U
The terms and definitions found in the glossary relate to the science of meteorology.
For descriptions and definitions of words used in the Bureau's forecasts and warnings, please refer to Weather Words.
- 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.
- Umbra
- That part of the shadow cast by the Earth or the Moon within which the Sun is completely hidden.
- Unsettled
- Changeable, variable (Oxford Dictionary).
- Unstable air
- Air in which static instability exists. This condition is determined by the vertical gradients of air temperature and humidity.
- Updraught
- Upward moving current of air of small dimensions. A rapidly moving vertical wind as found in thunderstorms.
- Upwelling
- The rise to the surface of cold, deep ocean waters.