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 extending over thousands of miles, usually toward the northeast from a point just off the coast of the United States at about 30°N. Most cyclones outside the tropics develop along the polar front from waves caused by the juxtaposition of cold air moving toward the equator and hot air moving toward the poles; the earth's rotation gives this air its cyclonic twist. See front.

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 indicates that no relative advance of either air mass is occurring. An occluded front is one in which a warm front has been completely undermined by cold air and is therefore positioned aloft. Since warmer air always overrides colder, denser air, the frontal boundary is sloped closer to the horizontal than the vertical. A mature cyclone usually involves all of the frontal types. The recognition of atmospheric fronts and their relative importance to weather forecasting came about only at the beginning of the 20th cent. as a result of publications by the meteorologists Vilhelm and Jakob Bjerknes.

Front de Libération du Québec (FLQ), Canadian separatist group formed in the 1960s to bring about the independence of Quebec, which has a French heritage, from the rest of Canada, which has a primarily British tradition. Using public demonstrations and terrorist activities, the FLQ attracted considerable attention. In 1970 members of the organization, demanding release of separatist prisoners, kidnapped a British official and the Canadian minister of labor; the latter was murdered. Their action persuaded Prime Minister Trudeau to institute martial law for six months.

Front Royal, city (1990 pop. 11,880), seat of Warren co., N Va., at the junction of South and North forks of Shenandoah River, inc. 1788. Furniture, lumber and building materials, automotive paints, and modular homes are among the local manufactures; limestone is quarried in the area. Tourism is also important; Front Royal is at the north end of Skyline Drive and Shenandoah National Park and near Skyline Caverns and part of the George Washington National Forest. Christendom College and the National Zoo's Conservation and Research Center are also nearby. Front Royal was the site of a Confederate victory in 1862 (see Jackson, Stonewall).

Front Range, an eastern range of the U.S. Rocky Mts., bordering the Great Plains and extending c.300 mi (480 km) S from SE Wyo. to the Arkansas River, S central Colo. It has several peaks, including Gray's Peak and Pikes Peak, that are more than 14,000 ft (4,270 m) high. The Arkansas and the South Platte rivers are the largest streams rising in the range. Most of Colorado's population is located along the range's eastern foothills. The Front Range was scouted by U.S. explorers Zebulon Pike, in 1806-7, and Stephen Long, in 1819-20. In 1858 gold was discovered at Cripple Creek, Colo., and goldseekers rushed into the S Front Range. Most of the range is in national forests; Rocky Mt. National Park is located in the north.

Imaginary surface that represents corresponding points of waves vibrating in unison. As identical waves from the same source travel through a homogeneous medium, corresponding crests and troughs are in phase at any instant; that is, they have completed the same fraction of their periodic motion. Any surface drawn through all points of the same phase constitutes a wave front.

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In European politics, any coalition of working-class and middle-class political parties united to defend democracy against an expected fascist assault. The policy of a “united front” against fascism was announced at the communist Third International (1935); it was to include not only communists and socialists but also liberals, moderates, and even conservatives. Popular-front governments were formed in France and Spain in 1936, but the financial consequences of the reforms undertaken by the French government, under Léon Blum, proved its undoing, and the Spanish government was brought down by Francisco Franco in the Spanish Civil War.

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In meteorology, the interface or transition zone between two air masses of different density and temperature. Frontal zones are frequently accompanied by low barometric pressure, marked changes in wind direction and relative humidity, and considerable cloudiness and precipitation.

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