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.

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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In telecommunication, the term front-to-back ratio has two meanings.

The first refers to when an antenna, the gain in a specified direction, i.e., azimuth, usually that of maximum gain, compared to the gain in a direction 180° from the specified azimuth. A front-to-back ratio is usually expressed in dB.

The second is a ratio of parameters used to characterize rectifiers or other devices, in which electrical current, signal strength, resistance, or other parameters, in one direction is compared with that in the opposite direction.

Source: from Federal Standard 1037C and from MIL-STD-188

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