there was cold air settling into the plains with warm chinook winds blowing off the black hills creating a strong temperature inversion. the south side of Spearfish South Dakota was 51 degrees and the north side 4 miles away was 16 degrees.

cold air is more dense than warm air and the cold air separated from the warm air and the 35 degree temperature difference was all within a few hundred feet difference in elevation.

below the warm air was completely calm but as soon as you rose in elevation into the inversion it instantly became very windy as well.

the boundary between the warm and cold air causes the speed of light to change and ends up bending the light rays causing a mirage