Documents for "Weather and Climate: Terms and Concepts":

• air mass large body of air within the earth's atmosphere in which temperature and humidity, although varying at different heights, remain similar throughout the body at any one height. Air masses form over parts of the earth's surface called source...

• altocumulus see cloud.

• altostratus see cloud.

• anthelion see halo.

• anticyclone region of high atmospheric pressure; anticyclones are commonly referred to as "highs." The pressure gradient, or change between the core of the anticyclone and its surroundings, combined with the Coriolis effect , causes air to circulate about the core in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. Near the surface of the earth the frictional...

• aureole in physics, luminous circle seen when the sun or other bright light is observed through a diffuse medium, i.e., smoke, thin cloud, fog, haze, or mist. It sometimes occurs as a series of concentric...

• aurora australis see aurora borealis.

• aurora borealis and aurora australis , luminous display of various forms and colors seen in the night sky. The aurora borealis of the Northern Hemisphere is often called the northern lights, and the aurora australis of the Southern...

• Beaufort scale a scale of wind velocity devised (c.1805) by Admiral Sir Francis Beaufort of the British navy. An adaptation of Beaufort's scale is used by the U.S. National Weather Service; it employs a scale from 0 to 12,...

• blizzard winter storm characterized by high winds, low temperatures, and driving snow; according to the official definition given in 1958 by the U.S. Weather Bureau, the winds must exceed 35 mi (56 km) per...

• chinook warm, dry air mass that descends the eastern slopes of the U.S. and Canadian Rocky Mts. after having lost moisture by condensation over the western slopes. Chinooks occur mainly in winter. They...

• climate average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain ), humidity , wind , barometric pressure, and other...

• cloud aggregation of minute particles of water or ice suspended in the air.

• Coriolis effect [for G.-G. de Coriolis, a French mathematician], tendency for any moving body on or above the earth's surface, e.g., an ocean current or an artillery round, to drift sideways from its course...

• corposant see Saint Elmo's fire.

• cumulonimbus see cloud.

• cumulus see cloud.

• cyclone atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect , causes air to circulate about the core of lowest pressure in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Near the surface of...

• dew thin film of water that has condensed on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of...

• dew point see dew.

• doldrums or equatorial belt of calms, area around the earth centered slightly north of the equator between the two belts of trade winds. The large amount of solar radiation that arrives at the earth in this area causes intense heating...

• drought abnormally long period of insufficient rainfall. Drought cannot be defined in terms of inches of rainfall or number of days without rain, since it is determined by such variable factors as the...

• dust devil see whirlwind.

• El Niño-Southern Oscillation (ENSO) , large-scale climatic fluctuation of the tropical Pacific Ocean. The El Niño [Span.,=the child] itself is a warm surface current that usually appears around Christmas in the Pacific off Ecuador and Peru and disappears by the end of March, but every two to seven years it...

• equatorial belt of calms see doldrums.

• fata morgana see mirage.

• foehn warm, dry wind that occurs on the leeward slopes of a ridge of mountains. The term was originally applied to a wind of the Alps but is now used as a generic term for all winds of this type. In...

• fog aggregation of water droplets or ice crystals immediately above the surface of the earth (i.e., a cloud near the ground). A light or thin fog is usually called a mist. Fog may occur when the...

• frigid zone see Arctic, the ; Antarctica ; zone.

• front in meteorology, zone of transition between adjacent air masses. If a cold air mass is advancing to replace a warmer one, their mutual boundary is termed a cold front; if the reverse, then the boundary is termed a warm front, whereas a stationary front...

• frost or hoarfrost, ice formed by the condensation of atmospheric water vapor on a surface when the temperature of the surface is below 32°F (0°C). In the formation of frost, a gas (water vapor) is changed directly to...

• Fujita scale or F-Scale, standard scale for rating the severity of tornadoes as a measure of the damage they cause, devised in 1951 by the Japanese-American meteorologist Tetsuya (Ted) Fujita (1920-98). It classifies...

• glaze in meteorology: see sleet.

• haboob see sandstorm.

• hail precipitation in the form of pellets composed of ice or of ice and snow, occurring at any time of the year, usually during the passage of a cold front or during a thunderstorm. Small hailstones...

