Weather Words - U | Weather | WGNTV.com | WGN TV

Weather Words - U

-- U --

Ubac: The shady, north-facing side of a mountain or of a mountain valley. Ubacs are colder, have more snow cover and a longer duration of snow cover. The opposite south-facing side is known as the adret.

Ultraviolet light: A part of the sun's electromagnetic spectrum not visible to human eyes but responsible for many biological effects, including sunburn, tanning, wrinkles, aging, skin cancer and cataracts.

Ultraviolet light: A part of the sun's electromagnetic spectrum not visible to the human eyes, but responsible for sunburn, tanning and skin damage. The three components of the ultraviolet spectrum, ranging from longest to shortest wavelengths, are UVA, UVB and UVC.

Ultraviolet light: A part of the sun's electromagnetic spectrum not visible to human eyes but responsible for many biological effects, including sunburn, tanning, wrinkles, aging, skin cancer and cataracts.

Ultraviolet light: A part of the electromagnetic spectrum not visible to human eyes but responsible for many biological effects, including sunburn, tanning, cellular and elastic tissue changes, wrinkles, aging, skin cancer and cataracts.

Ultraviolet radiation: Electromagnetic radiation which causes sunburn. Because its wavelength is outside the visible light range, we cannot see it. It constitutes 5 to 7 percent of the total energy emitted by the sun.

United States' average precipitation: The total amount of water the United States receives in an average year is equal to a layer of water about 30 inches thick over the entire country, or about 4,300 billion gallons of water per day.

United States' coldest December temperature: On this date (December 18) in 1924 the temperature at Riverside Ranger Station in Yellowstone National Park, Wyoming, plunged to 59 degrees below zero, setting the all-time December low temperature record for the continental U.S.

United States' first documented tornado: On July 8, 1680, a tornado touched down at Cambridge, Mass. The funnel was filled with "stones, bushes, boughs, and other things"; one person died.

United States' greatest annual snowfall: The greatest annual snowfall in the United States is 1,122 inches at Paradise Ranger Station on Mt. Rainier, Wash., during the 1971-72 snow season. Chicago's annual average snowfall: 38 inches.

United States' greatest single snowstorm: 189 inches (that's 16 feet) at the Mt. Shasta Ski Bowl in California. It was a seven-day storm beginning on Feb. 13, 1959, and finally ending on the 19th.

United States' greatest snow accumulation: The greatest accumulation of snow on the ground in the United States is 451 inches -- that's 37.6 feet -- recorded at Tamarack, Calif., on March 11, 1911. The high Sierra Nevada Mountains intercept moisture sweeping in the Pacific Ocean and huge winter snows occur.

United States' heaviest 24-hour rainfall: 43" during tropical storm Claudette at Alvin, Texas, in suburban Houston, during a 24-hour period from July 25 into the 26th, 1979. Winds gusted only to 52 m.p.h.

United states' heaviest 24-hour rainfall: On this date (July 25) in 1979 the greatest single 24-hour rainstorm in the United States began. In a 24-hour period from the 25th into the 26th, 43 inches of rain fell at Alvin, Texas.

United States' heaviest snowstorm: Feb. 13, 1959, marked the beginning of a seven-day snowstorm that established a U.S. and world single-storm snow record. The Feb. 13-19 event at the Mt. Shasta Ski Bowl in California resulted in an accumulation of 189 inches of new snow --nearly 16 feet--a record that still stands.

United States' highest air pressure: The air pressure at Norway, Alaska, rose to 31.85 inches of mercury on this date (January 31) in 1989, establishing an all-time high barometric pressure for North America. The temperature at the time was about 46 degrees below zero.

United States' highest temperature: 134 degrees at Greenland Ranch in Death Valley Calif., on July 10, 1913. The location is 178 feet below sea level.

United States' highest temperature in June: 127 degrees at Fort Mojave, Calif., on June 15, 1896. The site, now abandoned, was a military base.

United States' highest wind speed: A measured gust 231 m.p.h., April 12, 1934, at Mt. Washington, New Hampshire.; highest hurricane wind: estimated gust 200-250 m.p.h., Sept 2, 1935 at Craig Key, Florida.

United States' largest hailstone: On Sept. 3, 1970, the largest hailstone officially reported in the U.S. fell at Coffeyville, Kansas. The hailstone measured 17.5 inches in circumference and weighed 1.67 pounds.

United States' longest dry spell: (1) 767 days. Bagdad, California, received no precipitation for a period of 767 days, from October 3, 1912, through November 8, 1914 -- a record-long U.S. dry spell. (2) 767 rain-free days, from Oct. 3, 1912, through Nov. 8, 1914, at Bagdad, Calif. The town, on old Route 66 in the Mojave Desert in southern California, was abandoned by 1991.

United States' lowest surface elevation: At 282 feet below sea level, Death Valley, California, is the lowest place in the United States. It is also the hottest place in the nation.

United States' lowest temperature: (1) -80 degrees F at Prospect Creek, Alaska, on January 23, 1971; lowest in the 48 contiguous states: -70 at Rogers Pass, Montana, on January 20, 1954; lowest in the 48 contiguous states: -70 degrees at Rogers Pass, Montana, on Jan. 20, 1954. Chicago's lowest temperature (-27 on Jan. 20, 1985) is 53 degrees higher than the U.S. record low.

United States' most flood-prone state: Iowa. The Mississippi River is the primary culprit, but other rivers also flood frequently. Flat terrain allows overflows to spread far away from the rivers.

