The fall version of the marine layer

3:16 PM, Sep 27, 2013   |    comments
The clouds had a tough time breaking today, despite a relatively dry atmosphere in the DC Metro.
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You might remember the late spring phenomenon of the marine layer from earlier this year. The marine layer is a relatively thin but solid deck of cloud cover that forms due to the influence of the Atlantic Ocean, and it can wreak havoc on what would otherwise be a sunny and pleasant day. It can happen any time of year here in the DC Metro, but it's most common in the late spring and the early fall. 

The picture below is of a "skew-T" diagram (short for skewed temperature, logarithmic pressure diagram). It shows changes in temperature and dewpoint through a vertical profile of the atmosphere at one location. In this case, the location is Dulles Airport. Picture the bottom of the diagram as the ground, and the top of the diagram as the highest part of the atmosphere. Notice how the red line (temperature) and the green line (dewpoint) are far apart everywhere except in one thin sliver of the vertical profile. This is where the cloud cover formed- where the temperature and dewpoint are very close to each other. 

Why did such a consistent deck of cloud cover form here, and nowhere else? It's because of the unique geography of the DC Metro area, and the precise location of our weather elements today. 

An area of high pressure is centered over New England. Air rotates in a clockwise direction around high pressure, resulting in an east or northeast wind direction for us. The easterly wind brings some Atlantic moisture in to the DC Metro in the form of low clouds. You can see it on this surface map:

The arrow indicates the airflow off the Atlantic. You can see how the air travels across hundreds of miles of cool ocean water before it moves inland to Washington! The moisture doesn't condense into cloud cover until it starts to move into higher elevations to the west of the District, toward the Appalachians. Once the condensation process starts and clouds begin to form, the clouds start "back-building" toward DC. If you were to watch the clouds form and build on satellite, you would see the initial cloud formation close to the mountains, then expand eastward to cover a larger portion of the area. By sunrise, it was basically overcast through the Beltway area, and the clouds hung tough until the early afternoon. 

With similar conditions persisting into Saturday, we might see this same deck of morning clouds again. The center of high pressure starts to slide out of its current position on Sunday, so it looks like Sunday will be the sunniest day of the weekend!