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Weather Glossary - U



A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.

Ultraviolet radiation

Electromagnetic radiation with wavelengths shorter than visible light, but longer than x-rays. Exposure to too much UV radiation can cause skin cancer.


Changeable or variable weather.

Unstable air

Air in which static instability exists. This condition is determined by the vertical gradients of air temperature and humidity.


A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

Updraft base

Alternate term for a rain free base.

Upper level

The portion of the atmosphere above the lower troposphere. It is generally applied to levels above 850 hPa.

Upper level system

A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term is sometimes used interchangeably with impulse or shortwave.

Upslope flow

Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upslope fog

Fog that forms when warm moist air is forced up a slope by wind.


Toward the source of the flow, or located in the area from which the flow is coming.


The rise of colder, deep ocean waters along a coast due to the movement of surface water away from the coast.


Abbreviation for Universal Time Coordinate. See also Zulu, GMT.

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Weather News

Thundery wake up call for the Northern Rivers

13:02 EDT

Those in parts of the Northern Rivers had a thundery wake up call on Sunday morning as a storm rumbled overhead.

Outlook not great for SA firefighters

15:39 EDT

Conditions will not be favourable for fires burning north of Adelaide as warming temperatures, little rain and strengthening winds move across the region early next week.

Publican tells of "frightening" fire that swept through Olary

13:33 EDT

The publican in the South Australian town of Olary said historic buildings and cottages stood no chance against a fire that tore through the town.