Natural causes involve those that act on the roof because of the environment in which the roof must perform. Snow, ice, rain, hail, and wind can cause physical damage to roofing systems. Temperatures as high as 160 degrees harden and break down roofing materials. Temperatures below zero compel them to become hard. Ultraviolet radiation from the sun chemically alters roofing materials, weakening them. Air pollution combines with rain to form acids that attack roof surfaces. Oxidation weakens materials, allowing thermal stresses to generate tears and splits. Vegetation, fungus, and algae attack roofing materials, demolishing them down or causing physical damage that allows water to enter the roofing system. Ponded water can chemically alter and compress materials, reducing their strength and puncture resistance.