Hailstorm is a heavy and slow bomber that can deal great amounts of damage flying over units and structures.
It is mostly designed to destroy buildings, but used well it can also deal high damage to mobile units - try to intercept their movements.
It deals much higher damage than its T1 form (Whirlwind). As little as 3 Hailstorms can destroy a fusion powerplant on one flyby (4-5 for an adv. fusion). They can drop another round of bombs every 7 seconds. It is more expensive but also stronger than its Armada counterpart - Blizzard.