The electrical current in the atmosphere that is required to balance the observed air–earth current of fair-weather regions by transporting positive charge upward or negative charge downward. Accounting for the supply current has been for many years a key problem of the field of atmospheric electricity and has received much attention. A quasi-steady current of about 1800 A for the earth as a whole is estimated to be required to balance the air–earth current. Wilson (1920) suggested that the thunderstorms present in widely scattered regions of the earth at any one time might be responsible for the supply current. Although this suggestion has not been fully confirmed, there is growing conviction that this is correct. When one considers an average over many storms, thunderstorm lightning transports negative charge downward to earth, as does point discharge in the regions below thunderstorms. Also, positive ions flow upward above active thunderstorms. See air–earth conduction current, point discharge current.