It’s wind, not rain, that reduces air pollution. It sounds so simple, yet on rainy days, many people believe it’s the rain that’s “washing away” air pollution.
Data shows otherwise. Rain is poor at clearing air pollution from the skies. Most of the time, it’s wind that blows pollution away and lowers overall air pollution levels.
Blue Skies After a Storm? Thank the Wind.
An analysis of 6 years of pollution and weather data in Beijing found that wind was a much stronger factor than rain. Fine particulate pollution (PM2.5) correlated strongly with wind (r = -.37) but almost not at all with rain (r = -.04).
If so, why do so many people believe rain reduces pollution? A likely reason for this confusion is that wind normally accompanies rain. Rain often comes from changing weather fronts, which bring both rain and wind. It’s this same wind that blows out the pollution.
From our perspective on the ground, it’s easy to think the pollution has been “washed” away. In reality, most of the pollution actually gets blown away by the winds, resulting in lower air pollution levels and blue skies.
Before the storm:
After the storm:
Data Shows Wind Reduces Air Pollution
Researchers in Lanzhou charted how a cold front and its wind helps reduce air pollution levels. Here’s what happened to pollution when a cold front moved in one day in November:
When the wind picked up, PM2.5 fell approximately 66% from top to bottom. Compare that to the effect of rain. Moderate rainfall decreased particulate pollution by just 1.45%. Even the heaviest rains reduced fine particulate by just 8.71%.
Wind Direction Matters
When the wind kicks up, it can blow away pollution—especially the small particles of PM2.5. In Beijing, that works the best when the wind comes from the less-polluted areas to the north. But when the wind comes from the south, pollution actually gets worse.
Why would southern wind hurt Beijing air quality?
- The areas south of Beijing are heavily industrialized.
- Southern winds trap air against the mountains to the north and west of Beijing. Essentially, southern winds force us in Beijing to stew in our own pollution.
For more on the effect of wind, check out this analysis of wind and pollution in Shanghai.