Infrared (IR) satellite imagery measures the temperatures of the tops of clouds.  Typically IR satellite imagery is used to see how high into the atmosphere clouds extend.  The colder the top of the clouds are, the higher into the atmosphere clouds extend.  Thunderstorms typically have the coldest cloud tops as they can reach as high as 50,000 or 60,000 feet above the ground and reach temperatures as cold as -70 degrees Celsius.  When clouds are not present, then the IR satellite can measure the temperature of the ground and the air near the ground.  On some occasions, a strong front can be seen on the satellite as it measures differences in temperature ahead of and behind the front.

Below are two satellite pictures.  The first picture is the satellite image with the temperature (upper left number for each location) and dew point (lower left) readings across South Dakota at 9 pm Sunday evening.  The second picture includes the location of the cold front on the same picture.

After sunset on Sunday evening, April 1, a strong front was moving across South Dakota.  Ahead of the front, around Pierre and Winner, temperatures cooled rapidly in the evening because of clear skies and light winds.  Meanwhile, immediately behind the front, winds increased to 15 to 30 mph.  These stronger winds brought warmer air back to the surface immediately behind the front, as seen in Philip.  As seen on the second picture below, the front extended from just west of Mobridge, South Dakota, to just east of Philip, South Dakota.  Ahead of the front, especially around Pierre, the lighter grey shades indicated cooler temperatures where readings had cooled into the 60s.  Just behind the front, the darker shading indicated areas where temperatures were still in the 70s such as near Philip.  Using a conversion table, we found that the satellite measured a temperature difference of 7 to 11 degrees across the frontal boundary.  Going farther west toward Buffalo and Rapid City, notice that the IR imagery again becomes lighter grey as colder air begins to move into western South Dakota.  The observations from both locations showed that temperatures were even colder than in Pierre.  This cool air will gradually spread across Northern Plains tonight and Monday.

IR Satellite Image from 9 PM CDT 2012 April 01

IR Satellite Image with Cold Front from 9 PM CDT 2012 April 01


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