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U
 
UCP (Unit Control Position)The WSR-88D radar operator uses this to control the entire radar system.  One of the main things that the radar operator will do at the UCP is change volume scan strategies of the antenna.  These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1 degree beam at selected elevation angles.  The WSR-88D uses 3 scan strategies.  They are the following:  14 elevation angles in 5 minutes (this is used during severe weather situations), 9 elevation angles in 6 minutes (this is used when there is precipitation within 248 nautical miles of the radar), and 5 elevation angles in 10 minutes (this is used when there is no precipitation within 248 nautical miles).  The radar operator at the UCP can also adjust the radar products and help the users out with their communication problems.

UKMET:  A medium-range (3 to 7 day) numerical weather prediction model operated by the United Kingdom METeorological Agency.  It has a resolution of 75 kilometers and covers the entire northern hemisphere.  Forecasters use this model along with the European and MRF in making their extended forecasts (3 to 7 days). 

ULJAn acronym for Upper Level Jet.  See Jet Stream.

Unambiguous Range:  See maximum unambiguous range.

Undercurrent:  A current below the upper currents or surface of a fluid body.

Underflow:  The lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called subsurface storm flow.

UnimodalA distribution having only one localized maximum, i.e., only one peak.
 
Unit Hydrograph: The discharge hydrograph from one inch of surface runoff is distributed uniformly over the entire basin for a given time period. It is also known as a "Unitgraph".

Unit Hydrograph Duration: The time over which one inch of surface runoff is distributed for unit hydrograph theory.

Unit Hydrograph Theory: Unit Hydrograph Theory states that surface runoff hydrographs for storm events of the same duration will have the same shape, and the ordinates of the hydrograph will be proportional to the ordinates of the unit hydrograph. For example, the hydrograph from one-half inch of runoff will be half of that from the unit hydrograph.

Universal Type Weighting and Recording Gage:  A gage which collects precipitation and then converts the weight onto an inked pen movement which traces on graph paper fixed to a clock driven drum.

Unstable:  An atmospheric state warm air below cold air.  Since warm air naturally rises above cold air (due to warm air being less dense than cold air), vertical movement and mixing of air layers can occur.

UpdraftCurrent(s) of air with marked vertical upward motion.  If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or cumulonimbus.

Updraft Base:  Alternate term for a rain-free base.

Upper-Level Disturbance: A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.

Upper Level System:  A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

Upslope Flow:  Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upslope Fog: It forms as air is cooled adiabatically by blowing up sloping terrain. The upslope cooling may form clouds concurrently with the fog. As is the case with advection fog, upslope fog can form with moderate to strong winds under cloudy skies. In stable air, upslope fog will form as soon as the air is cooled to the surface dew point. Temperature and dew point close at about 4oF for each 1,000 feet rise in altitude. When air is unstable, convective clouds may form; fog forms at the surface where the ground level is at or above the condensation level.

Upstream:  Toward the source of the flow, or located in the area from which the flow is coming.

Upstream Slope:  The part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock riprap or concrete.

UpwellingThe process by which cold waters from the depths of a lake or ocean rise to the surface.

Urban & Small Stream Flood Advisory (FFW) This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area.  Issued when heavy rain will cause flooding of streets and low-lying places in urban areas.  Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.  This advisory is rarely used in the state of Michigan.  The reason for this is that we do not have the observational resolution necessary to determine whether flooding is an inconvenience or not.  As a result, we will usually go with a Flood Warning instead of a Urban and Small Stream Flood Advisory.

Urban Flash Flood Guidance:  A specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area.

Urban Flooding:  Flooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
 
Urban Heat Island: The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.

Urban/Small Stream Flooding: Flooding that occurs after heavy rains of relatively short duration, and it is generally not life threatening. It causes ponding of water in urban areas, especially in low places, and results in minor flooding of small streams and creeks.

U.S. Army Corps of Engineers (USACE):  As part of the Department of the Army, the Corps has responsibilities in civil and military areas. In civil works, the USACE has authority for approval of dredge and fill permits in navigable waters and tributaries there of; the USACE enforces wetlands regulations, and constructs and operates a variety of water resources projects, mostly notably levee, dams and locks.
 
