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Air Pollution on a Rainy Day: Understanding Weather & Air Quality

Ran Korber
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While the sources of air pollution themselves can be man-made (car traffic and industrial activity) or natural (pollen and bushfires) , weather can also determine the way we experience air pollution in a big way.

Weather can be a friend, helping to clear away pollutants and serving to improve  air quality, or it can make air pollution drastically worse by helping to create pockets of highly polluted air or by helping to spread bushfire smoke.

The clear impact of weather on air quality explains why our team uses this information (among many other data sources) to accurately calculate and predict air pollution (forecast & real-time).

Raining Outside? Go for a walk!  

For many, winter means dark days and dreariness. Here's a funny idea: Instead of retreating to your couch and blanket and reaching for the hot chocolate consider braving the elements and finding a park, trail, or another outdoor spot to visit.

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Rain might ruin a picnic, but when it comes to air pollution, it can actually be a really good thing. This is because, on rainy days, most of the common air pollutants and pollen in the air are washed away, helping to increase the quality of the air. This phenomenon is called Wet deposition.

To be more scientific: 'Wet deposition' refers to the natural processes by which material is removed ('scavenged') by atmospheric hydrometeors (cloud and fog drops, rain, snow ) and delivered to the Earth's surface. A number of different terms are used more or less synonymously to describe this phenomenon - such as 'precipitation scavenging', 'wet removal', 'washout', and 'rainout'. 

As a result, getting your steps in on days that are gray and cold can be especially important, especially considering the importance of exercise for mental and physical health.

To visualize 'washout' in action, you can see how the air pollution  dropped in the below locations while it was raining, compared to average recorded levels:

Boston

 St Petersberg

Wind Can (Sometimes) Help, by Blowing Pollution Away

Wind can sometimes help to ‘clear the air’ by helping to disperse pollutants, rather than allowing them to concentrate and build up in city centers. When the air is stagnant, pollutants and dust particles can gather to create low air quality areas. 

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Cities next to oceans can sometimes receive extra benefit this way if wind brings in comparatively cleaner air from the sea.

However...Wind Can Also Do the Opposite

Unfortunately, when it comes to air pollution, wind can  be a double-edged sword. In some cases, the effect of wind is to introduce new pollutants to your area and help to spread air pollution sourced far away to your area.

This can often be the case in the event of rapidly spreading bushfires. In fact, wind was a key reason why the California fires and smoke spread so far and wide in the summer of 2019. The West Coast of the US and Mexico often experience warm winds during this part of the year, commonly referred to as the Diablo and Santa Ana Winds. During fire season, dry and warm conditions combined with extreme gusts can create the perfect conditions for air pollution to spread - meaning people far away from the fire itself can still be impacted by toxic smoke.

 

Fun in the Sun or Smoggy Days of Summer?

Sunny days invite thoughts of picnics in the park and barbecues in the backyard. But hot weather can also mean the lowest air quality of the year in many cities.

Air tends to stagnate in the summer - with less wind to move it around, dangerous pollutants like nitrogen dioxide  and ground level ozone can settle and build up in certain areas. Ed Avol, Professor of Clinical Preventive Medicine at USC says ozone and other pollutants are most serious in cities like LA and Houston, “large urban areas that have a lot of sunlight, a lot of emissions and a lot of stagnation, causing things to cook and become new pollutants.”  

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Weather & Air Quality - An Unbreakable Link

We've seen the way weather impacts air pollution, but new research also shows that this connection works the other way around too.: It seems air pollution can impact the amount of sunlight that reaches the Earth's surface, because particulate aerosols like dust and ash play some role in dispersing it. 

As a result of the strong connection between weather and air quality, we're seeing weather providers like WeatherBug turn to us for accurate air quality data information in a bid to better inform their users and improve their overall product offering.

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