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Sand Stream

When this Pokémon is brought out, a permanent sandstorm covers the battlefield. It can only be removed by bringing out a Pokémon with Drizzle, Drought, or Snow Warning, or by using Rain Dance, Sunny Day, or Hail.

Smogon has an article on this topic.

When a Pokémon with this ability is first in the party, wild encounters have a lower chance of occurring, but only when venturing through an area affected by sandstorm.

Related Abilities

Drizzle is a rain equivalent.
Drought is a sun equivalent.
Snow Warning is a hail equivalent.

Pokémon Type Tier Abilities HP Atk Def SpA SpD Spe BST
Hippopotas Ground NU Sand Stream 68 72 78 38 42 32 330
Hippowdon Ground OU Sand Stream 108 112 118 68 72 47 525
Tyranitar Rock / Dark OU Sand Stream 100 134 110 95 100 61 600