ozone

A trioxigenic gas that is light blue in color and has a distinctive sharp smell. Ozone is a form of oxygen, but while an oxygen molecule contains two atoms, an ozone molecule contains three, which makes it less stable. Ozone is part of the earth's stratosphere, creating a protective layer that absorbs ultraviolet radiation. Occasionally it also occurs in the lower atmosphere, especially during thunderstorms. Ozone molecules are formed when electric charges pass through dry air, and although not visible, the gas will give the air a slightly pungent smell.