United States' record rainfall: The U.S. record for the most rainfall in one year is 739 inches -- that's 61 1/2 feet -- recorded in the 12-month period from Dec. 1981 through Nov. 1982 at Kuki, Maui, Hawaii. Chicago's greatest one-year precipitation total: about 4 feet (49.35 inches) in 1983.

United States' second-deadliest tornado: The Natchez, Miss., tornado of May 7, 1840, claimed 317 lives along a 35-mile path, including 7 miles on the Mississippi River. 48 died in Natchez, 269 on river boats.

United States' second-deadliest tornado: The Natchez, Mississippi, tornado of May 7, 1840, claimed 317 lives and injured 109 along a 35-mile path, including 7 miles on the Mississippi River. 48 died in Natchez and 269 on river flatboats and steamers.

United States' snowstorms: Each year an average of 105 snow-producing storms affect the continental U.S. A typical storm will have a snow-producing lifetime of two to five days and will bring snow to portions of several states.

United States' sunniest city: Yuma, Ariz. When the sun is above the horizon, it is shining 91 percent of the time at Yuma, the highest percentage of any U.S. location. At Chicago, the comparable value is 52 percent.

United States' third-worst tornado: The St. Louis/East St. Louis Tornado of May 27, 1896. It cut through St. Louis, crossed the Mississippi River, then tore across East St. Louis; 255 deaths, 1,000 injuries.

United States' worst avalanche: On March 1, 1910, the deadliest avalanche in U.S. history killed as many as 118 as it swept across two trains stalled on the grade leading to Stevens Pass, WA. The station house at Wellington, WA, was also swept away.

United States' worst flash flood: The Johnstown, Pennsylvania, flash flood of May 31, 1889. After 6-8 inches of rain, the poorly maintained South Fork Dam collapsed at 3:10 p.m., sending a 40-foot wall of water and debris down the Little Conemaugh River, claiming 2,209 lives at Johnstown and other riverfront communities.

United States' worst storm disaster: The Galveston, Texas, hurricane of Saturday, September 8, 1900. 120 m.p.h. winds and a massive storm surge devastated the city of 38,000; 5,000 people were injured and 6,000 perished, perhaps as many as 10,000; another 1,200 died outside the Galveston area.

United States' worst tornado: The Tri-State tornado of March 18, 1925--the largest, longest, fastest, most destructive, most deadly U.S. tornado. 695 deaths, 2000+ injuries; path length 219 miles from southeast Missouri across southern Illinois into southwest Indiana.

Universal Time: The number of hours, minutes, and seconds that have elapsed since midnight in the Greenwich, England, time zone. To convert from Universal Time to Central Daylight Time, subtract five hours.

Unseasonable: With regard to the weather, an event not characteristic of the time of year. Note: "Unseasonable weather" is a rather common occurrence in Chicago's highly variable climate.

Unseasonable weather: Weather that is not characteristic of the time of year. Because daytime temperatures in the 70s are rare in November in Chicago (only one day in 37 on average), they would be considered unseasonable.

Unstable air: A layer of air or an air mass in which vertical air motions are likely to occur, usually resulting when warmer air lies beneath cooler air. The lower few thousand feet of the atmosphere is more unstable in the summer than in the winter.

Updraft: (1) A relatively small-scale current of air with marked upward motion. The term is frequently used in describing rising air currents associated with cumulus and cumulonimbus (thunderhead) clouds. (2) A small-scale current of air with marked, sometimes violent, upward motion usually beneath or within convective (cumulus) clouds.

Upper-level low: A storm system, generally above 15 thousand feet, about which winds circulate in counterclockwise fashion; produces inclement, cold, unsettled weather at the surface.

Upper air: That portion of the atmosphere that is above the lower troposphere, and generally applied to a height in the atmosphere above about 5,000 feet above sea level.

Upper limit of the Earth's atmosphere: The atmosphere extends upward to an indefinite height, its density gradually diminishing to that of the density of particles in interplanetary space, and this occurs at a height of about 18,600 miles above the surface.

Upper trough: A huge U-shaped buckle in upper-level wind patterns. Troughs usually move from west to east, crossing the United States in 3-6 days.

Upslope flow: Air that blows toward higher terrain and is therefore forced to rise. This often results in widespread cloudiness and, if the air is stable, fog and steady rain; if unstable, thunderstorms.

Upslope fog: Where the land slopes gradually upward, as on the U.S. Great Plains, fog forms when winds blow upslope and gradually cool to saturation. This kind of fog often forms during strong upslope winds.

Upwelling: (1) The rising of cold water to the surface from the deeper regions of a body of water. In the summer, cold Lake Michigan water upwells at Chicago's beaches when strong land-to-lake winds push warm surface water off-shore. (2)The rising of cold water to the surface from deeper regions of a body of water. In the summer, cold Lake Michigan water upwells at Chicago's beaches when winds push warm surface water off-shore.

Urban canopy layer: The layer of air interspersed between the buildings, trees and other objects of which a city is composed and extending upward to the average height of the buildings and trees.

Urban climate: A climate that shows some modifications to the climate experienced in the surrounding countryside, entirely due to the effects of a city or town; it is usually warmer and less windy.

Urban flooding: Urbanization increases the runoff of rainwater up to six times what would occur on natural terrain. City streets can become swift-moving rivers and basements can become deathtraps as they fill with water.

Urban heat island: The tendency for the central portion of a large urban area to experience higher temperatures -- day and night, winter and summer -- than surrounding rural areas.

Urban heat island effect: The tendency for the central portion of a large urban area to experience higher temperatures -- day and night, winter and summer -- than surrounding rural areas.

Urban runoff: Storm water from city streets and gutters that usually contains a great deal of litter, bacterial wastes and oily residue.