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Ultramafic -  Rock that is rich in magnesium and iron content.
Ultraviolet Radiation -  Electromagnetic radiation with a wavelength between 0.1 and 0.4 micrometers (µm).
Unconfined Aquifer -  Aquifer that is not restricted by impervious layers of rock.
Unconfined Groundwater -  Groundwater that is not restricted by impervious layers of rock.
Unconformity -  A break in the sequence of sedimentary strata. Often the unconformity surface is the result of erosion.
Undercut Bank -  Steep bank found on the inside of stream meanders. Formed by the erosion that occurs when a stream channel moves horizontally.
Uniformitarianism -  Is a theory that rejects the idea that catastrophic forces were responsible for the current conditions on the Earth. The theory suggested instead, that continuing uniformity of existing processes were responsible for the present and past conditions of this planet.
Universal Time -  (UT) The mean solar time of the meridian at the Prime Meridian. Universal Time replaced the time standard known as Greenwich Mean Time in 1928. Universal Time is commonly used to denote solar time.
Universal Transverse Mercator -  (UTM) Grid System: Rectangular coordinate system used to find location of points on the Earth's surface. Based on the Universal Transverse Mercator projection system.
Universe -  All of the observable phenomena in the celestial cosmos.
Unloading -  The releasing of downward pressure on rocks because of removal of overlying material by erosion. Unloading can cause the development of horizontal bedding in once solid rock.
Unstable Atmosphere -  Condition in the atmosphere where isolated air parcels have a tendency to rise. The parcels of air tend to be warmer than the air that surrounds them.
Unstable Equilibrium -  In an unstable equilibrium the system returns to a new equilibrium after disturbance.
Updraft -  Upward movement of air.
Upper Air Westerlies -  Consistent winds that exist in the upper troposphere that flow east to west from about 20° of latitude to the poles.
Upper Mantle -  Layer of the Earth's interior extending from the base of the crust to 670 kilometers below the surface. Part of the Earth's mantle layer. The upper mantle is composed of peridotite, an ultramafic magma primarily made up of the minerals olivine and pyroxene. The top layer of the upper mantle, 100-350 km below surface, is called the asthenosphere.
Upslope Fog -  Fog produced by air flowing over topographic barriers. As the air is forced to rise, it is cooled by adiabatic expansion. Upslope fog is most common on the windward slopes of hills or mountains.
Upwelling -  The movement of nutrient-rich deep seawater to the ocean's surface.
Urban Area -  Geographic area with a high density of people over a limited area. Homes and other types of buildings tend to be close together. Urban systems also tend to differentiate themselves spatially into particular types of human activities.
Urban Heat Island -  Observed condition that urban areas tend to be warmer than surrounding rural areas.
Urbanization -  Expansion of cities into rural regions because of population growth. In most cases, population growth is primarily due to the movement of rural based people to urban areas. This is especially true in Less Developed Countries.