Hailstones develop due to the updrafts and downdrafts that are produced within cumulonimbus clouds during a thunderstorm.  Within these clouds exist supercooled water droplets.  These droplets are transformed into ice due to two factors: temperatures below 0 degrees Celsius and tiny particles of solid matter where the droplets can collect known as freezing nuclei.  As the supercooled water droplets collect onto the freezing nuclei, the ice crystals grow into hailstones.  An ice conglomeration can only be called a hailstone when it reaches 5 millimeters in diameter.  Typically, hail occurs in short outbursts, rather than as a steady precipitation.