338 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
Exe AA, BON 
AES ES SESE 
Ficure 2.—Formation of the Flattop mirage by stages. No. 1, Normal skyline; No. 2, 
feelers reach up to height of highest peak, and having reached it, flatten out; No. 3, 
steamers extend from flattened tops until they meet, also from end hills; No. 4, windows 
formed filling in; No. 5, windows completely filled giving a seeming flattop mesa replacing 
old skyline. About 75 percent of these mirages stop development between stages 3 and 
4. Unmaking exactly reverses routine of building. 
of the highest peak. Touching it, as though reaching a ceiling, the 
top flattens out. From the flattened tops streamers reach out until 
they meet others from right and left. The openings or windows 
thus formed fill in slowly, from all sides, like the diaphragm of a 
camera, until the window is closed and we have a seeming flattened 
mesa in place of the old irregular skyline. Streamers have also 
reached out from the end hill masses for perhaps a mile or two. Sec- 
tions of these streamers may lose contact with their base and join up 
again or, more rarely, persist by themselves after the parent mirage 
has ceased to be. In the mesa face, windows may appear and fill in 
again; the display is not static. Finally, usually within an hour, 
windows appear and grow, the streamers and flattened tops draw 
back, the feelers fade away, and we have our group of broken hills 
again. 
This building and unbuilding of the mirage must be the result of 
progressive buildup and breakdown in the temperature-inversion 
layer and in the air lens it creates. At some point in this develop- 
ment the lens becomes capable of producing the effect of feelers 
