Category: Chester County

PHILADELPHIA — It is rare to have an air quality alert in the winter; however, Philly has experienced four hazardous air days in a row. Normally air quality alert days are caused by ozone. These air quality alert days are caused by “particle concentrations”.

View the current air quality for Philadelphia.

Forecast Discussion: Tuesday is an Air Quality Action day for the Philadelphia metropolitan area for Unhealthy for Sensitive Groups (USG) or Code Orange particles. An unprecedented air quality event unfolded across the Delaware Valley beginning Saturday night and has continued into Monday. A very strong area of high pressure has created strong surface inversions that have trapped pollutants, allowing particle concentrations to reach the USG range on Sunday and continue into Monday. Typically these types of strong inversions do not develop until later in the winter, when there is snow on the ground. For Tuesday, another strong morning inversion is expected, and very light winds will become calm in the afternoon, which will continue to limit atmospheric mixing. This stagnation will allow USG particle concentrations to extend into Tuesday. **Extended Forecast: By Wednesday afternoon, surface winds will pick up, which should return particles to the upper Moderate range. But on Thursday, a warming trend combined with another episode of stagnating winds may push particles back into the USG range.

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