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U

Ultraviolet radiation
Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays, between 0.02 and 0.4 micron (200 and 4000 angstrom).

Uncertainty
The standard deviation of a sufficiently large number of measurements of the same quantity by the same instrument or method. The non-correctable part of the inaccuracy of an instrument, it represents the limit of measurement precision. The uncertainty of an instrument is caused by the unpredictable effects upon its performance of such factors as friction, backlash, and electronic noise.

Unprotected thermometer
A reversing thermometer (for seawater temperature) which is not protected against hydrostatic pressure. The mercury bulb is therefore squeezed, and the amount of mercury broken off on reversal is a function of both temperature and of hydrostatic pressure.

Updraft
A relatively small-scale, upward moving current of air.

Upper air
That portion of the atmosphere which is above the lower troposphere. Generally applied to levels above 850 mb.

Upper air observation
A measurement of atmospheric conditions aloft, above the effective range of a surface weather observation. Elements evaluated include temperature, humidity, pressure, wind speed, and wind direction.

Upward total radiation
Solar and terrestrial radiation directed upward (away From the earth's surface); outgoing radiation.

Upwind
In the direction from which the wind is blowing.

V

VDT
Video Display Terminal. An input and display device which includes a keyboard and a screen and allows a human to communicate with a computer.

Vane
See wind vane.

Variograph
A recording variometer.

Variometer
A instrument designed to study small fluctuations of some quantity. The microbarograph is an example of a recording pressure variometer.

Vectopluviometer
A rain gauge or array of rain gauges designed to measure the inclination and direction of falling rain.

Vector
Any quantity, such as force velocity, or acceleration, which has both magnitude and direction at each point in space, as opposed to scalar which has magnitude only. Such a quantity may be represented geometrically by an arrow of length proportional to its magnitude, pointing in the assigned direction.

Veering
A change in wind direction in a clockwise sense. The opposite of backing.

Venturi tube
A tube designed to measure the rate of flow of fluids. It consists of a tube having a constriction or throat at its midsection. The difference between the pressure measured at the inlet and at the throat is a function of the fluid velocity. Compare to Pitot tube.

Vernier scale
A small, moveable graduated scale adjacent and parallel to the main scale of an instrument. It provides a means for interpolating between the graduations of the main scale.

Vertical anemometer
General name for an instrument designed to measure the vertical component of the wind speed. See anemoclinometer.

Vertical-current recorder
General term for an instrument which records the vertical electric current in the atmosphere.

Vertical visibility
The distance that an observer can see vertically into a surface-based obscuring phenomenon such as fog, rain, or snow. The distance estimate must be based upon ceiling balloon ascensions or ceiling light projector measurements.

VFR
Abbreviation for visual flight rules, but commonly used to refer to the relatively favorable weather and/or flight conditions to which these rules apply.

Virga
Precipitation falling from a cloud, usually in wisps or streaks, but evaporating before it reaches the ground.

Virtual temperature
Temperature to which absolutely dry air would have to be brought in order for it to have the same density as moist air, considered at the same pressure.

Visibility
The greatest distance at which it is just possible to see and recognize with the unaided eye (1) in the daytime, a prominent dark object against the sky at the horizon, and (2) at night, a known, preferably unfocused, moderately intense light source.

Visibility meter
General term for an instrument used to make direct measurements of visual range or measurements of the physical characteristics of the atmosphere (or other medium) which determine the visual range.

Visibility sensor
General term for an instrument used to make direct measurements of visual range or measurements of the physical characteristics of the atmosphere which determine the visible range.

Visible radiation
Electromagnetic radiation lying within the wavelength interval to which the human eye is sensitive, the spectral interval from approximately 0.4 to 0.7 microns (4000 to 7000 angstroms). Bounded on the short-wavelength end by ultraviolet radiation and on the long-wavelength end by infrared radiation.

Visual range
The maximum distance, usually horizontally, at which a given object or light source is just visible under particular conditions of transmittance and background luminance.

VOLMET broadcast
Routine broadcast of meteorological information for aircraft in flight.

Vortex thermometer
A thermometer used in aircraft which automatically corrects for adiabatic and frictional temperature rises by imparting a rotary motion to the air passing the thermal sensing element.

Vorticity
A vector measure of local rotation in an airflow.