• halo in meteorology, short-lived circles or arcs, and less commonly spikes and crosses, of colored or whitish light surrounding the moon or sun or in clouds as seen from above. A halo occurs when the...

• haze suspension in the atmosphere of minute dust or salt particles that are not individually seen but that nevertheless reduce visibility. So-called damp haze and dry haze produce different optical...

• hoarfrost see frost.

• horse latitudes two belts of latitude where winds are light and the weather is hot and dry. They are located mostly over the oceans, at about 30° lat. in each hemisphere, and have a north-south range of about 5°...

• humidity moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity alone is used), the ratio of the actual water-vapor content of the air to its total capacity at the given temperature; specific humidity, the mass of water vapor per unit mass of natural air; and...

• hurricane tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds reach over 190 mi (289 km) per hr in some hurricanes. The term is often restricted to those storms occurring over the N...

• isobar or isobaric line , line drawn on a weather map through points of equal atmospheric pressure. Isobars are used to define cyclones (low-pressure regions) and anticyclones (high-pressure regions). Weather maps are...

• isotherm line drawn on a map of a particular region of the earth's surface connecting points of equal temperature; each point reflects one temperature reading or an average of several readings over a period...

• La Niña see El Niño-Southern Oscillation.

• lightning electrical discharge accompanied by thunder , commonly occurring during a thunderstorm. The discharge may take place between one part of a cloud and another part (intracloud), between one cloud and another (intercloud), between a cloud and the earth, or earth and cloud. Lightning may...

• looming see mirage.

• meteorology branch of science that deals with the atmosphere of a planet, particularly that of the earth, the most important application of which is the analysis and prediction of weather. Individual studies within meteorology include aeronomy, the study of the physics of the upper atmosphere; aerology, the study of free air not adjacent to the earth's surface; applied meteorology,...

• mirage atmospheric optical illusion in which an observer sees in the distance a nonexistent body of water or an image, sometimes distorted, of some object or of a complete scene. Examples of mirages are...

• mist see fog.

• monsoon [Arab., mausium =season], wind that changes direction with change of season, notably in India and SE Asia. To a lesser degree, monsoonal winds also develop in portions of all other continents except Antarctica. The...

• nimbostratus see cloud.

• nimbus in meteorology, low, dark, formless cloud covering the entire sky, from which rain or snow is steadily falling. The term is usually applied to any cloud from which rain descends. Modifications are cumulonimbus, fractonimbus (ragged, broken...

• northern lights see aurora borealis.

• parhelion see halo.

• polar front zone of transition between polar and tropical air masses. Its average position during the winter is at about 30° lat. and during the summer at about 60° lat. In the N Atlantic Ocean, for example, the polar front can often be traced as a continuous line...

• precipitation in meteorology, condensed moisture that falls to the surface of the earth in the form of rain , sleet , snow , hail , frost , or dew.

• rain precipitation in liquid form. It consists of drops of water falling from clouds; if the drops are very small, they are collectively termed drizzle. Rain plays a key role in the hydrologic, or...

• rainbow arc showing the colors of the spectrum , violet inside and red outside, which appears when the sun shines through water droplets. It often appears while the sun is shining after a brief thundershower in the late afternoon or on fog...

• rainmaking production of rain by artificial means now generally disregarded, though it is probable that rainmaking hastens or increases rainfall from clouds suitable for natural rainfall. Interest in rainmaking has been spurred...

• roaring forties name applied, especially by sailors, to the latitudes between 40°S and 50°S, where the prevailing westerly winds are strong and steady. Unlike the winds in the Northern Hemisphere, those in the...

• Saffir-Simpson scale standard scale for rating the severity of hurricanes as a measure of the damage they cause; it is based on observations of numerous North Atlantic Basin hurricanes. First developed in the late...

• Saint Elmo's fire luminous discharge of electricity extending into the atmosphere from some projecting or elevated object. It is usually observed (often during a snowstorm or a dust storm) as brushlike fiery jets...

• sandstorm strong dry wind blowing over the desert that raises and carries along clouds of sand or dust often so dense as to obscure the sun and reduce visibility almost to zero; also known as a duststorm. Such a wind is...

