U

Ultraviolet radiation

Electromagnetic radiation with wavelengths less than about 0.3 µm, that is shorter than visible radiation but longer than X-rays. It can be subdivided into UV-A, UV-B and UV-C.

Upslope wind

Wind directed up a mountain slope, component of the mountain-valley wind systems.

Upward motion

Upwelling

Ascending motion of cold, deep water to the surface to replace surface water; it is moved from the area of upwelling by divergent horizontal flow.

UV Index / UltraViolet Index

International standard measurement of the intensity of the UV radiation from the Sun, at a particular place, on a certain day.