V Notch Weir - A specific type of weir designed to measure stream discharge by partially obstructing the normal flow of a stream. It is named for the V-shaped notch found on the weir's length.

Vacuum - (1) A space that is devoid of atoms or molecules. (2) The removal of air from a space.

Valley - A relatively long 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 higher elevation areas during the daytime.

Valley Fog - A type of fog formed by the movement of cooler, denser air from higher elevations to the warm valley bottom.

Valley Glacier - See alpine glacier.

Valley Train - A linear accumulation of glaciofluvial outwash sediments found in a once glaciated valley. Also called a gravel train.

Valley Wall - The side slope of a stream or glacial valley.

Vapor Pressure - The 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 squares of each value in the data set, subtracted from the mean (average), divided by the number of observations in the data set minus 1.

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 and its surrounding environment.

Vascular Plant - A plant that has specialized vascular tissues used to transport water, nutrients, and other metabolic products.

Velocity - The speed of movement of an object in one direction. An important concept in physics.

Ventifact - A loose piece of rock that has been polished smooth by wind-transported particles. Common in arid environments.

Venturi Effect - The 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. In the Northern Hemisphere, the vernal equinox falls on either March 20 or 21 (changing each year). 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 hours) day and night.

Vertebrates  - Animals that have a backbone. Compare with invertebrates. See the Encyclopedia of Life for more information on this group of organisms.
https://eol.org/pages/2774383

Vertical Aerial Photograph - An aerial photograph taken from an overhead or near overhead angle from a platform in the atmosphere.

Vertisols - (1) Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. Tropical or sub-tropical soil with a high clay content and base cation status. Image Source: U.S. Department of Agriculture. (2) Soil order (type) of the Canadian System of Soil Classification. This soil has a high clay content and exhibits extreme contraction-expansion due to temporal changes in soil water content. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html.

Virga - Is rainfall from a cloud that evaporates or sublimates before it reaches the ground surface.

Virus - A fragment of DNA or RNA that depends on the infection of host cells for its reproduction. They are not cells. Viruses are thought to be parts of the genetic code found in either eukaryotic or prokaryotic cells that can exist on their own. At times, viruses are metabolically inert and technically nonliving.

Viscosity - The amount of resistance to flow in a fluid due to intermolecular friction. Viscosity is measured with instruments called viscometers and rheometers. 

Visible Light - A form of electromagnetic radiation that is visible to humans. There are two common wavelength descriptions for this type of radiation. (1) This radiation has a wavelength between 0.40 and 0.71 micrometers (µm). (2) This radiation has a wavelength between 0.39 and 0.74 micrometers (µm). Also called luminous energy and radiant energy.

Void Ratio - A measure of void space in a substance. It is determined by finding the ratio between the volume of void space and the volume of solid material. This measurement is commonly used in Soil Science, Geology, and Engineering. 

Volatile Organic Compounds (VOCs) - Organic molecules that are mainly composed of carbon and hydrogen atoms (hydrocarbons). The most common volatile organic compound released into the atmosphere is methane. VOCs contribute to the formation of photochemical smog.

Volatilization - The process by which a solid or liquid substance is converted into a gas.

Volcanic Ash - Small-sized loose fragments of solidified lava expelled from a volcanic crater during a volcanic eruption.

Volcanic Cone - A cone-shaped hill formed by volcanoes. Volcanic cones are created by the deposition of material (lava, tephra, and volcanic ash) released by a volcanic vent or many vents. These features vary significantly in size, from a few meters (10 to 15 feet) tall to over 8 kilometers (5 miles) tall. The tallest and largest volcanic cones are built mainly from fluid lava flows.

Volcanic Crater - A circular depression associated with volcanoes and found usually at the center of a volcanic cone. These features vary greatly in size. Inside the volcanic crater are one or many volcanic vents that can release lava, tephra, and volcanic ash.

Volcanic Eruption - An event where lava, tephra, volcanic ash, and/or gases are released or ejected from a volcanic vent or fissure.

Volcanic Fissure - A linear opening on a volcano through which lava and/or gases are released.

Volcanic Neck - A pipe-like mass of solidified lava (dyke) often associated with a volcano that reaches the surface of the Earth. Volcanic necks are significantly longer and larger than volcanic pipes and involve the transport of magma from the mantle. Compare with a volcanic pipe.

Volcanic Pipe - A relatively narrow pipe-like mass of solidified lava (dyke) often associated with a volcano that reaches the surface of the Earth. Compare with a volcanic neck.

Volcanic Vent - An opening on a volcano through which lava, tephra, ash, and/or gases are released or ejected. Also called a chimney.

Volcanism - A process or a thing that is related to a volcano.

Volcano - An elevated area of land created from the release of lava and ejection of volcanic ash and rock fragments from a volcanic vent. For a volcano to form, magma must be generated and then migrate upward through the crust to the Earth's surface. Geologic factors that can cause this process include subduction zones, continental rift zones, mid-oceanic ridges, and hot spots.

Volatilization - The process by which a solid or liquid substance is converted into a gas.

Volume - The occupation of space in three dimensions. Volume is normally measured in cubic units.

Vortex - A rapid spiralling motion of air or liquid around a center of rotation.

Vugh - A hollow in a rock that has a surface covered with precipitated minerals.

Vulcanism - See volcanism.

Vulnerable - One of the categories used by the International Union for Conservation of Nature's (IUCN) Red List to describe the status of a species. This category indicates that the species has been evaluated as having a high risk of extinction in the wild.