Hailstones are deemed to be either casual water or loose impediments, at the option of the player, according to the Definition of loose impediments at the beginning of the Rule Book
Casual water is an abnormal ground condition, and if the ball lies in the condition or it interferes with the player’s stance, relief may be taken.
More likely, the hailstones would be treated as a loose impediment. Loose impediments are naturally occurring objects such as leaves provided they are not adhering to the ball. Therefore, unless the ball is in a hazard, the hailstones can be brushed away. On the putting green if the ball is accidentally moved in the removal of the hailstones there is no penalty and the ball must be replaced.