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How Fog Forms!

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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.

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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.

Finally there are other types of fog include Advection Fog, Upslope Fog, Ice Fog, Freezing Fog, and Evaporation Fog whichall occur under different weather and environmental conditions. We don’t usually deal with upslope or ice fog in Texas but we do have to deal with freezing and evaporation fog on occasion. Freezing fog is simply fog that occurs while surface temperatures are below freezing. If dense enough it can act like light freezing rain and deposit a thin layer of ice on objects. Evaporation fog typically occurs here in Texas when are lakes or other bodies of water are warm with a very cold airmass in place. WAn example would be when our low temperature dips to 17 degrees fahrenheit but a lake temperature is still closer to 50 degrees fahrenheit. In that case you’ll see evaporative or mixing fog form just above the water’s surface. You can also see something similar occur when air inside a freezer condenses (turns to fog) when you expose it to the warmer air outside the freezer.

November 20th Texas Drought Monitor

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As of this week nearly a third of Texas was completely out of drought conditions. That’s the highest number we’ve seen in at least a year. While Southwest Texas and extreme Southwest Texas are not in drought conditions a majority of the state continues to experience a long-term drought. The worst drought classification currently resides over western North Texas and Northwest Texas where an exceptional drought continues. While snow did fall on Sunday across parts of the region the liquid equivalent was less than a tenth of an inch which is barely a drop in the bucket. Moderate to severe drought conditions continue across Northeast, Central, and South-Central Texas plus the Hill Country and Panhandle. The good news is we’re expecting at least one inch of widespread rain to fall over North Texas by this weekend which will fill a small hole in a very large crater. It’s better than no rain though. Here’s a summary that accompanied this week’s Texas drought monitor.

Southern Plains and Texas
Bitter cold — albeit dry — weather resulted in no change to the drought depiction except along the Texas Gulf Coast. Temperatures averaged 15 to 25°F below normal for the week, with some shallow snow noted over northern portions of the region at the end of the monitoring period. Despite the frigid, mostly dry conditions, some Abnormal Dryness (D0) was reduced along the southeastern coast of Texas where rainfall totaled locally more than 2 inches. Short-term drought remained most intense (Exceptional Drought – D4) along the Texas-Oklahoma border west of Wichita Falls, where 90-day precipitation has totaled less than 50 percent of normal. In contrast, many of the long-term drought areas (“L” designation) from Texas into Colorado have received above-normal precipitation over the past 90 days, but are still wrestling with the impacts of longer-term deficits (60-80 percent of normal over the past three years).

October 5/6, 2014 Storm Photography

Strong thunderstorms unexpectedly formed along a southward moving outflow boundary in Central Oklahoma late last night and continued into this morning. I ended up photographing four separate rounds of storms over the course of several hours. The first photo shows a strong multi-cellular thunderstorm taken on a high hill west of Norman, Oklahoma.

Strong thunderstorm west of Norman, OK late on October 5, 2014. This photo is owned by Paige Burress with all rights reserved.

Strong thunderstorm west of Norman, OK late on October 5, 2014. This photo is owned by Paige Burress with all rights reserved.

This second photograph was taken on a parking garage on the University of Oklahoma’s campus in Norman, OK sometime after 2 AM. It goes without saying these thunderstorms were very electrical in nature. Considering storms were not in the forecast at all this is easily one of the best photography events of the year for me.

Lightning from the University of Oklahoma's campus early on October 6, 2014. This photo is owned by Paige Burress with all rights reserved.

Lightning from the University of Oklahoma’s campus early on October 6, 2014. This photo is owned by Paige Burress with all rights reserved.

@TxStormChasers

/TxStormChasers

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