The Washington Post

Unhealthy air quality in December? What’s going on?


As of  10 a.m. this morning, the Air Quality Index in Baltimore stood at 122, passing the code orange threshold, indicating unhealthy air for sensitive groups (children, older adults, and individuals with respiratory illness). (In D.C., it was 97, just below code orange levels). In fact, since Monday, parts of Maryland and northwest Virginia have experienced compromised air quality in the code orange range.  We’re accustomed to code orange days on sweltering summer days.  But why now?

In short, the uptick in air pollution is due to an absence of wind.  We’ve been in between weather systems, with hardly any “flow” over the region.

Surface weather map shows the region between weather systems and in an area of weak flow (

The stagnant air has allowed pollutants in the region to gather and stay put – particularly in sheltered valleys. The map below shows highest forecast pollution levels today in mountain valleys west of the D.C. and Baltimore.

Today’s graphical air quality forecast for the Mid-Atlantic. Orange indicates unhealthy air quality for sensitive group.  Yellow indicates moderate air quality. (

When there’s wind, it mixes the air pollutants so it’s not too high in any one place and, like a fan, wind pushes pollution in and out so it doesn’t linger anywhere long.

Sunil Kumar, senior environmental engineer at the Metropolitan Washington Council of Governments, offers some more detail on the causes of this bad air in valleys to our west today (e.g. around Hagerstown):

Limited vertical mixing of air mass due to inversion, light surface winds that recirculate within the area, and warming temperatures. There may also be some smoke in the region that may be mixing down toward the surface too. Fine particles produced from previous days are also getting added to the current fine particle level.

Right now, the pollution we’re seeing is mostly from fine particles – which is not unusual in winter time.  Ozone pollution – which is exacerbated by heat and most common in summertime – is relatively low.

The good news for individuals sensitive to elevated particle pollution is that increasing winds from the south should reduce its concentration by tomorrow.  Whereas today’s AQI is around 100, tomorrow it should be closer to 60, which is moderate.

Related: Breathing easier: Washington, D.C.’s remarkable improvement in air quality

Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.
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