A once in a decade occurrance at the Grand Canyon...
Typically, the temperature of the air decreases with height. When the temperature of the atmosphere is "inverted," warm air sits on top of cooler air.
"The warm air above cooler air acts like a lid, suppressing vertical mixing and trapping the cooler air at the surface," explains the National Weather Service.
Temperature inversions happen once or twice a year, typically in the winter months. However, ranger Erin Whittaker told MailOnline that the most recent inversion only happens once every 10 years — it's special because the fog filled up the entire canyon (not just parts) and it happened on a cloudless day. As a result, the view was particularly beautiful....
http://www.businessinsider.com/temperature-inversion-creates-rare-sight-at-the-grand-canyon-2013-12
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Enjoy!