The Maryland Department of Environment predicted elevated levels of fine-particle pollution, with the worst conditions in urban areas.

If you looked up at the sky Monday, an unseasonably mild day in Washington, the haze was unmistakable.

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“As beautiful as the weather is in DC today, it’s weirdly hazy. Can someone explain this to me?” tweeted Laur_Katz.

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In its air quality discussion, the Maryland Department of Environment attributed this winter pollution episode to a “stout capping inversion” which trapped a cocktail of air pollutants from wood burning and vehicle emissions near the ground.

Inversions occur when the air temperature warms as it rises. (Normally, the air cools with altitude.) When warmer air sits above a layer of colder air near the ground — which is heavier — pollutants get stuck.

“Any local pollutants emitted were staying nearby and very close to the surface,” said Dan L. Goldberg, an atmospheric scientist specializing in air quality at Argonne National Laboratory, in email.

Such a capping inversion is strongest during the morning. Into the afternoon, Goldberg explained, the pollutants are able to disperse more as the sun heats up the ground and the inversion erodes.

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The conditions that allowed this inversion to develop began with Friday’s snowfall.

“The fresh snow traps cold air near the surface very well,” Joel Dreessen, a meteorologist with Maryland’s Department of Environment, said in an email. “Particles jumped dramatically Saturday (in comparison to Friday) due to the inversion which set up. This very stout near-surface inversion was/is in place through Monday due to ongoing high pressure in the region.”

A Code Orange alert was first issued for the Baltimore region Sunday before it was expanded to the Washington region for Monday. On Sunday, Goldberg and Dreessen said, the observed particle pollution stayed just below Code Orange level in the Baltimore region but exceeded this level in south-central Pennsylvania.

Dreessen said that ode Orange conditions in winter are “fairly rare” and that the last instance occurred Dec. 4, 2017. In all, there have been three instances of these particle pollution episodes during the winter months since 2014, he said.

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These winter particle pollution cases — tied to inversions — are different from typical summer pollution, which is mostly the result of ground-level ozone spurred by hot weather and stagnant air.

The air quality is expected to improve through midweek, as the inversion is expected to weaken by Tuesday morning and then dissipate altogether after a cold front passes.