With quiet weather and several mornings in a row of fog I thought today would be a good time to type up our next Weather Wednesday article! These articles help explain interesting weather phenomena. Because we’ve had so much fog this week I thought you might be interested in knowing how fog forms and why!
Let’s start off by explaining why we’ve been dealing with fog this week. To do that I picked up a graphic depicting weather balloon data from Tuesday morning in Fort Worth. This is when very dense fog had the airports socked in and made for a nasty day to be flying.
This type of graphic is called a SKEW-T and would normally display the following data: air temperature, dewpoint temperature, wet-bulb temperature, wind direction, and wind speed among other data in a 2D representation of a 3D environment (altitude). With that said I’ve removed all but the air temperature and dewpoint temperature since that’s all that we really need for this explanation. I could have left the winds in for data but it’s just easy enough to say that calm or nearly calm winds are best for most types of fog to form.
The type of fog we’ll be talking about is Radiation Fog. No that does not mean you’re being radiated by toxic fog. Per the National Weather Service this type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and is always found at ground level and usually remains stationary. It goes without saying this type of fog can reduce visibility to near zero as some of you have found out this week.
You can see on the SKEW-T (above) that the dewpoint temperature and air temperature come together in the lowest few hundred feet of the atmosphere. Above that the air temperature quickly increases and the dewpoint temperature decreases. That’s a pretty strong inversion but you may know it better as a cap. This cap kept cooler/moist air confined to the lower atmosphere and helped fog form. You may also notice this type of inversion when smoke has a tendency to stick closer to the ground instead of rising. We’ve had a fairly stagnant weather pattern this week which has kept surface winds light and our air fairly still. Once the sun rises and begins to warm the atmosphere fog will usually dissipate as humidity levels drop. The opposite occurs at night when the surface or low levels of the atmosphere are moist. That can happen after rain saturates the soil or when frontal boundaries are in the area resulting in one airmass at the surface with another air mass a few thousand feet above it like in the SKEW-T I posted with this blog. Here’s a great photo from Justin Terveen (an amazing photographer) showing the dense fog in the lowest few hundred feet of the atmosphere. Above it was nothing but clear skies! This photo was taken on Tuesday in Dallas.
Here’s another photo this time from an aircraft flying above D/FW. Those buildings are actually all you can see of Downtown Dallas. Everything else is shrouded in a few hundred feet of dense fog. Most commercial aircraft flying today have the ability to continue operations in dense fog. However airport operations will still be delayed quite a bit since they’ll only be able to operate with runways featuring specialized equipment. The same goes for aircraft and cockpit crew. That explains why some flights got in yesterday morning while others were either cancelled or diverted.