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Warm front
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==Development== [[Image:Airmassesorigin.png|250px|thumb|right|Different air masses that affect North America, as well as other continents, tend to be separated by frontal boundaries.]] Air masses are large bodies of air with similar properties of [[temperature]] and [[humidity]] that form over source regions. The warm air mass behind a warm front is not only warmer, but often (but not always) also higher in humidity than the colder air preceding it. Because of a warm air mass’s higher temperature and thus lesser [[density]], mixing between the two air masses is unlikely. Being light, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 1:200 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, any [[water vapor]] that is present will [[Condensation|condense]] and form extensive [[cloud]] cover. The first clouds that indicate an approaching warm front tend to be mostly high [[Cirrus cloud|cirrus]] at first, changing to [[Cirrostratus cloud|cirrostratus]] as the front approaches. However, if [[Cirrocumulus cloud|cirrocumulus]] also appears, there is greater airmass instability approaching ahead of the front. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 8 hours away.<ref name="Justice Perrin">{{cite web|url=http://www.weatheronline.co.uk/reports/wxfacts/Sometimes-a-bit-fishy.htm|title=Mackerel sky|publisher=Weather Online|access-date=21 November 2013}}</ref> A thickening and lowering of these high clouds into middle-stage [[Altostratus cloud|altostratus]] or [[Altocumulus cloud|altocumulus]] is a good sign the warm front or low has moved closer and precipitation may begin within less than six hours. Once the clouds have thickened to {{convert|2500|m|ft}} from the earth’s surface, precipitation can begin to fall from heavy [[nimbostratus cloud|nimbostratus]]. If unstable altocumulus castellanus accompanies or takes the place of the main altostratus layer, cumulus congestus or cumulonimbus producing showers or thunderstorms may follow. Low stratus and stratocumulus commonly form underneath the main precipitating clouds. A warm front is also defined as the transition zone where a warmer air mass is replacing a cooler air mass. Warm fronts generally move from southwest to northeast. If the warmer air originates over the ocean, it is not only warmer but also more moist than the air ahead of it.
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