U.S. Bureau of Reclamation (USBR) The Federal agency whose mandate was to reclaim the arid west of the United States. Operating in 17 western states, this agency builds, operates and maintains a variety of irrigation, power, and flood control projects.

User AgencyA public fire service or wildlands management agency, Federal or non-Federal, which regularly requires and uses NWS fire and forestry meteorological services.
 
USFS:  The U.S. Forest Service
 
U.S. Geological Survey (USGS):  The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation's rivers and streams that are useful for mitigation of hazards associated with floods and droughts.

UV (Ultraviolet) IndexThis index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays.  It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA).  Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities.  The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country. A radiative transfer model is then used to determine the amount of UV radiation reaching the ground from 290 to 400 nm in wavelength, using the time of day (solar noon), day of year, and latitude. As an example, assume the following UV levels for each wavelength are predicted for a given location (these are totally made up numbers, and not even the ratios represent reality):

          Wavelength        Incidence
             290nm                   10
             350nm                   20
             400nm                   50

This information is then weighted according to how human skin responds to each wavelength; it is more important to protect people from wavelengths that harm skin than from wavelengths that do not damage people's skin. The weighting function is called the McKinlay-Diffey Erythema action spectrum. For illustration purposes only (these numbers are not correct), assume 290nm radiation causes three times as much damage as 350nm radiation and five times as much damage as 400 nm radiation. Then, if in some unit 290nm UV radiation did 15 units of damage, 350nm radiation would do 5 units and 400 nm radiation would do 3 units. At each wavelength, multiply the actual incoming radiation level by the weighting:

          Wavelength       Incidence    Weight      Result
              290nm                 10               15             150
              350nm                 20                 5             100
              400nm                 50                 3             150
 

These weighted irradiances are summed up, or integrated, over the 290 to 400 nm range resulting in a value representing the total effect a given day's UV radiation will have on skin. For our example, the total is 400.  These estimates are then adjusted for the effects of elevation and clouds. UV at the surface increases about 6% per kilometer above sea level. Clear skies allow 100% of the incoming UV radiation from the sun to reach the surface, whereas scattered clouds transmit 89%, broken clouds transmit 73%, and overcast conditions transmit 31%. If we assume that the example location is at 1 kilometer in elevation, and that there will be broken clouds, then the calculation is:

          400 x 1.06 x 0.73 = 309.5

Once adjusted for elevation and clouds, this value is then scaled (divided) by a conversion factor of 25 and rounded to the nearest whole number. This results in a number that usually ranges from 0 (where there is no sun light) to the mid teens. This value is the UV Index. Thus, the UV Index for the example city would be:

          309.5 / 25 = 12.4, rounded to 12

Currently, the computation of the UV Index does not include the effects of variable surface reflection (e.g., sand, water, or snow), atmospheric pollutants or haze.   

By following the few simple precautions in the table below, you can greatly reduce your risk of sun related injuries (blistering sunburns, as well as longer-term problems like skin cancer and cataracts).
 

What does the UV Index Mean?
Exposure Category UV Index Value Time to Burn* Actions to take at noon
Minimal  0-2 60 minutes Apply SPF sunscreen
Low 3-4 45 minutes Apply SPF sunscreen, wear a hat.
Moderate 5-6 30 minutes Apply SPF 15, wear a hat
High 7-9 15-24 minutes Apply SPF 15 to 30 sunscreen, wear a hat and sunglasses. Limit midday exposure
Very High 10+ 10 minutes Apply SPF 30 sunscreen. Wear a hat, sunglasses, and protective clothing. 
* "Time to burn" and "actions" apply to people with a Type II, fair skin that sometimes tans and usually burns. People with lighter skin need be more cautious. People with darker skin may be able to tolerate more exposure. But even dark skin can burn. 

 

When the Index is High or Very High, try to minimize your outdoor activities between the peak hours of 10 AM and 4 PM when the sun is most intense. When the Index is 10 or higher, stay indoors if possible, otherwise be sure to take all the other necessary precautions. Watch closely for the UV Index reports in your local newspapers and on television, and remember to Be Sun Wise!

UVM An acronym for Upward Vertical Motion.

UVVAn acronym for Upward Vertical Velocity  


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