pratt:

Cloudscapes: Japanese architect Tetsuo Kondo and a German climate engineering firm Transsolar came together to put a cloud in a large interior space called the Corderie, a 316-meter-long space where ropes for Venetian ships were made. Visitors to the exhibit can walk through the cloud via a circular ramp that ascended 4.3 meters high. This feat of climate engineering is produced by blowing three layers of air into the space at different temperatures. Cool dry air at the bottom layer keeps the cloud up; warm, humid air in the middle creates the dense fog; hot, dry air sits on top.