Have you felt like you’ve been stuck in a fog all week? No, I’m not talking about the the type of fog you feel when you haven’t had your morning coffee yet. I’m talking about the fog you can SEE. Well, perhaps I should phrase it like the fog you can’t see through when you leave for work in the morning.
This week has been incredibly foggy for millions east of the Mississippi. Day after day places like Chicago, Raleigh, Charleston and Washington, D.C., have experienced relentless fog that lasts late into the morning and even into the afternoon. Visibilities have dropped to near zero at times, causing ground stops at airports in Chicago and Philadelphia, and traffic jams in Houston and Charlotte.
Where are the crystal clear and cold December nights we’re used to for this time of year? Well this lack of wintry cold air is partly the reason why the dense fog has been so prolific all week. In fact, unusual warmth all week from the Midwest to the Northeast has our days and nights feeling more like September and October rather than December, and we can partly blame this fall-like warmth for the fog.
First of all, what is fog? It’s a very basic answer: a cloud that forms at or near the surface of the earth, and it can form one of several ways.
Advection fog forms when warm, moist air moves over a cold land area (or vice versa) causes the air to cool quickly down to the dew point which causes condensation into a cloud. Example: San Francisco.
Topographic fog, or “upslope fog,” occurs when air pushed up against a mountain cools and condenses into a cloud. Example: Denver in the springtime when easterly winds force moist air up the east side of the Rockies. “River or valley fog” occurs when dense cold air drains and gets trapped in these low-lying areas again cooling to the dew point producing a low cloud. Example: California’s “Tule Fog.”
Inversion fog forms when high pressure causes an inversion layer (warm air aloft) which traps colder air at the surface. This cold air cools the air near the surface which then condenses into a cloud. Example: Washington, D.C. earlier this week.
Radiation fog occurs during calm and clear nights when heat from the ground radiates out to space. This cools the air at the surface to the dew point promoting condensation and formation into a cloud. Example: Chicago.
So how does fog go away? You’ve likely heard the phrase, “the fog is burning off.” This is an incorrect description as fog does not “burn away” but instead evaporates, or dissipates once the sun rises and warms the air above the dew point. This dries out the air allowing the fog to dissipate.
The neat thing about fog and why it can make for such picturesque photos is the fact that it is often very shallow, sometimes only hundreds of feet in depth. That’s why the top of Trump Tower in the photo above, standing at nearly 1,400 feet, can be seen shining in blue sky above the fog layer. In fact, some of the most jaw-dropping weather photos out there are of fog taken from the tops of buildings or airplanes where only the tallest of skyscrapers can be seen peeking above the clouds.
The tallest buildings in #Pittsburgh rise above the fog this morning as the first light of day creeps into the sky pic.twitter.com/q87TXrzvby
— Dave DiCello (@DaveDiCello) December 7, 2015
Fog is beautiful#Chicago #Timelapse12/4/15
Posted by David Harpe Photography on Saturday, December 5, 2015
So if you happen to be one of millions waking up in a fog yet again this morning and you’re in a city with tall skyscrapers, head to the tallest building and see if you can get above the clouds.
Weather is awesome. #cwgpicoftheweek
