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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

Tropical Cyclone Nora: BOM issues warning as storm approaches Gulf of Carpentaria

01:08 EDT

Tropical Cyclone Nora is passing by north-east Arnhem land, before moving to the Cape York Peninsula and intensifying over the weekend.

Cyclone Marcus and Cyclone Nora joined by two other systems as storms converge on Australia

00:36 EDT

A very active cyclone season is stepping up around the Australian tropics, with the Bureau of Meteorology monitoring four significant systems spiralling around the country's north.

Cyclone Nora: BOM warns system may brew into category four storm in Gulf of Carpentaria

21:55 EDT

A severe category four tropical cyclone could develop off the Queensland coast on Saturday, the Bureau of Meteorology (BOM) has warned.