• seasons divisions of the year characterized by variations in the relative lengths of day and night and in the amount of heat received from the sun. These variations depend on the inclination of the equator...

• simoom or simoon: see sandstorm.

• sirocco [Ital., from Arab. sharq =east], hot, dust-laden, dry, southerly wind originating in the N African desert (most commonly in the spring) and reaching Italy and nearby Mediterranean areas. The term more generally denotes any...

• sky apparent dome over the earth, background of the clouds, sun, moon, and stars. The blue color of the clear daytime sky results from the selective scattering of light rays by the minute particles of...

• sleet precipitation of small, partially melted grains of ice. As raindrops fall from clouds, they pass through layers of air at different temperatures. If they pass through a layer with a temperature...

• snow precipitation formed by the sublimation of water vapor into solid crystals at temperatures below freezing. Sublimation resulting in the formation of snow takes place about a dust particle, as in...

• southern lights see aurora borealis.

• storm disturbance of the ordinary conditions of the atmosphere attended by wind, rain, snow, sleet, hail, or thunder and lightning. Types of storms include the extratropical cyclone , the common, large-scale storm of temperate latitudes; the tropical cyclone, or hurricane , which is somewhat smaller in area than the former and accompanied by high winds and heavy rains; the tornado , or "twister," a small but intense storm with very high winds, usually of limited duration; and the thunderstorm , local in nature and accompanied by brief but heavy rain showers and often by hail. The term storm is also applied to blizzards, sandstorms, and dust storms, in which high wind is the dominant meteorological element. A storm surge, sometimes called a tidal wave, is a flood of ocean or lake water that occurs in areas subject to tropical storms and bordering on shallow waters, but any strong low-pressure system in a coastal...

• storm surge see under storm.

• stratocumulus see cloud.

• stratus see cloud.

• temperature inversion condition in which the temperature of the atmosphere increases with altitude in contrast to the normal decrease with altitude. When temperature inversion occurs, cold air underlies warmer air at...

• temperature-humidity index see humidity.

• thunder sound produced along a path of a lightning flash, caused by the rapid heating and expansion of the adjacent air. Rolling thunder occurs either as a result of the time difference between sounds from...

• thunderstorm violent, local atmospheric disturbance accompanied by lightning , thunder , and heavy rain, often by strong gusts of wind, and sometimes by hail. The typical thunderstorm caused by convection occurs when the sun's warmth has heated a large body of moist air near the ground. This air rises and is cooled by expansion. The cooling condenses...

• tornado dark, funnel-shaped cloud containing violently rotating air that develops below a heavy cumulonimbus cloud mass and extends toward the earth. The funnel twists about, rises and falls, and where it...

• torrid zone see tropics.

• trade winds movement of air toward the equator, from the NE in the Northern Hemisphere and from the SE in the Southern Hemisphere. The trade winds originate on the equatorial sides of the horse latitudes , which are two belts of high air pressure, one lying between 25° and 30° north of the equator and the other lying between 25° and 30° south of it. The high air pressure in these belts forces air to...

• typhoon see hurricane.

• waterspout tornado occurring at sea or over inland waters. The characteristic funnel-shaped cloud is formed at the base of a cumulus-type cloud and extends downward to the water surface, where it picks up spray...

• weather state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer ), wind, humidity, cloudiness, and precipitation. The term weather is restricted to conditions...

• weather map see weather.

• whirlwind revolving mass of air resulting from local atmospheric instability, such as that caused by intense heating of the ground by the sun on a hot summer day. Examples of whirlwinds are waterspouts , tornadoes...

• wind flow of air relative to the earth's surface. A wind is named according to the point of the compass from which it blows, e.g., a wind blowing from the north is a north wind.

• wind chill the cooling effect of wind and temperature combined, expressed in terms of the effect produced by a lower, windless temperature, also called wind chill factor, wind chill temperature, wind chill...

• wind shear a sudden, drastic change in wind direction or speed over a comparatively short distance. Most winds travel horizontally, as does most wind shear, but under certain conditions, including...

• zone [Gr.,=girdle], in geography, area with a certain physical and/or cultural unity that distinguishes it from other areas. The division of the earth into five climatic zones probably originated (5th...