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Weather Words - An-Az

-- An-Az --

Anabatic wind: A wind blowing up a mountain slope. It is usually a warm wind (the opposite of katabatic wind: a wind, usually cold, blowing downslope).

Anaerobic: Characterizing an environment that lacks air or free oxygen; also, characterizing organisms that do not need free oxygen for their metabolism. Contrast aerobic.

Ancraophobia: The irrational or morbid fear of wind.

Anemo-: A prefix meaning wind.

Anemometer: A weather instrument that measures the wind speed. Wind vanes measure the direction of the wind.

Anemophobia: The irrational or morbid fear of wind.

Aneroid: The literal meaning is NOT WET; containing no liquid. Usually it describes a kind of barometer that contains no liquid (mercury): an aneroid barometer.

Aneroid barometer: An instrument that measures air pressure by detecting the movements of a thin, hollow, partially evacuated metal disk that expands/contracts as air pressure changes; requires frequent recalibration.

Anomalous propagation: (1) The transmission of a radar beam through the atmosphere when density discontinuities cause the beam to move along a path other than straight line; also, the false echoes resulting from that non-linear propagation. Radar beams are usually line-of-sight.

Anomalous propagation: (2) A radar term for false (non-precipitation) echoes occurring when the radar beam is bent into the ground by certain atmospheric conditions (usually temperature inversions).

Anomaly: The deviation of temperature or precipitation (or some other atmospheric variable) in a given region over a specified period of time from the long-term average value for that region.

Antarctic winter: It's summer here, but Antarctica and the Southern Hemisphere is now in its winter season. The world's lowest temperature, -129 degrees F, was recorded at Vostok, Antarctica, on July 21, 1983.

Antarctica: The world's highest, coldest and driest continent; average elevation 8,000 feet; average interior winter temperature -80 degrees F; average annual precipitation (water content) is less than three inches.

Antarctica's highest temperature: 59 degrees F at Vanda Station on Jan. 5, 1974. When it's winter in the Northern Hemisphere, but summer is in full swing in the Southern Hemisphere.

Antarctica's temperature extremes: A range of 188 degrees F from 59 at Vanda Station on Jan. 5, 1974, to -129 at Vostok Station on July 21, 1983. The Vostok reading is also the world's lowest recorded temperature.

Anthropogenic climate change: Climate change that occurs as a result of human activities. Contributing factors are addition of greenhouse gases to the atmosphere and changes (such as deforestation) of land surfaces.

Anthropogenic heat: Heat released into the atmosphere as a result of human activities. Sources of such heat include engines, heating and air conditioning units of buildings, fires and even the human body itself.

Anticyclone: A high pressure system; a region of the atmosphere characterized by relative high air pressure and wind movement (in the northern hemisphere) in a clockwise sense when viewed from above.

Anticyclonic rotation: Rotation about a vertical axis in the opposite direction as the Earth's rotation. When viewed from above, it is clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

Antifreeze: A substance, often a liquid such as ethylene glycol or alcohol, mixed with another liquid, such as water, in order to lower the freezing point of the resulting mixture.

Antlophobia: The irrational or morbid fear of floods.

Anvil cloud: The popular name applied to the upper, flattened ice-crystal portion of a cumulonimbus (thunderhead) cloud formed when the storm's updraft encounters a layer of stable air and spreads horizontally.

Anvil crawlers: Lightning that branches upward and outward along the sides and tops of large thunderstorms. The lightning seems to travel or crawl across the sky, branching as it goes; also known as spider lightning.

Anvil lightning: A cloud-to-ground lightning bolt that arcs horizontally away from the parent cloud to strike the ground some distance away, sometimes directly below clear skies overhead.

Anvil zits: A slang term used by storm chasers to describe frequent (often continuous or nearly continuous), localized lightning discharges occurring within the anvil portion of a thunderhead.

Aphelion: The point on the Earth's annual orbit around the sun at which the Earth is farthest from the sun. It occurs on about July 5, when the Earth is 94,452,000 miles from the sun, or about 3,110,000 miles farther from the sun than at perihelion (the point when the Earth is closest to the sun).

Aphotic zone: That portion of the ocean environment that does not receive enough sunlight to allow plant photosynthesis to take place. In clear tropical waters it begins at a depth of about 350 feet, but in middle latitudes is commonly starts at about 175 feet.

Apogee: The point on the orbit of the moon (or any other Earth satellite) that is farthest from the Earth; opposed to perigee.

