The roof of the dome was covered by a water-vapor fog sculpture, designed by Fujiko Nakaya, titled Fog Sculpture #47773. An ever-changing cloud bank was generated by a system using pure water and impaction pin nozzles, designed and manufactured by Tom Mee, that introduced tiny water droplets into the air, that partially evaporated to increase the humidity  so that the tiny fog droplets could hover in the air.  As with natural clouds, the character of the fog depended upon the weather; as wind and humidity conditions varied, the shape, longevity, and position of the cloud changed. 

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​The system used 2500 impaction-pin nozzles that were installed one foot apart in ten foot long pipes, and the pipes were placed along the exterior ridges and the trough lines of the building's panels. The number of fog nozzles required to maintain the 150-foot cloud depended on the wind and humidity. In order not to fog in all the surrounding area, anemometers (wind gauges), humidity gauges, and thermometers were placed on the roof, to monitor wind and temperature conditions. Nine different sections of fog strands and nozzles  could be activated or de-activated to reduce or increase the amount of fog based on these measurements.

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​​The  impaction-pin nozzles consisted of a smooth, short, straight-through orifice with a specially engineered impaction pin in front of the orifice. A fine jet of water was emitted, which immediately would strike the pin, causing the water to form a cone of micro-fine particles. Pumps produced water at a pressure of 500 pounds per square inch so that water droplets exiting the nozzle and hitting the pin were ejected three to six feet out from the nozzle, hovering in the air to produce the cloud.