Sky voltage refers to an electrostatic potential difference (voltage) that is present in the free air of the atmosphere with respect to the earth's ground surface. The voltage varies with atmospheric humidity, dropping lower on days with high humidity, and higher in very dry air, averaging about 120 volts per meter.
Tall insulated metallic objects can absorb these voltage gradients out of the atmosphere, and conduct the voltage to a terrestrial collection and measurement station.
Low-power electrostatic motors and toys can be driven from this collected sky voltage when one side is connected to earth ground, and the other is connected to the sky collector, such as a kite or balloon with a foil collector and fine wire running down to the ground. A collector 200-300 ft high can develop a sky to ground potential as high as 20,000 volts. 
Sky voltage and the electrical neutral wire
Sky voltage was a problem for early electrical power and communications grids which used suspended insulated bare wire to transport current. These suspended wires also acted as sky voltage collectors, and could result in dangerous electric sparking from electrical devices to nearby earth grounded metal, or sparking to the people touching the electrical devices. Due to having wires spanning tens and hundreds of miles, the wire has the capacitive potential to store large charge quantities across the system that could prove lethal to energy customers. Lightning striking near an electrical power device also had the potential to leak dangerous high voltages into the system that could spread throughout the system in moments.
In order to safely dissipate this collected sky voltage, one current-carrying wire of the electrical grid is connected to a grounding rod at regular intervals throughout the power system. This serves to discharge and bleed off any large voltage charges being collected from the suspended wires. This wire is called the neutral, since its voltage is zero relative to the earth ground.
Modern three-phase transmission lines also include a fourth suspended grounding wire which carries no load, but helps to absorb the sky voltage and prevent electrostatic buildup in the other poly-phase current-transporting wires.
- Popular Science magazine, The Amazing Motor that Draws Power from the Air, April 1971, p80-81 p154. Direct link to cited article, scanned by Google Books: http://books.google.com/books?id=kwAAAAAAMBAJ&pg=PA80&lpg=PA80
- Popular Science magazine, Electrostatic Motors You Can Build, May 1971, p95-97 p114. Direct link to cited article, scanned by Google Books: http://books.google.com/books?id=ngAAAAAAMBAJ&pg=PA95