Apparent temperature: The heat index; using air temperature and relative humidity, how hot it feels to an average person (147 pounds) wearing light clothing and walking 3.1 mph in a 5 mph wind.

Apparent Wind: Wind relative to a boat's speed and direction. For example, a 10 m.p.h. wind blowing from the east and a boat moving to the east at 10 m.p.h. gives an apparent wind from the east at 20 m.p.h.

April: The 4th month of the year and 2nd month of meteorological spring. With a 77-year average temperature (Midway) of 49.5 degrees F, precipitation of 3.52 inches and snowfall of 1.3 inches, it is Chicago's 7th warmest, 4th wettest and 6th snowiest month.

April 1: (1) On this date in 1970, Chicago received 8.2 inches of snow; in 1960, Tiros I, the world's first weather satellite, was launched; in 1946, a tsunami up to 55 feet in height killed 156 in and around Hilo, Hawaii.

April 1: (2) A tornado touchdown during snow squalls near Bay Mills, Wisc. (1987); Chicago snowstorm (1970), 10.7 inches (8.2 inches on 1st, 2.5 inches on 2nd); Tiros I, first weather satellite, launched (1960); a 55-foot tsunami killed 159 in/near Hilo, Hawaii (1947).

April 10, 1930: Chicago's temperature zoomed to 90 degrees F at the University of Chicago (the site of the city's official weather observation station at that time) and to 91 at Midway Airport. This is Chicago's earliest-occurring spring temperature of 90.

April 12, 1934: An important day in meteorology because it is the anniversary of the world's highest measured wind speed -- a gust to 231 mph on the 6,288-foot summit of Mount Washington, New Hampshire.

April 21, 1967, tornadoes: 48 tornadoes across the Upper Midwest with 16 in northern Illinois, including two F4 tornadoes (winds 207-260 mph), in Belvidere (24 deaths) and Oak Lawn/Hometown/Evergreen Park (33 deaths). April, 1970, snowstorm: On April 1 in 1970, a major snowstorm began in Chicago. A total of 10.7 inches of damaging, wind-driven snow blasted the city on April 1-2 (8.2 inches on the 1st, 2.5 inches on the 2nd); thundersnow occurred on both days, and high winds caused drifts 2 to 3 feet.

April tornadoes: In metropolitan Chicago, more tornado deaths (65) have occurred in April than in any other month.

Arctic Circle: The line of latitude about 66 degrees 33 minutes north of the Equator. To the north of this line the sun does not set on at least one day in summer, nor does it rise during at least one day in winter.

Arctic air: A type of air mass whose characteristics are developed in the cold season over ice- and snow-covered areas of the far north. It has low temperatures, low moisture content, and is very stable. Air masses of arctic origin cover Chicago about six percent of the time.

Arctic outbreak: The rapid movement of a mass of air originating above the Arctic Circle southward to the Gulf of Mexico, or farther. Arctic outbreaks bring Chicago its lowest autumn and winter temperatures; also known as a polar outbreak.

Arcus: A low, dense, horizontal roll cloud, associated most often with the leading edge of cool thunderstorm outflow (the gust front) or more rarely with a very well-defined cold front.

Areic region: A region whose climate is so arid that it contributes no water to surface drainage. Portions of the southwest and west U.S. are areic. The vast reaches of north Africa's Sahara Desert comprise the world's largest areic region.

Argon: A colorless, odorless, inert gaseous element that is the third most abundant constituent of the atmosphere (behind nitrogen and oxygen), amounting to almost one percent by volume of dry air.

Arid: Lacking in sufficient precipitation to support vegetation; more generally, lacking in effective, life-promoting moisture. The most arid climate in the United States is found in Death Valley, California.

Arid climate: A climate lacking in sufficient precipitation to support vegetation; more generally, a climate lacking in effective, life-promoting moisture.

Aridity: The degree to which the climate of a location lacks effective, life-promoting moisture. A location is termed arid when its rainfall is insufficient to support vegetation.

Aristotle: (384-322 B.C.) The author of the earliest known systematic treatises on meteorology, the Meteorologica. For the next 2,000 years, the science of meteorology did not progress substantially beyond the point to which Aristotle carried it.

Arizona's and Florida's highest temperatures: 128 degrees F at Lake Havasu City, Ariz., on June 29, 1994; 109 at Monticello, Fla., on June 29, 1931.

Arizona's lowest temperature: -40 degrees F at Hawley Lake on Jan. 7, 1971. The Hawley Lake station is located in the mountains of northeast Arizona at an elevation of 8,180 feet.

