It's raining hard yet the AQI is over 100 in my area

It’s raining hard here in the San Francisco Bay Area, when I checked the purpleair map I was surprised to see the AQI over 100 on average. I thought the rain would have the AQI closer to 5 or 10.

Can someone explain why I’m seeing these higher numbers? It’s raining hard in this entire area. Here’s a screen grab.

I’ve seen plenty of high numbers in my area when it rains. Often after the rain it will go down, if the rain was a bringing a front in, with cleaner air behind it - center of country, southerly winds bring bad air typically, while rain will often bring a cold front from Canada, with change to northerly winds and cleaner readings (often dramatically within in a short time after the rain stops).

I’m not sure if your assumption, that rain will wash the pollution out of the air is valid. Seems intuitive, but might be a more complicated scenario where particulate matter is concerned/remains.

India has a lot of monsoon rains, but their meters are always outrageously high, and unfortunate.


Hi Robert, thank you for your reply. I don’t think the storm front brought in the higher pollutants. Looking at the map, there are a few AQI numbers showing 0 and those line up with our coastal mountain ridges, these are at 2000 foot height, I don’t think it was raining at that elevation.

I just checked and the humidity at the time was 87 where the over 100 AQI is being shown. I suspect the sensors are giving false information during a heavy rain storm. I will keep an eye on the relationship between humidity and AQI numbers and report back.

I have the same suspicion – I’ve seen a huge AQI spike during heavy rain with high humidity on my sensor as well. AirNow for that same area showed great air quality.

I saw this online article and thougt it could be of interest to your topic:
A more technical paper:
Also, an FYI: PurpleAir typically reports higher than federal regulatory monitors (FRMs) or federal equivalent monitors (FEMs). To better compare PurpleAir values on the PurpleAir map to the EPA AirNow map (which uses FRM/FEMs) one should apply the EPA correction factor in the drop down menu of the PurpleAir map.

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Thank you, Jasmine. That first link explains:

Data shows that rain has a relatively small impact on reducing air pollutants.