Infrared satellite images shows how cold it was Tuesday morning

Infrared satellite picture from 8 am January 22, 2008

Infrared satellite pictures are typically used to measure the temperature at the top of clouds.  This helps meteorologists discern cumulonimbus clouds, which are colder and higher in the atmosphere and associated with thunderstorms, from stratus clouds which are lower in the atmosphere and warmer.  Typically, most meteorologists look for clouds that are colder than -20 C or -4 F.  So temperatures that are warmer than -20 C are generally colored grey. Colder cloud temperatures show up in different colors so they can more easily be seen. 

On the picture above, however, skies are clear yet there is coloring on the satellite pictures.  In this case, the IR satellite picture is displaying the temperature near the ground.  Because temperatures across the area are between 0F and -10F, only the coldest areas are colored in the above picture.  In many cases, these colder areas are coincident with river valleys across the area.  Three river valleys are most apparent on the image - the Big Sioux River Valley which is along the eastern border of South Dakota, the Missouri River Valley which forms the northeastern border of Nebraska and the Little Sioux River valley in northwest Iowa.  Other areas in northern Iowa and southern South Dakota which are colder are not necessarily associated with large river valley but are areas with relatively lower elevation.

This is typical during nights with clear skies and light winds.  Cold air will tend to settle in areas where the elevation is lowest. Therefore valley locations will typically be as much as 5 or even 10 degrees colder than higher elevations - even in the eastern Plains where there is not a lot of topography.  Because of the frigid temperatures, this phenomenom was evident on this morning’s satellite picture.

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