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Vacuum -  (1) Space devoid of atoms or molecules. (2) Emptying of air.
Valley -  A linear depression in the landscape that slopes down to a stream, lake or the ocean. Formed by water and/or ice erosion.
Valley Breeze -  Local thermal circulation pattern found in areas of topographic relief. In this circulation system, surface winds blow from the valley bottom to areas of higher elevation during the daytime.
Valley Fog -  Fog formed by the movement of cooler, more dense air from higher elevations to the warm valley bottom.
Valley Train -  A linear accumulation of glaciofluvial outwash sediments found in a once glaciated valley.
Valley Wall -  The side slope of a stream or glacial valley.
Vapor Pressure -  Pressure exerted by water vapor molecules in a given quantity of atmosphere.
Variance -  A statistical measure of the dispersion of observation values in a data set. The variance of a sample is the sum of the square of each value in the data set subtracted from the mean divided by one less than the total number of observations in the data set.
Varve -  A thin yearly deposit of sediment found on the bottom of a lake. Within each yearly varve, there are variations in the color and the texture of the material deposited. The thickness of the varve and its associated layers can be used to reconstruct past environmental conditions influencing the lake.
Velocity -  The speed of movement of an object in one direction.
Ventifact -  A loose piece of rock that has been polished smooth by wind transported particles. Common in arid environments.
Venturi -  An increase in the velocity of a fluid or gas due to the constriction of flow.
Vernal Equinox -  One of two days during the year when the declination of the Sun is at the equator. The vernal equinox denotes the first day of the spring season. For the Northern Hemisphere, the date of vernal equinox on either March 20 or 21 (changes yearly). September 22 or 23 is the date of the vernal equinox in the Southern Hemisphere. During the vernal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night.
Vertical Aerial Photograph -  Photograph taken from a overhead or near overhead angle from a platform in the atmosphere.
Viscosity -  The amount of the resistance to flow in a fluid due to intermolecular friction.
Volcanic Ash -  Small sized particles ejected from explosive volcanoes.
Volcanic Pipe -  A dyke reaches the surface of the Earth. Also called volcanic neck.
Volcanic Vent -  An opening on a volcano through which lava is released and rock fragments and ash are ejected.
Volcano -  An elevated area of land created from the release of lava and ejection of ash and rock fragments from and volcanic vent.
Volume -  The occupation of space in three dimensions. Measured in cubic units.
Vortex -  A rapid spiraling motion of air or liquid around a center of rotation.