We estimate the voltage relative to the Earth inside thunderstorms and at cloud top from balloon soundings of electric field through convective regions and stratiform clouds. These are the first estimates we know of that give the voltage through the entire depth of storm clouds. Cloud top voltages were available for 13 soundings and ranged between -23 and +79 MV, with an average of +25 MV. The average cloud top voltage among the nine cases with positive values was +41 MV. These values support the Wilson <1920> hypothesis that thunderstorms drive the global electric circuit. The average cloud top voltage above electrified stratiform clouds was +32 MV, so these clouds may also make a substantial contribution to the global electric circuit. Within clouds the voltage values ranged between -102 and +94 MV in the 15 soundings. The voltage difference associated with intracloud and cloud-to-ground lightning flashes estimated from our soundings is in the range of about 20 to 130 MV. From the voltage differences inside storms we estimate the energy available for weak and strong lightning flashes as approximately 107 and 1010 J, respectively. ¿ 2001 American Geophysical Union