Arroyo: A stream channel in an arid region, generally dry because the eroding stream flows only after rains. Violent, life-threatening flash floods can sweep through an arroyo following heavy thunderstorm rains in the drainage area.

Artesian well: A drilled well from which water is forced out at the surface because its source is under such great pressure.

Ash: Airborne particulates resulting as a byproduct of combustion. It greatly reduces air quality and its emission into the atmosphere is often regulated.

Asia's highest temperature: On June 21, 1942, the temperature at Tirat Tsvi, Israel, soared to 129 degrees F., the highest temperature ever recorded on the continent of Asia.

Asia's lowest temperature: -90 degrees F, first recorded on Sunday, February 7, 1892, at Verkhoyansk, Russia, then recorded again on Monday, February 6, 1933, at Oimekon, Russia.

ASOS: Acronym for Automated Surface Observing System, a package of automated weather observing instruments that is in widespread use by the National Weather Service and Federal Aviation Administration.

Asteroid: Any of the tens of thousands of rocky or metallic minor planets that orbit the sun, mainly between the orbits of Mars and Jupiter, and detectable because of their size. As many as 350 may be in Earth orbit-crossing trajectories.

Astraphobia: The morbid or irrational fear of thunder, thunder, or thunderstorms.

Astrapophobia: The morbid or irrational fear of thunder and lightning.

Astrophobia: The irrational or morbid fear of the stars.

Atlantic hurricane season: (1) The 6-month season ends Nov. 30. The 2002 season brought 14 tropical depressions (winds 38 mph or less), 8 tropical storms (39-73 mph) and 4 hurricanes 74 mph or higher).

Atlantic hurricane season: (2) The six-month period June 1 through November 30.

Atlantic Ocean coastline: Because of its many arms and appendages (like the Caribbean Sea and Mediterranean Sea), the Atlantic Ocean has more miles of coastline than the Pacific and Indian Oceans combined.

Atmometer: The general name for instruments that measure the evaporation rate of water into the atmosphere. Atmometry: The science of measuring the rate and amount of evaporation of water.

Atmosphere: (1) The gaseous fluid surrounding a planet. The Earth's atmosphere consists of a mixture of gases: nitrogen (78 percent), oxygen (21 percent), argon (1 percent), carbon dioxide (0.03 percent) and several trace gases. Water vapor is a variable constituent, up to 4 percent.

Atmosphere: (2) The gaseous fluid surrounding a planet. By weight, 99 percent of the Earth's atmosphere lies within 18 miles (95,040 feet) of the surface, and 99.999 percent lies within 50 miles.

Atmosphere: (3) The gaseous fluid surrounding a planet. By weight, only one percent of the Earth's atmosphere lies above a height of 18 miles (95,040 feet), and only 0.001 percent lies above a height of 50 miles.

Atmosphere: (4) The gaseous fluid surrounding a planet; also the average pressure (an atmosphere) exerted at sea level by the Earth's atmosphere, equal to a barometric pressure of 29.92 inches of mercury.

Atmosphere, weight of: A sea level, the atmosphere pushes on the surface with a pressure of 14.7 pounds per square inch, or about 2100 pounds per square foot. The atmosphere's total weight is about 5.75 quadrillion (5,750,000,000,000,000) tons.

Atmospheric circulation: The large-scale, general circulation of the atmosphere consisting of the equatorial doldrums, the trade winds, the horse latitudes, mid-latitude westerlies and the polar vortices. Most of the United States lies within the mid-latitude westerlies.

Atmospheric extinction: The apparent dimming of a celestial object as it nears the horizon, owing to its appearance at that angle through more of the densest layers of the Earth's atmosphere, resulting in greater scattering and absorption of the light.

Atmosphere, upper limit of: 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.

Atmospheric greenhouse effect: The heating effect of the Earth's atmosphere upon the planet. Sunlight readily passes through the atmosphere and warms the surface, but the atmosphere retards the escape of heat back into space.

Atmospheric size scales: Four scales of size into which all atmospheric phenomena are classified: microscale (up to a few miles), mesoscale (a few miles to a few hundred miles), synoptic scale (a few hundred miles to about two thousand miles), and macroscale (continental to global).

Atmospherics: Radio-frequency electromagnetic radiations that originate mainly from thunderstorm lightning. Atmospherics are heard as the static in AM radios.

Attenuation: Reduction of radar beam power due to reflection or absorption of energy when it strikes a target. The greatest attenuation occurs when the radar beam goes through very heavy rain.

Audible range of thunder: The sound of thunder usually travels no more than about 10 miles, but the audible range can be much less if the sound passes through heavy rain.

