Ultraviolet Radiation - radiation having a wavelength shorter than wavelengths of visible light and longer than those of X rays

Universal Coordinated Time (UTC) - a global time scale based on the local time observed on the Prime Meridian; also called Greenwich Mean Time (GMT) or Zulu (Z) time

Unstable Air - generally, air which will continue to rise and accelerate when briefly forced upward

Updraft - a relatively small-scale current of air with marked upward motion

Upslope Flow - air that flows toward higher terrain and, hence, is forced to rise

Upslope Precipitation - precipitation that forms due to moist, stable air gradually forced upward along an elevated plain (e.g., the Rocky Mountains)

Upstream - toward the source of the flow, or located in the area from which the flow is coming

Upwelling Radiation - the component of radiation (either reflected solar or emitted terrestrial) directed upward from the earth's surface; opposite of downwelling radiation