Correct Answer: Inversion

Solution : The correct answer is Inversion.

Temperature inversion is a phenomenon in which temperature increases with increasing altitude. This phenomenon is visible in hilly areas and is the main reason for formation of fog. During the night time in hilly areas of higher latitudes have lower temperature than lower valleys, because of which the air in the upper location of hill become dense. This density variation causes the denser air to come down to valley and the warmer air of valleys goes up. This phenomenon is known as temperature inversion.