U-Shaped Valley - A valley that has a U-shape rather than the typical V-shape. This shape suggests the valley was created by erosion associated with a glacier.

Ultisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. A tropical or subtropical soil that is in an advanced state of development. These soils have pronounced eluviation, clay accumulation in a subsurface layer, and are often poor in base cations. Also called ferrisols. 

Ultramafic - A rock that is rich in magnesium and iron content.

Ultramafic Magma - A type of magma that is relatively poor in silica (less than 45%) but rich in magnesium and iron content. This type of magma solidifies to form dark-colored igneous rocks rich in magnesium and iron but relatively poor in silica, aluminum, and calcium. Ultramafic magmas are very hot (>1500°C), and current conditions do not allow for the formation of ultramafic rocks on the Earth's surface.

Ultraviolet Radiation - Electromagnetic radiation with a wavelength between 0.1 and 0.4 micrometers (µm).

Unconfined Aquifer - A type of 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 - A steep bank found on the inside of stream meanders. Undercut banks are formed by the erosion that occurs when a stream channel moves horizontally.

Underfit Stream - A type of misfit stream that is too small to be responsible for eroding the valley that surrounds it. This type of stream is common in landscapes where past glacial erosion is responsible for forming the valley.

Underplating - A tectonic process associated with the subduction of an oceanic plate underneath continental crust. This subduction causes the accumulation of partially melted magma beneath the continental crust and just above the descending oceanic plate. The partially melted magma that accumulates is mafic or ultramafic in chemical composition. Partial melting occurs because of the lowering of the melting temperature of the rock from the input of water and other substances by the process of subduction. Underplating helps explain the observation of mountain building and volcanic activity along the edges of many continental boundaries. 

Uniclinal - Layers of rock strata that dip evenly in one direction.

Uniformitarianism - Is a theory that rejects the idea that catastrophic forces were responsible for the current conditions on the Earth. The theory instead suggested that the continued uniformity of existing processes was responsible for the present and past conditions of this planet.

United Nations Framework Convention on Climate Change (UNFCCC) - Is a United Nations-backed international environmental treaty (convention) negotiated at the Conference on Environment and Development (UNCED), also known as the Earth Summit, held in Rio de Janeiro from June 3 to 14, 1992. The focus of this treaty was to stabilize atmospheric greenhouse gas concentrations at levels that would prevent hazardous anthropogenic modification of the Earth's climate system. However, this convention is not legally binding and imposes no mandatory limits on greenhouse gas emissions for individual countries. This convention does provide for legally binding ratified agreements, called protocols, such as the Kyoto Protocol, that could set mandatory emission limits at the national level. See the following website for more information: http://unfccc.int. 

United States Natural Resources Conservation Service Soil Classification System - A hierarchical system used in the United States to classify soils. This system has six levels: order, suborder, great group, subgroup, family, and series. At the order level, 12 types of soils are recognized: gelisols, oxisols, aridisols, mollisols, alfisols, ultisols, spodsols, entisols, inceptisols, vertisols, histosols, and andisols.

Universal Time (UT) - The mean solar time of the meridian at the Prime Meridian. Universal Time replaced Greenwich Mean Time as the time standard in 1928. Universal Time is commonly used to denote solar time.

Universal Transverse Mercator (UTM) Grid System - A rectangular coordinate system is commonly used on maps to find the location of points on the Earth's surface. Based on the Transverse Mercator Map Projection system.

Universe - All of the observable things in the celestial cosmos.

Unloading - The releasing of downward pressure on rocks because of the removal of overlying material by erosion. Unloading can cause the development of horizontal bedding in once solid rock.

Unstable Atmosphere - A condition in the atmosphere where isolated air parcels tend to rise, because the parcels of air are warmer than the air that surrounds them. Compare with a neutral atmosphere and a stable atmosphere.

Unstable Equilibrium - In this type of equilibrium, the system returns to a new measured system state after a disturbance.  

Updraft - Upward movement of a mass of air in the broader atmosphere. Compare with a downdraft.

Upper Air Westerlies - Consistently flowing winds that exist in the upper troposphere and have a mostly west-to-east direction around our planet. The upper air westerlies are normally found from about 20°N to the North Pole and 20°S to the South Pole. The subtropical and polar jet streams are part of this wind system.

Upper Mantle - The layer of the Earth's interior extending from the base of the crust to 670 kilometers (415 miles) below the ground surface. Part of the Earth's mantle layer. The upper mantle is made of peridotite, an ultramafic magma primarily composed of the minerals olivine and pyroxene. The top layer of the upper mantle, 100 to 350 kilometers (62 to 217 miles) below the surface, is called the asthenosphere. Compare with the lower mantle.

Upslope Fog - A type of fog produced by air flowing over topographic barriers. As air rises, it cools through 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.

Ural Glacier - A type of small alpine glacier where its mass balance is supplied with significant amounts of wind-blown snow from adjacent locations. This added snow is essential for maintaining the glacier. 

Urban Area - A 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 spatially differentiate into particular types of human activities.

Urban Heat Island - A climatological condition observed in some urban settlements where the temperatures in the human-built area tend to be warmer than the surrounding rural region.

Urbanization - The expansion of cities into rural regions because of population growth. In most cases, population growth in urban areas is driven by the migration of people from rural areas. This is especially true in Less Developed Countries.