During the summer of 1986, the air conductivity and the vertical electric field were measured over thunderstorms from a high-altitude U-2 airplane. The conductivity near 20 km was found to be relatively steady above storms with variations less than ¿15%. In addition, the positive and negative conductivities were at all times approximately equal and displayed a very similar temporal/spatial behavior. While traversing thunderstorms, sustained upward directed electric fields were maintained along the flight path over over distances of 20 to 40 km, and electric fields in excess of 5 kV/m were often observed while passing over very intense storm centers. Estimates of the storm generator current and the Wilson conduction current were obtained for 15 storm over-flights using the current densities derived from the conductivity and field measurements and integrated over area. Area-integrated Maxwell currents of 0.09 to 5.9 A were observed with an average of 2.2 A. The U-2 data show that this measure of storm current varies linearly with flash rate, which suggest that the average charge transfer per discharge is independent of storm development or activity. The Wilson conduction current, which contributes to the global circuit, varied between 0.09 and 3.7 A above storms with an average of 1.7 A. The conduction current was not found to be linearly related to the total flash rate but, instead, leveled off with increasing flash rate. This result may indicate that the relative efficiency of a thunderstorm to supply current to the global electric circuit has an inverse relationship to the strength of the cloud electrical generator. ¿ American Geophysical Union 1989