Weather is one of Pokemon\'s deepest strategic systems. Five weather conditions exist: Sun, Rain, Sandstorm, Hail/Snow, and the special Strong Winds. Each fundamentally changes how the game plays out.
Sun (Harsh Sunlight)
Sun boosts Fire moves by 50% and halves Water moves. It also enables one-turn Solar Beam and activates Chlorophyll (2x Speed on Grass Pokemon), Solar Power (+Sp.Atk on Fire Pokemon), and Growth (+2 instead of +1). Set by the Drought ability (Ninetales, Torkoal, Charizard-Y).
Rain
Rain boosts Water moves by 50% and halves Fire moves. It enables Thunder (100% accuracy), Hurricane (100% accuracy), and Swift Swim (2x Speed). Set by Drizzle (Politoed, Pelipper, Kyogre).
Sandstorm
Sandstorm damages non-Rock/Ground/Steel Pokemon by 1/16 HP each turn. It also boosts Rock types\' Special Defense by 50%. Enables Sand Rush (2x Speed) and Sand Force (+30% Rock/Ground/Steel moves). Set by Sand Stream (Tyranitar, Hippowdon).
Hail / Snow
Hail damages non-Ice Pokemon by 1/16 each turn. In newer games, Hail was replaced by Snow, which boosts Ice types\' Defense by 50% instead of damaging. Enables Slush Rush (2x Speed on Ice), Ice Body (healing), and Blizzard (100% accuracy).
Building a Weather Team
Pick a weather setter with good bulk, then stack 4-5 Pokemon that benefit from that weather. Example: Politoed + Swift Swim Ludicolo + Swift Swim Kabutops + Choice Scarf attacker + Water immunity pivot. Weather teams trade some flexibility for massive offensive pressure.