From gopher@esdim.noaa.gov Wed Jun 18 22:54:27 1997 Received: from iisc.ac.in by ces.iisc.ac.in (ERNET-IISc/SMI-4.1) id WAA26771; Wed, 18 Jun 1997 22:54:27 +0500 Received: from esdim.noaa.gov by iisc.ac.in (ERNET-IISc/SMI-4.1) id WAA28213; Wed, 18 Jun 1997 22:53:20 +0530 Received: by esdim.noaa.gov (4.1/SMI-4.1) id AA29397; Wed, 18 Jun 97 12:22:33 EDT Date: Wed, 18 Jun 97 12:22:33 EDT From: gopher@esdim.noaa.gov (ESDIM Gopher) Message-Id: <9706181622.AA29397@esdim.noaa.gov > Subject: Weatherguide Glossary Apparently-To: cestvr@ces.iisc.ac.in Status: R WEATHERGUIDE GLOSSARY acid rain Precipitation that carries to earth sulfuric and nitric acid accumulated from air pollutants. airmass Large body of air whose temperature and moisture distribution is shaped by its original geographic location. aphelion The point on a planet's or comet's orbit farthest from the sun. For Earth, it presently occurs about July 1st. apogee The point on the moon's orbit, or that of any other Earth satellite, which is the farthest from the Earth. aquifer An underground geologic formation transmitting water in sufficient quantity to support wells and springs. astronomy The scientific study of the universe beyond the Earth, extending to the outermost range of telescopes. aurora borealis Also called "northern lights." Glow of rarified gases in upper atmosphere induced by bursts of material from the sun. They are frequently visible over the northern United States and Canada. barometer An instrument for measuring atmospheric pressure and used in predicting weather changes. blizzard A severe winter storm characterized by winds of 35 mph or more, with or without falling snow, but most often with blowing snow. Blue Moon The name given to the second full moon in a given month, which occurs about once every three years. Celsius A temperature scale in which the melting point of ice is 0 degrees; boiling point of water at standard pressure is 100 degrees. climate The long-term display of weather conditions in a specified area. cloud Visible, water droplets and/or ice particles in the air. Fog is a cloud touching the Earth's surface. compost Organic matter, such as grass clippings, decayed by nitrogen and sunlight into a nutrient-rich material for mulching. condensation The process by which a vapor becomes a liquid. In meteorology it occurs when water vapor changes to dew, fog, or becomes a cloud. In the winter, water vapor turns to ice or snow. conjunction Time of apparent closest approach to each other of any two heavenly bodies. contrail Cloud-like streamer forming behind high-flying aircraft in cold, clear, humid air. Also known as "condensation" or "vapor" trail. deciduous Trees and shrubs that shed their leaves annually. degree day A measure of the departure of the mean daily temperature from a given standard. For heating and cooling the base is 65 degrees; one degree day for each degree above or below. Degree days are accumulated over a "season" at any point during which the total can be used as an index of past temperature relations with some quantity, such as a plant growth, fuel consumption, power output, etc. dew Water vapor condensed onto grass and other objects near the ground. dew point The temperature at which water vapor will condense on an object Dew point is used as a measure of moisture in the air. Higher dew points indicated more moisture. Dew points above 60 degrees are uncomfortable for most people. downburst A severe downrush of wind from a severe thunderstorm. Sometimes referred to as straight line winds. Occasionally these winds can exceed 100 mph and cause damage to trees and structures. Doppler Radar A type of radar that can detect wind velocity and circulation, in addition to precipitation intensity. ecology The science of the relationships between plants, animals, and the environment. El Nino A vast ocean atmosphere interaction manifesting itself as an expanding area of warm water extending hundreds of miles off the coast of Ecuador and Peru. It causes changes in atmospheric circulation effecting the weather over vast regions of the earth causing droughts in some areas and extreme moisture in others. equinox "Vernal equinox" is approximately March 20 when the sun crosses the equator going north and is the beginning of spring. "Autumnal equinox is about September 22, when the sun crosses the equator going south and heralds the beginning of fall. evaporation The process by which a liquid is transformed to a vapor. The opposite of condensation. flash flood A flood that rises and falls rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. fog A cloud touching the Earth's surface. When made up of ice crystals it's called an ice fog. fossil fuels Combustible materials resulting from the remains of plants and animals deposited in past geologic eras. They include coal, oil, natural gas, and peat. When burned they release various compounds of carbon, nitrogen, and sulphur into the atmosphere. fronts The line along which two air masses of different density and temperature meet, generally identified as either a "warm front," "cold front," or "stationary front." frostbite The freezing of exposed parts of the body, usually fingers, toes, and the face, causing damage to the tissues. glaze A coating of ice, generally clear and smooth, formed by rain or drizzle on the surface. It is denser, harder and more transparent than either rime or hoarfrost. greenhouse effect Carbon dioxide, methane, chlorofluorocarbons and water vapor are atmospheric gasses transparent to radiation, reaching the earth from the sun. However, they trap heat emanating from the earth's surface, much as a window traps solar heat inside a greenhouse. "The Greenhouse Effect" is the theorized heating of the earth's atmosphere from increase in, primarily, carbon dioxide from burning fossil fuels and deforestation. halo A ring or arc of white or colored light around the sun or moon caused by ice particles in the