(Note: I updated this post several times, after initially posting it, by accident, sooner than I had meant to.)

After a weekend that really started to feel like summer, it was cold this morning in New York City this morning, with a thick fog. Here is a photo I took from the George Washington Bridge at about 11:30 AM, looking south along the Hudson. See the boat in there?

The weather service’s forecast discussion calls it an “advection fog”, which means it happens when warm, moist air moves over a cooler surface, and ascribes it to a “back door cold front”. The modifier “back door” refers to the fact that while most cold fronts – like most weather of any kind at our latitude – come from the west, this one came from the east. You can see it in this image, a map of the flow this morning. The arrows show the flow at 1000 hPa (near the surface), and the colors show the temperature there. You can see the cool air blowing into NYC from offshore:

The definition of advection fog is that it forms when relatively warm, moist air moves over a cooler surface, so that it cools by contact with that surface and eventually reaches saturation. I’m not sure if that’s happening here, at least not in the short term. It looks as if the air was already cool before it blew onshore. But the key thing is that the layer of cool, moist air over the sea is very shallow and topped by a temperature inversion, so that warmer air overlies it. This means the boundary layer is very stable, and the air near the surface won’t easily mix with that above. Thus while over the ocean, the air took on more and more water vapor and couldn’t get rid of it, and eventually reached saturation. The sounding from this morning at Upton, NY in Long Island shows it very clearly:

Look at the bottom: there is only a single white curve until somewhere above 950 hPa (see the blue numbers at left which give the pressure) This means the temperature and dew point are the same, which means the air is saturated. I.e., fog. Then the steep jump to the right means a temperature inversion; it’s a good couple of degrees C warmer at 900 hPa (roughly 1 km up) than at the surface. Now look at the wind barbs at bottom right, showing the easterly flow just right near the surface, taking that cool foggy air in from offshore, while just a little ways up we have west to northwesterlies.

To get even nerdier:

Actually, there are at least four distinct layers in this sounding. 1. The cool fog layer at the surface. 2. Above the inversion, just above 950 hPa, a layer that is close to saturated, but not quite (the two white lines are separate, but near each other, indicating temperature is just a little greater than dew point. The lapse rate is a little steeper than moist adiabatic (temperature angling to the left of the white dashed curve) which, given how close to saturation, suggests this air is almost unstable to a little elevated convection? 3. Atop that, between around 700-600 hPa, a roughly isothermal layer – very stable, close to being another inversion in that there is a slight temperature increase – in which the humidity drops steeply. 4. Above that, an atmosphere that is close to moist adiabatic in its temperature structure, but very dry.

I won’t try to do a whole analysis of this structure here, but it’s fascinating!