National Weather Service United States Department of Commerce

Weather Glossary: U's

Ultraviolet Radiation
The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.
An acronym for Upper Level Jet. See Jet Stream.
Unambiguous Range
See maximum unambiguous range.
A current below the upper currents or surface of a fluid body.
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.
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 Air
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.
Current(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.

When air is unstable, convective clouds may form; fog forms at the surface where the ground level is at or above the condensation level.

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.
The 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.
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, ungagged 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. 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.
Coordinated Universal Time. The time in the zero-degree meridian time zone.
UV (Ultraviolet) Index

This 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 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.

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).

The UV index and suggestions on how to protect yourself
UV Index Value Exposure Category Time to Burn Actions to take at noon
10+ Very High 10 minutes Apply SPF 30 sunscreen. Wear a hat, sunglasses, and protective clothing.
7-9 High 15-24 minutes Apply SPF 15 to 30 sunscreen, wear a hat and sunglasses. Limit midday exposure
5-6 Moderate 30 minutes Apply SPF 15, wear a hat
3-4 Low 45 minutes Apply SPF sunscreen, wear a hat.
0-2 Minimal 60 minutes Apply SPF sunscreen
Note: 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.

An acronym for Upward Vertical Motion.
An acronym for Upward Vertical Velocity.