>>11791985From Bulbapedia:
Generation II
In the Generation II games, three moves were introduced that could change the weather during a battle to alter the playing field slightly. These moves were Sunny Day, which caused intense sunlight for five rounds, which allowed a Pokémon to attack with SolarBeam without waiting a turn; Rain Dance, which caused heavy rain for five rounds, which increased Thunder's accuracy to 100%; and Sandstorm, which caused a sandstorm. These three moves can change the course of a battle, with intense sun and heavy rain powering up Fire- and Water-type moves, respectively, and powering down the other, and a sandstorm slightly damaging any Pokémon that are not Rock-, Ground-, or Steel-type.
Although weather conditions were introduced in this generation, they could not be seen in the overworld until the next generation.
Generation III
The Generation III games introduced a new weather condition: hail. Hail is caused by the identically titled move Hail, which, like Sunny Day, Rain Dance, and Sandstorm before it, changes the weather for five turns. Hail, similar to sandstorm, slightly damages all Pokémon that are not Ice-type each turn.
