When a Pokémon with Delta Stream enters the battle or a Pokémon gains the Ability, a unique weather condition called mysterious air current is created. In a Rotation Battle, Delta Stream will not activate if the Pokémon is sent out in a rotated-out position, nor will it activate when that Pokémon is rotated in. In addition, Delta Stream will not activate if a mysterious air current is already present on the battlefield or if the Pokémon's Ability is suppressed prior to it gaining Delta Stream.
Once in effect, as long as there is a Pokémon on the field with the Ability Delta Stream active (even if it is not the Pokémon who created it), the mysterious air current will remain in effect unless it is replaced. (If the only Pokémon on the field with Delta Stream is rotated out or switched out, or has its Ability suppressed or replaced, the mysterious air current will dissipate.) The mysterious air current can be replaced by Desolate Land or Primordial Sea.