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In terms of moves affected by the sand, Solar Beam gets a reduction in power and Thunder gets a reduction in accuracy. Restorative moves like Moonlight, Synthesis, and Morning Sun only recover 25 percent of the user's hit points.
With the right Pokemon, sand can be devastating. The ability Sand Veil increases a Pokemon's evasion rate during a sandstorm. Several Pokemon have this ability, including the powerful Garchomp. This Pokemon is an offensive menace; combined with a high evasion rate, Garchomp can be especially nasty during a sandstorm. Tyranitar, a ridiculously powerful Pokemon as it is, benefits greatly from sandstorms as well. Since it's a Rock Pokemon, it gets the defensive boost on top of its already high defense.
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One way to exploit the sand is to use moves that incrementally drain your opponents Pokemon. Moves like Toxic or Leech Seed are particularly filthy if your foe is using Pokemon that aren't resistant to sandstorms; the hit points lost from the weather and from a degenerative move will make things extremely difficult for your opponent or force a switch.
Sandstorms are arguably the toughest weather effect to deal with in this generation of Pokemon. If you're caught with your pants down in a sandstorm (not literally), you really only have two choices. You can switch out Pokemon, which essentially gives your opponents a free turn, or you can change the weather to something more preferable to your team. Many players that are new to competitive play terribly underestimate weather-control moves like Rain Dance and Sunny Day. Getting slaughtered by the doubles team of Garchomp and Tyranitar during a sandstorm will quickly teach them how vital it is.
