U
- umbra
the darkest part of a shadow (p. 800)
- uniformitarianism
the principle that geologic processes operating today also operated in the past (p. 661)
- unsaturated hydrocarbon
a hydrocarbon that contains one or more double or triple bonds (p. 265)
- unsaturated solution
a solution in which more solute can be dissolved at a given temperature (p. 236)
- upwelling
the movement of water from the deep ocean to the ocean's surface (p. 728)
V
- valence electron
an electron that is in the highest occupied energy level of an atom (p. 139)
- valley glacier
a long, narrow mass of moving ice and snow that usually begins near a mountain peak and winds down from the mountains through a valley formed originally by a stream (p. 719)
- vaporization
the phase change in which a substance changes from a liquid into a gas (p. 88)
- vapor pressure
the pressure caused by the collisions of particles in a vapor with the walls of a container (p. 89)
- vector
a quantity that has a direction associated with it (p. 330)
- velocity
the speed and direction an object is moving, measured relative to a reference point (p. 336)
- vent
an opening in the ground where magma escapes to Earth's surface (p. 691)
- virtual image
a copy of an object formed at the point from which light appears to be coming (p. 571)
- viscosity
the tendency of a liquid to keep from flowing; resistance to flowing (p. 45)
- vitamin
an organic compound that organisms need in small amounts, but cannot produce (p. 284)
- volcanic neck
a structure that forms when magma hardens in a volcano's pipe (p. 696)
- volcano
a mountain that forms when magma reaches the surface (p. 690)
- voltage
potential difference, the difference in electrical potential energy between two places in an electric field (p. 606)
- volume
the amount of space taken up by an object (p. 16)
W
- warm front
a front that occurs when a warm air mass overtakes a cold air mass (p. 768)
- waste heat
thermal energy discharged into an area at a lower temperature without being converted into useful work (p. 483)
- water cycle
the continuous movement of water in all its forms among Earth's oceans, atmosphere, and land (p. 705)
- watershed
the area of land that contributes water to a river system (p. 706)
- water table
the top surface of the saturated zone (p. 707)
- watt (W)
the SI unit of power, equal to one joule per second (p. 415)
- wavelength
the distance between a point on a wave and the same point on the next cycle of the wave (p. 505)
- weak nuclear force
a powerful attractive force that acts over a short range (p. 380)
- weather
the condition of the atmosphere at a particular place at a particular time (p. 749)
- weathering
the process by which rocks are chemically altered or physically broken into fragments (p. 710)
- wedge
a V-shaped object whose sides are two inclined planes sloped toward each other (p. 431)
- weight
the force of gravity acting on an object (p. 367)
- wheel and axle
a simple machine that consists of two rigidly attached disks or cylinders, each one with a different radius (p. 430)
- white dwarf
the small, dense, but dim remains of a low- or medium-mass star (p. 839)
- wind
the mainly horizontal movement of air from areas of higher pressure to areas of lower pressure (p. 757)
- work
the product of distance and the force in the direction an object moves (p. 412)
- work input
the work done on a machine as the input force acts through the input distance (p. 419)
- work output
the work done by a machine as the output force acts through the output distance (p. 420)
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