How high into the sky can thunderstorms develop – what are the highest even measured thunderstorms?
The strongest updrafts reach the tropopause and produce overshooting tops. The highest storms in the world have been recorded in the tropics, where the tropopause is the highest (15-18 km). Thunderstorms reaching 20-22 km high have been recorded there.
July 23, 2010, Vivian, South Dakota severe supercell thunderstorm, which produced the world record hailstone (20 cm in diameter!) reached a maximum height of 17 km! This is all the more remarkable, since this storm was far from the tropics, with the tropopause at a lower altitude, indicating an exceptionally strong updraft.
Recently, on May 26, 2016, a severe supercell thunderstorm on the Mexico-USA (Texas) border was measured at 20.7 km peak height; the storm was so intense it produced lightning up to 95 km from its core!
Keep in mind that the average cruising altitude of commercial airplanes is 11-12 km, so these thunderstorms are up to twice as high as you would normally go on your flights!