atmosphere. hoarfrost Ice crystals formed on objects exposed to the air such as tree branches, plant stems, wires, poles, etc. It is the result of water vapor passing from a gas to a solid form. humidity A measure of the water-vapor content of air. hurricane A tropical storm with winds of 75 mph and over with heavy rains. In the western Pacific area it is often referred to as a "typhoon," and in the Indian Ocean it is called a "cyclone." hypothermia A fall in the temperature of a body below the usual level, brought on when the homeostatic mechanisms fail to maintain adequate production of heat under conditions of extreme or prolonged cold. ice crystals Ice particles that form in the air during cold weather as a result of the passage from water droplets of vapor to ice. They may fall from a cloud or form a cloudless sky and are visible especially in direct sunlight or in a artificial light beam. Indian Summer A period in mid or late autumn of abnormally warm weather, generally clear skies, sunny but hazy days and cool nights. Often a killing frost and a stretch of normally cool weather precedes this warm spell. It does not occur every year. inversion An increase in temperature with height, which is reverse of the norm, and is visible over cities where it causes smoke and pollution to be trapped next to the ground. isobar A line of equal or constant barometric pressure. isotherm A line of equal or constant temperature. jet stream An apparent meandering current of high-speed winds 3 to 10 miles above the Earth's surface, blowing from a westerly direction and often exceeding 250 mph. lightning Visible electrical discharge produced by thunderstorms, cloud-to- cloud, cloud-to-earth, or cloud-to-air. magnetic pole The north magnetic pole is now about 200 miles north of the Boothia Peninsula in Canada, while the south magnetic pole is in South Victoria Land on the Antarctic continent. The two poles do not lie at extremes of a diameter of the earth, for the line joining them misses the exact center of the earth by about 750 miles. mesostorm An intermediate-sized storm system between the mirco scale (tornadoes) and synoptic scale (cyclones). Mesostorms are usually more localized and are within the range of 10 to 200 km. meteor A streak of light seen at night when a particle from space enters the Earth's atmosphere and burns, usually called a shooting or falling star. meteorite That portion of a meteoroid which survives its passage through the atmosphere and lands on Earth. meteorology The science of weather and weather forecasting. Milky Way The spiral-shaped galaxy of which our Earth is a part. On a clear night away from city lights, its main band of light is seen across the night sky. opposition Time when two heavenly bodies appear on opposite sides of the sky, or when they are farthest apart. parhelion Also called "mock sun" or "sun dog." Either of two colored bright spots appearing at points 22 degrees on both sides of the sun and at the same elevation as the sun. Often seen on clear, cold winter days and caused by light refraction through ice crystals. perigee The point in the orbit of the moon or any other satellite, closet to the Earth. perihelion The point on the Earth's orbit which is nearest the sun. phenology The science of tracking biological and physical events of nature in relationship to weather and climate, events such as the flowering, breeding, and migration, along with ice-out dates and freezeovers of lakes and streams. rainbow An arc, usually from ground to ground, with bands of color ranging from red on the inside to violet on the outside, formed opposite the sun by the reflection ad refraction of the sun's rays inside raindrops. Most often seen following the rain. relative humidity The ratio of the air's water vapor content to the maximum water vapor it could hold at a given temperature. rime A white granular deposit of ice on the windward sides of objects, always out directly against the wind. It is denser and harder than hoarfrost, but lighter, softer and less transparent than glaze. shower Precipitation characterized by rapid starts and stops and by rapid changes in intensity. smog Originally, a mixture of smoke and fog. Currently, any air pollution, regardless of high humidity or fog. snowburst or thundersnow An intense convective snow squall accompanied by lightning and thunder. Snowfall rates in snowbursts often reach 1 to 3 inches per hour." solstice The point on the sun's path when it is farthest north or south of the Earth's equator. The "summer solstice" is about June 21 and the "winter solstice" is about December 22 in the Northern Hemisphere. Times of sunrise and sunset remain fairly constant for several weeks around either solstice, which comes from the latin and means "to make stand." sublimation The process where snow and ice pass directly to the vapor stage without first melting. Much of our snow and ice disappears this way in the winter. thunder The sound caused by rapidly expanding gases along the channel of a lighting discharge. thunderstorm A local storm produced by a cumulonimbus cloud and always accompanied by lighting and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. tornado A violently rotating column of air dropping from a cumulonimbus cloud to the ground. It is the most violent of all atmospheric phenomena. twilight The intervals of incomplete darkness following sunset and preceding sunrise, caused by the bending of the sun's light by the atmosphere. virga Precipitation falling from a cloud that evaporates before reaching the ground. waning moon Shrinking from "full" to "new" moon. waxing moon Growing from "new" to "full" moon. wind-chill index The cooling effect of any combination of temperature and wind on bare skin. wind rose A circle from which 4 to 36 lines emanate, one for each compass point. The length of each line is proportional to the frequency of wind from that direction, usually averaged for a month.