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OCCLUDED FRONT – A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.
OFFSHORE WATERS – Ocean waters from 60 nm to 250 nm.
OMEGA – A term used to describe vertical motion in the atmosphere. The “omega equation” used in numerical weather models is composed of two terms, the “differential vorticity advection” term and the “thickness advection” term. Put more simply, omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere. On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.
OPEN WAVE– A wave of low pressure that does not have a complete circulation around it; also called a short wave trough.
OROGRAPHIC UPLIFT – The vertical forcing of air by terrain features such as hills or mountains. This can create orographic clouds and/or precipitation.
OUTFLOW – Air that flows outward from a thunderstorm.
OVERCAST – Sky condition when greater than 9/10 of the sky is covered
OVERSHOOTING TOP – A ‘bubble’ of cloud sticking up above the anvil of a thunderstorm, due to a vigorous updraft within the storm.
OZONE – A form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering out much of the sun’s ultraviolet radiation.