Illustration of a cumulonimbus hail storm cloud. Hail is a form of precipitation made up of balls of ice and snow. Hail forms in cumulonimbus clouds due to the violent convection currents present (blue and red arrows). Particles of rain or snow are drawn vertically up through the cloud and collide with supercooled water and other particles. These collisions lead to the particles sticking together. When they become too heavy to be supported by the air currents, they fall to earth. Some hailstones can be over 10 centimeters across.