August: The 8th month of the year and 3rd month of meteorological summer. With a 77-year average temperature (Midway data) of 73.7 degrees and precipitation of 3.49 inches, it is the 2nd warmest and 5th wettest month.

August 17: In 1969, the storm surge and 190 mph winds of Hurricane Camille claim 256 lives; in 1917, the 43rd (and last) consecutive day of maximum temperatures 120 degrees F or higher occurs at Death Valley, Calif.

August snow: It was reported, but never confirmed, that ships plying northern Lake Michigan were covered with six inches of slushy snow on Aug. 8, 1882. Snow has never fallen in Chicago in August.

Aurora: A luminous phenomenon in the night sky consisting of overlapping curtains of green, white, red and blue light.

Aurora Borealis-the northern lights or the aurora of the Northern Hemisphere; Aurora Australis-the southern lights or the aurora of the Southern Hemisphere.

Aurora borealis: (1) Common across Alaska and northern Canada, on Monday night it was visible as far south as Alabama and Georgia.

Aurora borealis: (2) A luminous phenomenon in the night sky consisting of overlapping curtains of green, white, red and blue light.

Aurora, Ill: Site of the heaviest rain ever to occur in Illinois. From the morning of Wednesday, July 17, to the morning of July 18, 1996, 16.91 inches of rain fell at Aurora, establishing an Illinois state record.

Australia's highest temperature: On Jan. 16, 1889, the temperature at Cloncurry, Queensland, Australia, soared to 128 degrees F, the highest temperature ever recorded on the continent of Australia.

Australia's lowest temperature: On June 29, 1994, the temperature at Charlotte Pass, New South Wales, Australia, plunged to -9 degrees F, the lowest temperature ever recorded on the continent of Australia.

Autan: In France, the name given to foehn winds (warm, dry winds that blow downslope on the lee (downwind) slopes of a mountain range). In the United States and Canada, foehn winds are known as chinooks.

Automated Surface Observing Systems (ASOS): A system deployed by agencies of the U.S. federal government designed to provide automated measurements of several weather variables at airports. Operational results have been very disappointing, especially for climatological purposes, and (as of 2007) manual augmentation of the automated weather observations is necessary.

Autumn: The season of the year comprising the transition from summer to winter. Meteorological autumn (September, October, November) starts today. Astronomical autumn begins about Sept. 21st, the autumnal equinox.

Autumnal: Pertaining to the autumn season. The autumnal equinox occurs on or about Sept. 22 in the Northern Hemisphere and on or about March 21 in the Southern Hemisphere; contrast vernal.

Autumnal equinox: (1) The time (usually on Sept. 22) when the sun, approaching the Southern Hemisphere, crosses the plane of the Earth's equator, making night of equal length all over the world; the start of astronomical autumn.

Autumnal equinox: (2) The point on the sun's apparent annual path at which it is directly overhead at the Earth's equator as it is retreating to the south. The Northern Hemisphere autumnal equinox falls on about September 22.

Avalanche: (1) The rapid, violent movement of large masses of snow, ice or rock and debris down steep mountain slopes. An avalanche may fall, slide, roll or flow. Snow avalanches rarely occur on slopes of less than 30 degrees, occurring mainly on slopes of 30-45 degrees.

Avalanche: (2) The rapid, violent movement of large masses of snow, ice or rock and debris down steep mountain slopes. Worldwide, about one million snow avalanches occur per year.

Avalanche blast: The powerful, destructive blast of wind that occurs when an avalanche is stopped abruptly, as in the case of an almost vertical fall onto a valley floor.

Avalanche victims: Typically, a male in his 20s; he is an experienced skier, good at his sport, and with good alpine skills -- but lacking avalanche knowledge. One-third of avalanche victims are killed striking trees or rocks on the way down.

Average: The arithmetical mean; the result obtained by summing the value of individual numerical quantities and dividing the sum by the number of individual quantities. An average over a specific 30-year period is considered to be a climatological normal.

Average daily temperature: The value obtained by summing the highest and lowest temperatures that occur during a calendar day (midnight to midnight local standard time), dividing the sum by two, and rounding to a whole number.

Aviation: Orville and Wilbur Wright initiated the age of aviation on Dec. 17, 1903, at Kitty Hawk, NC, with a 12-second, 120-foot powered, controlled flight. The growth of aviation was an important factor in the development of weather forecasting.

Aviation and weather: Weather is a factor is about 30 percent of all aviation accidents, and weather-related flight delays cost about $2 billion per year.