A rotor ventilator comprising a ventilator stator for mounting to a structure and a ventilator rotor for mounting and rotation with respect to the ventilator stator is disclosed. The ventilator further comprises a cover for positioning over the ventilator rotor and mounting to the ventilator stator. The cover comprises a central portion surrounded by an outer substantially concave portion. The outer concave portion of the cover may protect the ventilator from environmental conditions/elements, such as by redirecting particles, water droplets, debris and dust, etc away from the interior of the ventilator. The outer concave portion may act as a trough, from which the particles can then be directed off the cover. The trough may also act to prevent the particles from escaping over the sides of the cover, and into the ventilator.