From AMS Glossary
Weakly luminous upward propagating discharges, blue in color, emanating from the tops of thunderstorms.
Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15° full width at vertical speeds of roughly 100 km s-1 (Mach 300), fanning out and disappearing at heights of about 40–50 km. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few millijoules per cubic meter. Blue jets are not aligned with the local magnetic field.