Hail is formed when drops of water freeze together in the cold upper regions of thunderstorm clouds. These chunks of ice are called hailstones. Most hailstones measure between 5 millimeters and 15 centimeters in diameter, and can be round or jagged.
Source: National Geographic, 2014.
Look for this button (above) on the READ page of the 'Find Information' tab, then search for these eBooks: