{"id":1607,"date":"2022-07-10T11:09:51","date_gmt":"2024-07-10T11:23:51","guid":{"rendered":"http:\/\/ialert.com\/blog\/?p=1607"},"modified":"2026-04-28T02:10:25","modified_gmt":"2026-04-28T02:10:25","slug":"glossary-of-weather-terms-beginning-with-u","status":"publish","type":"post","link":"https:\/\/ialert.com\/blog\/weather-articles\/weather-glossary\/glossary-of-weather-terms-beginning-with-u\/","title":{"rendered":"Glossary of Weather Terms – Beginning with “U”"},"content":{"rendered":"

Curious about what an updraft is<\/strong> inside a thunderstorm, what the urban heat island effect<\/strong> means for your city’s forecast, what the UV index numbers actually mean<\/strong>, how an upper level trough<\/strong> triggers severe weather, or what causes upslope fog<\/strong> along the Rockies?<\/p>\n

Jump to weather terms beginning with the letter:<\/h3>\n

A<\/a> | B<\/a> | C<\/a> | D<\/a> | E<\/a> | F<\/a> | G<\/a> | H<\/a> | I<\/a> | J<\/a> | K<\/a> | L<\/a> | M<\/a> | N<\/a> | O<\/a> | P<\/a> | Q<\/a> | R<\/a> | S<\/a> | T<\/a> | U<\/a> | V<\/a> | W<\/a> | X<\/a> | Y<\/a> | Z<\/a><\/p>\n

\n

Get Severe Weather Alerts for Your Location<\/p>\n

When any NWS watch, warning, or advisory is issued for your area, iAlert sends you an immediate notification by email or text, 24 hours a day, 7 days a week.<\/p>\n

Sign Up for Severe Weather Alerts →<\/a>\n<\/div>\n

\n

Weather Terms Beginning with “U”<\/strong><\/h3>\n

Updraft<\/strong><\/h4>\n

An updraft is a strong column of rising air within a thunderstorm, driven by the positive buoyancy of warm, moist air that is lighter than the surrounding environment. It is the engine of all convective storms. In an ordinary thunderstorm, updraft speeds reach 40-60 mph. In a violent supercell thunderstorm, the updraft can exceed 100-150+ mph vertically and extend to 60,000+ feet altitude, well into the stratosphere. The updraft lifts water vapor to the freezing level where it forms ice crystals, and carries hailstones aloft repeatedly until they grow heavy enough to fall. A rotating updraft, called a mesocyclone, is the defining feature of a supercell thunderstorm and a prerequisite for significant tornado development. Updraft strength is measured by Convective Available Potential Energy (CAPE), values above 3,000 J\/kg support violent, intense updrafts and a high potential for large hail and tornadoes.<\/p>\n

See also: What is a Supercell thunderstorm?<\/a> | Life cycle of a thunderstorm<\/a><\/p>\n

Upper Level Trough<\/strong><\/h4>\n

An upper-level trough is a southward-extending “dip” in the upper-level flow pattern at 500 mb (approximately 18,000 feet altitude), associated with a region of relatively low pressure aloft. Troughs promote rising motion at the surface by drawing air upward to fill the divergence aloft, air spreads away from the trough axis at that level, and surface air must rise to replace it. Upper-level troughs are the primary triggering mechanism for developing surface low-pressure systems, severe weather outbreaks, and heavy precipitation events across the United States. They are shown on 500-mb weather maps as areas where the height contours dip southward in a distinctive “U” or “V” shape. Troughs move generally eastward and forecast meteorologists track them closely as they approach from the Pacific, often days in advance of the weather they will produce.<\/p>\n

Upslope Fog<\/strong><\/h4>\n

Upslope fog forms when moist air flows upslope and cools to its dew point through adiabatic lifting, as the air rises along terrain, it expands and cools at a rate of approximately 5.5 degrees F per 1,000 feet of altitude gained. When it cools to the dew point, water vapor condenses into fog. Upslope fog is extremely common along the Front Range of the Rocky Mountains when easterly upslope flow lifts Gulf of Mexico moisture up the gentle eastern slope toward the Continental Divide. It can persist for days when an upper-level high pressure system locks the upslope flow pattern in place. Upslope fog is also common along the Appalachians and in any terrain where stable, moist air is forced uphill by the prevailing wind. It reduces visibility to near zero over large areas simultaneously, a much broader and more persistent hazard than the patchy radiation fog that forms on flat terrain.<\/p>\n

Urban Heat Island (UHI)<\/strong><\/h4>\n

The urban heat island effect is the measurable phenomenon where urban areas are significantly warmer than surrounding rural areas, caused by the replacement of vegetation, soil, and water bodies with heat-absorbing materials, concrete, asphalt, brick, dark roofing, and building materials. Cities can be 2-10 degrees F warmer than surrounding rural areas, with the greatest difference occurring at night when heat stored in urban materials slowly radiates back into the air, while rural areas cool rapidly. The UHI raises overnight low temperatures, increases air conditioning energy demand, and can enhance local thunderstorm development over and immediately downwind of large cities where the excess heat and urban roughness provide lift. Mitigation strategies include light-colored “cool roofs,” urban trees, green roofs, and permeable pavements that allow evapotranspiration. The National Weather Service accounts for UHI effects when issuing temperature forecasts for urban areas.<\/p>\n

UV Index<\/strong><\/h4>\n

The UV Index is a scale developed jointly by the World Health Organization (WHO) and NOAA that measures the intensity of ultraviolet radiation reaching Earth’s surface at solar noon on any given day, expressed on a scale from 0 to 11+. The NWS issues UV Index forecasts daily for major US cities as part of its Hazardous Weather Outlook products. The scale breaks into five risk tiers: 0-2 (Low), 3-5 (Moderate), 6-7 (High), 8-10 (Very High), and 11+ (Extreme). At UV Index 6+, unprotected skin on light-complexioned individuals can begin to burn in less than 20 minutes. The index is highest in summer, at midday (10 AM, 4 PM local solar time), at high elevations (UV increases approximately 10% per 1,000 feet of altitude), near the equator, and when stratospheric ozone levels are depleted. Reflected UV from sand, water, snow, and concrete significantly increases exposure beyond what the direct index number suggests, fresh snow reflects up to 80% of incoming UV radiation.<\/p>\n\n\n\n\n\n\n\n\n\n
UV Index Scale and Recommended Protection<\/caption>\n
UV Index<\/th>\nRisk Level<\/th>\nTime to Burn (Light skin)<\/th>\nRecommended Protection<\/th>\n<\/tr>\n<\/thead>\n
0-2<\/td>\nLow<\/td>\n60+ minutes<\/td>\nSunscreen optional; wear sunglasses<\/td>\n<\/tr>\n
3-5<\/td>\nModerate<\/td>\n45-60 minutes<\/td>\nSPF 15+ sunscreen; UV sunglasses; hat<\/td>\n<\/tr>\n
6-7<\/td>\nHigh<\/td>\n25-35 minutes<\/td>\nSPF 30+ sunscreen; protective clothing; limit midday sun<\/td>\n<\/tr>\n
8-10<\/td>\nVery High<\/td>\n15-25 minutes<\/td>\nSPF 50+ sunscreen; protective clothing; avoid midday sun<\/td>\n<\/tr>\n
11+<\/td>\nExtreme<\/td>\nLess than 15 minutes<\/td>\nMaximum protection; avoid outdoor exposure if possible<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

See also: Hot Weather Survival Tips in Extreme Summer Heat<\/a> | Protection Against Extreme Heat<\/a> | Heat Index Calculator<\/a><\/p>\n

Atmospheric Instability<\/strong><\/h4>\n

Atmospheric instability is a condition in which a parcel of rising air remains warmer, and therefore lighter, than the surrounding environment, causing it to continue accelerating upward rather than sinking back to its original level. It is the primary driver of all convective weather, including thunderstorms, tornadoes, and heavy showers. Instability is measured by several indices: CAPE (Convective Available Potential Energy) in joules per kilogram, the Lifted Index (LI, where negative values indicate instability), and the K-Index. “Convective instability”, also called “potential instability”, describes a layer of air that may appear stable in place but becomes unstable when lifted as a whole, a common setup ahead of major severe weather outbreaks. Forecasters identify “trigger mechanisms”, fronts, dry lines, outflow boundaries, and upper-level disturbances, that initiate lifting and release the stored instability, converting it into active storm energy.<\/p>\n

UTC \/ Zulu Time<\/strong><\/h4>\n

Coordinated Universal Time (UTC), also called Zulu (Z) time in aviation and meteorology, is the universal time standard used in all weather observation, forecast, and warning products worldwide to eliminate confusion between time zones. Weather balloons are launched twice daily simultaneously across the globe at 0000Z and 1200Z, midnight and noon UTC. All NWS warning issuance times, model run initialization times, and satellite imagery timestamps are expressed in UTC. To convert from UTC to US local time during Standard Time: Eastern = UTC, 5 hours; Central = UTC, 6; Mountain = UTC, 7; Pacific = UTC, 8. During Daylight Saving Time, subtract one fewer hour from each. A weather product issued at “2100Z” is 4:00 PM Eastern Standard Time. Storm chasers, meteorologists, and weather enthusiasts always reference UTC when discussing model runs and forecast data to avoid ambiguity.<\/p>\n

Upper Atmosphere<\/strong><\/h4>\n

The upper atmosphere, in operational meteorology, generally refers to the portion of the atmosphere above the planetary boundary layer and lower troposphere, typically above 3,000-5,000 feet altitude. “Upper air” products derived from twice-daily weather balloon soundings show temperature, wind speed and direction, and moisture at standard pressure levels: 850 mb (approximately 5,000 feet), 700 mb (10,000 feet), 500 mb (18,000 feet), 300 mb (30,000 feet), and 200 mb (39,000 feet). Upper-air patterns, jet stream position, upper-level ridges and troughs, and temperature anomalies at various levels, are the primary drivers of surface weather systems. The 500-mb level is the traditional “steering level” for surface weather systems, as upper-level winds at that altitude largely determine the direction and speed at which surface lows, fronts, and thunderstorm complexes move. Understanding the upper atmosphere is essential to medium-range weather forecasting beyond 1-2 days.<\/p>\n

Ocean Upwelling<\/strong><\/h4>\n

Ocean upwelling is the process by which cold, nutrient-rich water from the deep ocean rises to the surface along coastlines, driven by offshore winds that push surface water away from shore; the displaced surface water is replaced by cold water from below through the principle of conservation of mass. Along the California coast, prevailing northwesterly winds push surface water offshore, causing persistent upwelling that produces sea surface temperatures 5-15 degrees F colder than you would expect for that latitude, often only 50-58 degrees F in summer even near Los Angeles. This cold water chills the marine layer of air above it, producing the persistent marine layer fog and stratus cloud that characterizes the coastal California summer climate and suppresses afternoon temperatures dramatically compared to inland areas just 20-30 miles away. Upwelling also delivers nutrient-rich water that supports some of the world’s most productive marine ecosystems. Pacific Coast upwelling is a dominant factor in the cool, foggy summer climate of coastal California and Oregon.<\/p>\n

\n
\n

Severe Weather Alerts, Any Location, Any Time<\/p>\n

From a UV Advisory to a Tornado Warning, iAlert monitors every NWS alert and notifies you the moment one is issued for your area. Set up alerts for home, work, school, or anywhere your family is.<\/p>\n

Get Weather Alerts →<\/a>\n<\/div>\n

\n

Frequently Asked Questions, “U” Weather Terms<\/strong><\/h3>\n

What is an updraft in a thunderstorm?<\/h4>\n

An updraft is the strong column of rapidly rising warm, moist air that powers a thunderstorm. In an ordinary thunderstorm, updraft speeds reach 40-60 mph. In a violent supercell thunderstorm, updraft speeds can exceed 150 mph vertically, extending up to 60,000 feet. The updraft is responsible for holding hailstones aloft while they grow, lifting water vapor that condenses into the storm’s anvil cloud, and, when the updraft begins rotating due to environmental wind shear, creating the mesocyclone that can produce tornadoes. The atmosphere’s CAPE value is the best single measure of updraft potential.<\/p>\n

What is the urban heat island effect?<\/h4>\n

The urban heat island (UHI) effect is the measurable temperature difference between urban areas and surrounding rural areas, caused by the replacement of vegetation and natural surfaces with heat-absorbing materials like concrete, asphalt, and brick. Cities can be 2-10 degrees F warmer than surrounding countryside, especially overnight when stored heat radiates from pavements and buildings. This warmer baseline raises overnight low temperatures, increases air conditioning demand, and can enhance thunderstorm development over cities. Mitigation strategies include cool roofs, urban trees, green roofs, and permeable pavements.<\/p>\n

What is the UV Index and what do the levels mean?<\/h4>\n

The UV Index measures the intensity of UV radiation from the sun on a scale from 0 to 11+. The higher the index, the faster unprotected skin burns. At UV Index 3-5 (Moderate), use SPF 15+ sunscreen and sunglasses. At 6-7 (High), use SPF 30+ and limit midday sun exposure. At 8-10 (Very High), use SPF 50+, wear protective clothing, and avoid midday hours. At 11+ (Extreme), the time to sunburn for light skin is under 15 minutes, stay indoors during peak hours or cover completely. UV levels are highest at midday, in summer, at high elevation, near the equator, and near reflective surfaces like snow and sand.<\/p>\n

What is an upper level trough and how does it affect weather?<\/h4>\n

An upper-level trough is a southward-pointing dip in the upper atmosphere’s pressure pattern at about 18,000 feet altitude. Troughs promote rising air at the surface by creating divergence aloft, air spreads away from the trough axis, drawing surface air upward to replace it. This rising motion leads to cloud development, precipitation, and storm intensification. When a strong upper-level trough approaches from the west and encounters warm, moist surface air, it can trigger explosive severe weather outbreaks. Forecasters track upper-level troughs on 500-millibar weather maps as primary drivers of day-to-day weather patterns.<\/p>\n

What is UTC (Zulu) time and why do weather forecasts use it?<\/h4>\n

UTC (Coordinated Universal Time), also called Zulu or Z time, is the universal time standard used by meteorologists, aviators, and scientists worldwide to eliminate time zone confusion. All weather balloon launches, satellite imagery timestamps, model runs, and NWS warning times are recorded in UTC. This ensures that a weather observation from London, Tokyo, and Houston taken at the same moment all share the same timestamp. To convert UTC to US Eastern Standard Time, subtract 5 hours (4 hours during Eastern Daylight Time). A weather product timestamped “1800Z” is 1:00 PM Eastern Standard Time.<\/p>\n

What is upslope fog?<\/h4>\n

Upslope fog forms when moist air is pushed uphill by the wind and cools to its dew point through adiabatic (pressure-related) cooling. As the air rises along a slope, it expands and cools at about 5.5 degrees F per 1,000 feet. When it cools to the dew point, fog forms. Upslope fog is a common winter weather hazard along the Front Range of the Rocky Mountains in Colorado and Wyoming, where easterly winds push Gulf of Mexico moisture westward and uphill. Dense upslope fog can persist for multiple days and reduce visibility to near zero across a wide area simultaneously, unlike the patchy radiation fog of flat terrain.<\/p>\n

\n
\n

Never Miss a Severe Weather Alert<\/p>\n

iAlert monitors National Weather Service watches, warnings, and advisories around the clock and sends you an immediate notification the moment your area is threatened. Works for any U.S. location.<\/p>\n

View All Alert Services →<\/a>  Severe Weather Alerts →<\/a>\n<\/div>\n

\n

Related Articles<\/strong><\/h3>\n