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UPPER AIR.—In synoptic meteorology and weather observing, that portion of the atmosphere which is above the lower troposphere. No distinct lower limit is set, but the term is generally applied to the levels above 850 mb.

UPPER ATMOSPHERE.—The general term applied to the atmosphere above the troposphere. 

UPPER FRONT.—A front that is present in the upper air but does not extend to the ground.

UPPER-LEVEL HIGH.—(also called upper-level anticyclone, upper high, high aloft) An anticyclonic circulation existing in the upper air. This often refers to such highs only when they are much more pronounced at upper levels than at the surface.

UPPER-LEVEL LOW.—(also called upper-level cyclone, upper cyclone, high-level cyclone, low aloft) A cyclonic circulation existing in the upper air, specifically as seen on an upper-level constant-pressure chart. This term is often restricted to such lows having little cyclonic cir-culation in the lower atmosphere.

UPPER-LEVEL RIDGE.—A pressure ridge existing in the upper air, especially one that is stronger aloft than near Earth’s surface.

UPPER-LEVEL TROUGH.—A pressure trough existing in the upper air. This term is sometimes restricted to those troughs that are much more pronounced aloft than near Earth’s surface.

UPSTREAM.—In the direction from which a fluid is flowing.

UPWIND.—In the direction from which the wind is blowing.

VECTOR.—Any quantity, such as force, velocity, or acceleration, that has both magnitude and direction at each point in space, as opposed to a scalar, which has magnitude only. Geometrically, it is represented by an arrow of length proportional to its magnitude, pointing in the assigned direction.

VEERING.—A change in wind direction in a clockwise sense in the Northern Hemisphere and counterclockwise direction in the Southern Hemisphere.

VERNAL EQUINOX.—For either hemisphere, the equinox at which the Sun’s most direct rays approach from the opposite hemisphere. In northern latitudes, this occurs approximately on 21 March; the Sun’s most direct rays are centered over the equator and moving north.

VIRTUAL TEMPERATURE.—In a system of moist air, the temperature of dry air having the same density and pressure as the moist air. It is always greater than the actual temperature.

VORTEX.—In its most general use, any flow possessing vorticity. More often the term refers to a flow with closed streamlines.

VORTICITY.—A vector measure of local rotation in a fluid flow.

WARM-CORE HIGH.—At a given level in the atmosphere, any high that is warmer at its center than at its periphery.

WARM-CORE LOW.—At a given level in the atmosphere, any low that is warmer at its center than at its periphery.

WARM FRONT.—Any non-occluded front or portion thereof that moves in such a way that warmer air replaces colder air.

WARM SECTOR.—That area within the cir-culation of a wave cyclone where the warm air is found. It lies between the cold front and the warm front of the storm; and, in the typical case, the warm sector continually diminishes in size and ultimately disappears (at the surface) as the result of occlusion.

WARM TONGUE.—A pronounced poleward extension or protrusion of warm air.

WAVE CYCLONE.—A cyclone that forms and moves along a front.

WAVE THEORY OF CYCLONES.—A theory of cyclone development based upon the principles of wave formation on an interface between two fluids. In the atmosphere, a front is taken as such an interface.

WEATHER.—The state of the atmosphere, mainly with respect to its effect upon life and human activities.

WEATHER RADAR.—Generally, any radar that is suitable or can be used for the detection of precipitation or clouds. 

WESTERLIES. —(also known as circumpolar westerlies, counter-trades, middle-latitude westerlies, midlatitude westerlies, polar westerlies, subpolar westlies, subtropical westerlies, temperate westerlies, zonal westerlies, and zonal winds) Specifically, the dominant west-to-east motion of the atmosphere, centered over the mid-dle latitudes of both hemispheres. At the surface, the westerly belt extends, on the average, from about 35° to 65° latitude. At upper levels, the westerlies extend farther equatorward and poleward. The equatorward boundary is fairly well defined by the subtropical high-pressure belt; the poleward boundary is quite diffuse and variable. 

WHITEOUT.—An atmospheric optical phenomenon of the polar regions in which the observer appears to be engulfed in a uniformly white glow. Shadows, horizon, and clouds are not discernible; sense of depth and orientation are lost; only very dark, nearby objects can be seen.

WIND-CHILL FACTOR.—The cooling ef-fect of any combination of temperature and wind, expressed as the loss of body heat, in kilogram calories per hour per square meter of skin sur-face. The wind-chill: factor. is based on the cooling rate of a nude body in the shade; It is only an approximation, because of individual body variations in shape, size, and metabolic rate. 

WIND ROSE.—Any one of a class of diagrams designed to show the distribution of wind direction experienced at a given location over a considerable period; it thus shows the prevail-ing wind direction. The most common form con-sists of a circle from which 8 or 16 lines emanate, one for each compass point. The length of each line is proportional to the frequency of wind from that direction, and the frequency of calm condi-tions is entered in the center.

WINTER SOLSTICE.—For either hemi-sphere, the solstice at which the Sun is above the opposite hemisphere. In northern latitudes, the time of this occurrence is approximately 22 December.

ZONAL.—Latitudinal; easterly or westerly; opposed to meridional.

ZONAL FLOW.—The flow of air along a latitude circle; more specifically, the latitudinal (east or west) component of existing flow.

ZONAL INDEX.—A measure of strength of the midlatitude westerlies, expressed as the horizontal pressure difference between 35° and 55° latitude or as the corresponding geostrophic wind.

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