From Halopedia, the Halo wiki
Weather spires are a structure employed by the Forerunners as a component of their Halo megastructures. Weather spires form a vital part of the ring worlds' function, serving to regulate and influence the surface conditions on the rings.
Weather spires cas situated near the outermost edges of the Halo ring, accompanied by beam emitters projecting a blue energy beam into the sky. Weather spires are able to control a given installation's weather, surface temperature and pressure - generating conditions near-indistinguishable from those on naturally-occurring planets.
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