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CINDY DAY: Cleaning the air, one drop at a time

Plain old rain can “clean” the air.
Plain old rain can “clean” the air. - 123RF Stock Photo

For many, Thursday’s cold front was, well, a breath of fresh air. It certainly put an end to the oppressive heat and humidity.

I overheard a number of people saying they hoped the storms would “clear the air.” I grew up hearing people say that too, but I’ve come to learn that it doesn’t always. Unless the thunderstorm is triggered by a cold front that’s ushering in a drier air mass, a summer thunderstorm can make things worse! 

If you suffer from any kind of respiratory issues, you were likely very happy to see the wind swing around to the north on Friday; that drier air is easier to breathe! The cold front really did clear the air.

Now that we’ve established what can clear the air, we’re learning that plain old rain can “clean” the air. As a raindrop falls through the air, it can attract hundreds of tiny aerosol particles to its surface before it reaches the ground. This process is called coagulation and it’s a natural phenomenon that can actually clear the air of things like soot, sulfates and organic particles.

Given the altitude of a cloud, the size of its droplets, and the diameter and concentration of aerosols, experts can predict the likelihood that a raindrop will sweep a particle out of the atmosphere.

From the measurements, scientists can calculate the rain’s coagulation efficiency or, more simply, the drop’s ability to attract particles as it falls. 

Going over the report, I came across two interesting bits of information:

  1. The smaller the droplet, the more likely it is to attract a particle. 
  2. Conditions of low relative humidity also seemed to encourage coagulation.

So why is this important? Well, these values can be extrapolated to predict rain’s potential to clear a wide range of particles in various environmental conditions. It can help address issues of air quality and human health.

Cindy Day is the chief meteorologist for SaltWire Network.

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