800px-Occluded_mesocyclone_tornado5_-_NOAA bw.jpg

Caption: 
Occluded mesocyclone tornado. Occluded means old circulation on a storm; this tornado was forming while the new circulation was beginning to form the tornadoes which preceeded the F5 Oklahoma City tornado, 1999
Credit: 
OAR/ERL/National Severe Storms Laboratory (NSSL)
Sustainable development theme: 
Country / location: 
Anadarko, Oklahoma, USA