Va-Vm

 

VACM

See Vector Averaging Current Meter.

 

Vaisala frequency

See buoyancy frequency.

 

Valanginian

The second of six ages in the Early Cretaceous epoch, lasting from 138 to 131 Ma. It is preceded by the Berriasian age and followed by the Hauterivian age.

 

VAS

Abbreviation for VISSR Atmospheric Sounder, a combined imaging and sounding device on the GOES series of geosynchronous satellites operated by NOAA. It is an advanced version of the VISSR flown on earlier geosynchronous satellites that retains the imaging function while expanding the number of infrared channels that allow the 3-D structure of the atmospheric temperature and water-vapor distribution to be determined. It has one channel in the visible spectrum (0.55 m to 0.75 m) which comprises signals from eight detectors and six infrared detectors which make up 12 channels by use of a filter wheel. The spectral channels are chosen to permit the measurement of atmospheric profiles of temperature and humidity as well as surface temperature. The vertical flux divergence of infrared radiation, i.e. the cooling-rate profile, can also be estimated. See the VAS Web site.

 

vascular plant

A plant containing vascular tissue, the conducting system which enables water and minerals to move through it.

 

vardarac

A cold northerly wind which blows from the mountains down to the valleys of Macedonia, especially through the Morava-Vardar gap, in the rear of a depression and affects the Thessalonika region. It is a type of ravine wind. It is similar to the mistral wind.

 

varve

A sedimentary bed or lamina or sequence of laminae deposited in a body of still water within one year's time. More specifically, a thin pair of graded glaciolacustrine layers seasonally deposited, usually by meltwater streams, in a glacial lake or other body of still water in front of a glacier. The lower layer is coarser material deposited by rapid melting of ice in the warmer months, and the upper layer more fine-grained deposited from suspended particles in quiet water in the winter months.

 

VCI

Abbreviation for vegetation condition index, an index derived from AVHRR data for drought detection and tracking in many countries of the world. See Kogan (1990).

 

Vector Averaging Current Meter

A current meter used in oceanography that senses speed with a Savonius rotor and direction relative to its case with a vane assembly. An internal compass senses the orientation of the case relative to magnetic north. Temperature is also recorded. The data recorded by the internal electronics is a true vector average with the sampling rate for speed and direction partly determined by the rotor rotation rate. See Heinmuller (1983).

 

Vector Measuring Current Meter

A current meter used in oceanography that is effective at measuring near-surface currents since it is not as susceptible as some other meters to contamination from vertical currents and high frequency horizontal currents. It uses two orthogonal propellors to achieve insensitivity to current flow at right angles to the propellor axis. See Weller and Davis (1980).

 

veering

Said of the clockwise change of the direction of a wind, as opposed to backing.

 

vegetation

The kind of plant cover in an area, e.g. forest, grassland, etc., as opposed to the flora. See Collinson (1988).

 

vegetation physiognomy

The suite of morphological and physiological characteristics that have evolved in resonse to climate variables, notably temperature and rainfall. Salient characteristics include leaf longevity, leaf dimension, and temperature sensitivity. See Woodward (1992).

 

VEI

Abbreviation for volcanic explosivity index. See Newhall and Self (1982).

 

velocity potential

A scalar function that exists in irrotational fluid motion. If we denote the velocity potential by , then it is defined by the equation

where is the velocity vector. This equation implies that is normal to the equipotential lines and is directed from high to low potential.

 

VEMAP

Acronym for the Vegetation/Ecosystem Modeling and Analysis Project, a research group formed to compare biogeography and biogeochemistry models in the context of global climate change. The project objectives are the comparison of model output for current conditions, intercomparison of the climate and CO2 sensitivity of three biogeochemistry models and three vegetation lifeform distribution models, and linking vegetation distribution and ecosystem physiology models to evaluate the joint response of these processes to altered conditions. VEMAP is a designated activity of the GCTE Core Project. See Group (in press) and the VEMAP Web site.

 

vendavales

A name given to strong, squally, southwest winds in the Straits of Gibraltar and off the east coast of Spain. It is associated with depressions and occurs mainly between September and March. They usually bring stormy weather and heavy rain.

 

ventilated thermocline

See Luyten et al. (1983) for the original concept and Huang (1991) and Pedlosky (1990) for reviews.

 

vernal equinox

See equinox.

 

VERTEX

A multidisciplinary study of VERtical Transport and EXchange of material in the upper ocean performed in the California Current. See Martin et al. (1987).

 

VHF

Abbreviation for very high frequency, an electromagnetic spectrum waveband ranging from 30 to 300 MHz.

 

Vienna Convention

Known fully as the Vienna Convention for the Protection of the Ozone Layer, this was a meeting that took place in March 1985 which started an intensive effort by chemical manufacturers to produce alternative refrigerants to ozone-depleting CFCs.

 

VIRR

Abbreviation for Visible and IR Radiometer, an instrument that flew aboard SEASAT. It produced imagery for the identification of cloud and geographic features.

 

virtual potential temperature

A temperature defined to include the buoyant effects of liquid water in the air. It is calculated identically to the virtual temperature.

 

virtual temperature

The temperature a sample of dry air at pressure P would have in order that its density equal that of the sample of moist air at temperature T, pressure P, and water vapor mixing ratio r. It is given by

a If there is also liquid water in the air this is modified as

where is the liquid water mixing ratio (in grams of liquid water per gram of dry air). The virtual temperature is thus defined because it allows the ideal gas law to be used for situations in which the air is not dry, i.e. moist air of temperature T behaves identically to dry air of temperature .

 

viscous sublayer

That part of a boundary layer where the viscous stress is much larger than the Reynolds stress. See Kagan (1995).

 

VISSR

Abbreviation for Visible Infrared Spin-Scale Radiometer, an imaging device on the METEOSAT, GOES and GMS platforms. It is used for obtaining 2-D cloud cover pictures from a geosynchronous altitude and provides for both day and night time observations of clouds and the determination of temperatures, cloud heights, and wind fields. The instrument consists of a scanning system, a telescope, and infrared and visible sensors. It is a radiometer with 2.5 x 2.5 km resolution in the visible and 5 x 5 km resolution in the infrared. The spectral range is 0.5 to 12.5 m and the temporal resolution 30 minutes (48 images per day). See the VISSR Web site.

 

VLBI

Abbreviation for Very Long Baseline Interferometry.

 

VLF

Abbreviation for very low frequency, an electromagnetic spectrum waveband below 30 kHz.

 

VMCM

See Vector Measuring Current Meter.