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WeatherChannel - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 30/09/2008, USA Words: 751

vertical wind profile
A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

virga
Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.

visibility
A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visi...

visible light
The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays. Related term: light waves

visual flight rules (vfr)
Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles. Related terms: MVFR and IFR

vortex
Any circular or rotary flow in the atmosphere that possesses vorticity.

vorticity
The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have...

walker circulation
A deep east-west overturning in the atmosphere normally confined to within about 20 degrees latitude of the equator extending from low-levels to near the tropopause.

wall cloud
An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. Forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. It often develops before strong or violent tornadoes.

warm
To have or give out heat to a moderate or adequate degree. A subjective term for temperatures between cold and hot. In meteorology, an air parcel that is warm is only so in relation to another parcel.

warm advection
The horizontal movement of warmer air into a location. Related term: cold advection

warm front
The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a col...

warm high
A high pressure system that has its warmest temperatures at or near the center of circulation. Contrast with a cold high. Related terms: cut-off high and omega block

warm low
A low pressure system that has its warmest temperatures at or near the center of circulation. Also referred to as a warm core low. Related term: cold low

warning
A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

wasatch winds
Strong winds blowing easterly out of the Wasatch Mountains in Utah, sometimes reaching speeds greater than 75 miles per hour.

watch
A forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows.

water
Refers to the chemical compound, H₂O, as well as its liquid form. At atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life-sustaining part of life on earth.

water cycle
The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas. Related term: hydrologic cycle

waterspout
A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.

water vapor (h2o)
Water in gaseous form. It is one of the most import constituents of the atmosphere. Due to its molecular content, air containing water vapor is lighter than dry air. This contributes to the reason why moist air has a tendency to rise.

wave(s)
In general, any pattern with some roughly identifiable periodicity in time and/or space. It is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. In meteorology, this applies to atmospheric waves, such as long waves and short waves. In oceanography, this applie...

wave cyclone
A cyclone which forms and moves along a front. The circulation around the cyclone's center produces a wavelike deformation on the front. May also be call a migratory cyclone or low.

wave length
The least distance between particles moving in the same phase of oscillation of a wave. In oceanography, it is the horizontal distance between the highest parts of two successive wave crests above the still water level, separated by a trough that is below the still water level, and it is measured in meters.

weather
The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind,

weathering
The decay and breakup of rocks on the earth's surface by natural chemical and mechanical processes. The mechanical action includes large changes of temperature, extreme temperatures, frost, or the impact of wind borne sand or water. Chemical action includes the chemical reactions between atmospheric constituents in a moist environments or in rain w...

weather vane
Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses.

wedge
Primarily refers to an elongated area of shallow high pressure at the earth's surface. It is generally associated with cold air east of the Rockies or Appalachians. It is another name for a ridge, ridge line, or ridge axis. Contrast with a trough. Wedge is also a slang term for a large, wide tornado with a wedge-like shape.

westerlies
Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere. Near the earth's surface, the westerlies extend from approximately 35 to 65 degrees latitude, while in the upper levels they extend further polarward and equatorwa...

west virginia high
An area of stagnant high pressure located over West Virginia during Indian Summer.

wet bulb depression
Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

wet bulb thermometer
A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

whirlwind
A small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear, dry, hot afternoons. Often called a dust devil when visible by the dust, dirt or debris it picks up. Also slang for a landspout or a tornado.

whiteout
When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.

wind
Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

wind chill index
The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature. Related term: Wind Chill Index

wind direction
The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc.).

wind shear
The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.

wind shift
The term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. It may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt.

wind speed
The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

wind vane
An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position. Related term: weather vane

windward
The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.

wind wave
An ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its fetch area, it is considered a swell.

winter
Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August...

winter storm
Any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice. Related terms: blizzard, ice storm, and nor'easter

zenith
The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. The point on any given observer's celestial sphere that lies directly above him. The opposite of nadir.

zodiac
The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.

zonal flow
The flow of air along a latitudinal component of existing flow, normally from west to east. Related term: meridional flow

zonal index
The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind.

d-value
The deviation of actual altitude along a constant pressure surface from the standard atmosphere altitude of that surface.

k index
The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer.