(margotbeezley/Twitter)

This weather is not normal for the second week of January, obviously. Everyone who walked out in a regular button-down shirt — no coat — and still wound up sweating by the time they got to work this morning will concur. It’s pretty gross.

It’s also making the Potomac look like it’s being boiled.

The super-cold outbreak of the past two weeks froze the Potomac River solid in some places. The ice was so thick that people were able to skate on it and play hockey for a day or two.

Now there’s another (giant) area of low pressure moving in, and — as they always do — winds have turned around to come out of the south ahead of that system’s cold front. It’s pumping a lot of warm air into the Mid-Atlantic, which is why temperatures went from the 20s to the 60s in the matter of just a few days.

The combination of river ice and warm air is causing the Potomac to go up in smoke.

Fog develops under a couple of different circumstances. What’s happening on the river is advection fog. Warm air is blowing in from the south. When it hits the cold, icy Potomac, it cools down, which lowers the temperature to the air’s dew point.

The dew point (something we talk a lot about in the summer) is a measure of how much moisture is in the air. It’s literally the temperature at which dew will form on the grass. It’s also the temperature at which we can say the air is “saturated,” meaning it’s holding as much moisture as it possibly can.

When cool air hits its dew point, the moisture condenses into tiny water droplets. That’s the fog we’re seeing over the Potomac. It looks like steam coming off a simmering pot, but it’s actually quite cold.

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