Question

What are all the effects that Hail has while in use?

Answer 1

All Pokemon except Ice types and those with the Ice Body or Snow Cloak abilities lose 1⁄16 of their max HP at the end of each turn.

Pokemon with the Ice Body ability recover 1⁄16 of their max HP each turn.

Pokemon with the Snow Cloak ability increase their evasion by 20%.

From Generation 4, Blizzard has 100% accuracy.

In Diamond/Pearl only, Blizzard has a 25% chance to break through Protect and Detect.

The move Weather Ball changes to Ice type and doubles in power.

The power of SolarBeam decreases by 50%.
The moves Synthesis, Morning Sun and Moonlight recover 1⁄4 of the user's max HP (instead of the usual 1⁄2 HP).

Castform changes to its Hail form.

Source

Answer 2

Damages all Pokémon who are not Ice-type.
Does not damage Pokémon with the following Abilities: Ice Body, Snow Cloak, Magic Guard, Overcoat.
Allows Blizzard to bypass accuracy check in Generation V.
In Pokémon Diamond and Pearl, allows it to hit through Protect and Detect 25% of the time.
Causes SolarBeam to deal only half of its normal damage.
Causes Moonlight, Synthesis, and Morning Sun to recover ¼ of max HP.
Activates the Abilities: Forecast, Ice Body, Snow Cloak.
Changes Weather Ball to an Ice-type move and doubles its power, and Castform to its Hail form.
Can be lengthened from 5 to 8 turns with the use of the Icy Rock.