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Offshore breeze – a wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore forecast – a marine weather forecast for the waters between 60 and 250 miles off the coast.

Onshore breeze – a wind that blows from a body of water towards the land. Also known as a sea breeze.

Outflow – air that flows outward from a thunderstorm.

Outflow winds – winds that blow down fjords and inlets from the land to the sea.

Overcast – sky condition when greater than 9/10ths of the sky is covered by clouds. In Canada overcast means grey and dull skies, with extensive cloud cover.

Overrunning – a condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.

Ozone – a form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. It is also a primary component of smog.

Ozone hole – a thinning of the ozone layer over Antarctica, which occurs each spring.