As late summer gives way to fall in the nation’s capital, brilliant colors will emerge, the air will turn crisp and festivals will be numerous. Despite the trend toward cooler temperatures and fall festivals, the season is also the favored time for fog formation that can slow the commute to work or school.
Fog is a subtle weather condition that can quickly become dangerous for commuters, walkers, bikers and runners without advanced warning. It doesn’t come with a bang like a thunderstorm or can be seen slowly piling up like snow, but rather slowly and quietly develops, usually at night, and can cause serious and abrupt visibility problems the following morning.
Fog is typically found in the cooler season because of the longer nights, which allow the temperature to drop to the dew point and condense suspended moisture in the air. Also, stronger storm systems often occur later in the fall (November particularly) which can produce foggy scenarios.
A recent study from the National Weather Service in Blacksburg, Va., showed that nearly 65% of fog events for the Blue Ridge foothills and Piedmont occurred November through February. Only 13% widespread dense fog events happen in the spring and summer.
In most of the fog events studied, a key feature was the position of a strong surface high pressure. High pressure produces sinking air, which results in clear skies and light wind (the perfect recipe for fog). When high pressure is anchored along or off the East Coast, the Mid-Atlantic tends to have foggy nights.
The upcoming pattern this week favors a similar set up where high pressure will be focused across the Mid-Atlantic and southern New England.
The type of fog most common across the Mid-Atlantic from April to August is Radiation Fog. Clear skies and calm wind at night allow temperatures to drop to the dew point. During times when the ground is wet from recent rain or a surplus in precipitation (as it is now across the Washington area), the surface dew point increases, making it easier for the temperature to drop to the dew point before the sun rises.
This often occurs along river valleys or near lakes and bays due to the high moisture content that is able to condense as suspended moisture, resulting in morning fog. Wind tends to be calm in valleys and higher in speed in the mountains, so that is why the Shenandoah Valley (highlighted in the image below) and towns adjacent to the Chesapeake Bay are favored for fog formation as well. This type of fog is usually shallow and lifts quickly following sunrise.
Another way in which fog forms is by means of a warm front. Fall is the time of year when the contrast in temperature in the Northern Hemisphere increases, resulting in stronger storms. The Mid-Atlantic is prone to the influence of warm fronts ahead of low pressures later in the fall season (see image below).
Warm air overrides cooler air at the surface producing widespread thick layer of fog that can linger for many hours. Southeast winds ahead of warm fronts in our region often produce upslope fog. An already moist air mass originating from the Chesapeake Bay and Atlantic Ocean glides up the eastern side of the Blue Ridge (western Frederick and Loudoun County, northern Fauquier County) and the air condenses to form fog just on the eastern flank of the mountain. This often limits visibility on Interstate 70 between Frederick and Hagerstown, for instance.
So, as the daylight shrinks in the coming months, be aware that morning fog will become more common across the Washington area. As always, get your latest 7-day forecast here.