.<\/p>\n\n\n

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\"Beijing<\/figure><\/div>\n\n\n

2. Sunlight on polluted days turns a yellow or orange color. Clean sunlight is clear and white, almost colorless.<\/h3>\n\n\n\n

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.<\/p>\n\n\n

\n
\"air<\/figure><\/div>\n\n\n

See how the sunlight on a polluted day (left) looks more orange or yellow? Now in Beijing, I can tell if it\u2019s a bad day just by looking at the color of the sunlight coming through my window.<\/p>\n\n\n\n

\"Beijing<\/figure>\n\n\n\n

3. Air pollution makes clouds look fuzzy<\/h3>\n\n\n\n

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\u2019s air for several weeks. See how the edges of the clouds are pretty distinct in the sky on an unpolluted day in Beijing:<\/p>\n\n\n

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\"small<\/figure><\/div>\n\n\n

Now here\u2019s a worse day. The edges of the clouds against the sky are starting to look fuzzier, less distinct.<\/p>\n\n\n

\n
\"edges<\/figure><\/div>\n\n\n

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).<\/p>\n\n\n

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\"Beijing<\/figure><\/div>\n\n\n

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\u2019d appreciate.<\/p>\n\n\n

Bottom Line<\/div>
Bottom line: How air pollution affects the color of the sky<\/div>
If you train your eyes, you\u2019ll 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.<\/div>
Smart Air<\/div><\/div>\n\n\n\n

Note: <\/strong>I\u2019m not an expert in air pollution. I\u2019m just a data nerd who got into making DIY purifiers<\/a> and testing whether purifiers could clean the air in my home<\/a> after coughing through a Beijing winter. If you\u2019re living in a city with dangerous air, check out my open data tests of air purifiers<\/a> and the specs on no-nonsense purifiers<\/a> before big companies convince you clean air needs to cost $1,000.<\/p>\n\n\n\n

Breathe safe!<\/p>","protected":false},"excerpt":{"rendered":"

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\u2019ve been amazed at how well we can detect air pollution … <\/p>\n

Read more<\/a><\/p>\n","protected":false},"author":1035,"featured_media":6759,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_relevanssi_hide_post":"","_relevanssi_hide_content":"","_relevanssi_pin_for_all":"","_relevanssi_pin_keywords":"","_relevanssi_unpin_keywords":"","_relevanssi_related_keywords":"","_relevanssi_related_include_ids":"","_relevanssi_related_exclude_ids":"","_relevanssi_related_no_append":"","_relevanssi_related_not_related":"","_relevanssi_related_posts":"25918,27350,20786,17908,39424,6248","_relevanssi_noindex_reason":"","footnotes":""},"categories":[17,257,261],"tags":[272,75,89,27,74,425,37],"class_list":["post-6749","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-air-pollution","category-pm2-5","category-weather","tag-air-pollution","tag-beijing","tag-comparison","tag-delhi","tag-pm2-5","tag-sky","tag-smog","resize-featured-image"],"acf":[],"_links":{"self":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6749","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/users\/1035"}],"replies":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/comments?post=6749"}],"version-history":[{"count":15,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6749\/revisions"}],"predecessor-version":[{"id":39903,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6749\/revisions\/39903"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media\/6759"}],"wp:attachment":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media?parent=6749"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/categories?post=6749"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/tags?post=6749"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}