What do you call the thingy on top of a barn?
Barn with Cupola
The small round dome on top of the barn is a cupola. They are also square and rectangular.
Square cupola on barn
Barn cupolas grew in popularity after the Civil War. While they may add some light into the barn, the primary purpose is to allow ventilation. This helps to keep stored hay dry, venting built-up heat, and deterring spontaneous fires.
Larger barns have more than one cupola.
Barn with two cupolas
Abandoned, old, and interesting Kansas 