Hail is precipitation in the form of spheres, or balls, of ice. Hail is usually associated with thunderstorms. Hail begins as a tiny ice crystal in a dark, towering cumulonimbus cloud. Updrafts and downdrafts are characteristic of cumulonimbus clouds. Swirling winds carry hailstones up and down through different regions of the cloud. Every time a hailstone goes through the lower part of the cloud it picks up a fresh coating of moisture. This moisture freezes as the hailstone is carried to the upper part of the cloud. The hailstone grows larger as it travels up and down. This gives hail a layered look, similar to an onion. Eventually the hailstone becomes heavy enough to fall to Earth's surface.