The definition of "gravity wave"from Wikipedia is :waves generated in a fluid medium or at the interface between two media of cloud..The technical definition is kind of complicated. Just Imagine you drop something in water and the ripples spread out. In the air, clouds form on those ripples, called gravity waves.Perhaps,Iwe can call it "a drops from heaven".
Near Smyer, Texas. Picture Credit: WTM.
Location:Northeast from Reese Center.
Gravity wave from eastern Hockley County at 7:30 am. Northeast towards Smyer, Texas.Credit: WTM.
Gravity wave with roll cloud feature (looking northwest) passing across Reese Center (Lubbock County).
Satellite image of gravity waves over the Arabian Sea.By:NASA-GSFC
The detail statistic about the illustration 1-4 can be found at:http://www.mesonet.ttu.edu