Monthly Storm Reports and Storm Data
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Are you interested in what happened during a recent event? Check out the report below.
 
Freezing Fog on December 12, 2012
 
The pattern at 600 pm CST on 12/08/2012. Cold high pressure built over Arkansas during the predawn hours of December 12th. Winds were nearly calm (no mixing) and the sky was mostly clear (no blanket of clouds). These are optimum conditions for temperatures to drop, and they did. Readings in much of  the region dipped into the teens and 20s.
In the picture: Fog formed along the Arkansas River as viewed from Burns Park (Pulaski County) early on 12/12/2012. Click to enlarge.

 

With subfreezing air hovering over the much warmer water of the Arkansas River (water temperature near 60 degrees), steam fog formed. The fog engulfed bridges in central Arkansas, including the Interstate 430 bridge in the Little Rock (Pulaski County) area.

The Interstate 430 bridge, which is about 6 miles west-northwest of downtown Little Rock (Pulaski County), spans a wide spot along the Arkansas River.
In the picture: The Interstate 430 bridge, which is about 6 miles west-northwest of downtown Little Rock (Pulaski County), spans a wide spot along the Arkansas River. A large fetch of water makes it easier to create steam fog when cold air settles over the area.

 

About Steam Fog

On December 12th, cold conditions over much warmer water created an unstable situation. Air near the water's surface had a tendency to rise (because it was warmer/less dense), and carried moisture upward. The added moisture saturated the environment, and this led to fog. There was enough vertical motion and available moisture to create a thick fog bank. Much the same process occurs when thunderstorms pop up, with updrafts driving moisture aloft and creating big cauliflower clouds.

 

A graphicast was issued by the National Weather Service in Little Rock (Pulaski County) early on 12/12/2012 to warn drivers of freezing fog and ice.
In the picture: A graphicast was issued by the National Weather Service in Little Rock (Pulaski County) early on 12/12/2012 to warn drivers of freezing fog and ice.
 

Because bridges are directly exposed to the surrounding air, water (from the fog) that deposited on the pavement froze. Freezing fog made the Interstate 430 bridge icy, and several traffic accidents resulted. The bridge was shut down temporarily.

 

Remember that bridges will ice before the (non-elevated) road because they are exposed to the surrounding air (no ground underneath to provide insulation).

This is a fairly common occurrence during the late fall and winter. People are reminded that when subfreezing temperatures are in the forecast and there is fog around, be extremely careful around bridges and overpasses! Many vehicles have temperature displays, and these can come in handy when you are nearing a problem area.

In the picture: Remember that bridges will ice before the (non-elevated) road because they are exposed to the surrounding air (no ground underneath to provide insulation).

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