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A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform, and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
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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 condense and form extensive clouds. The first clouds to form along the sloping surface of the cold high pressure air are high cirrus, which thicken to cirrostratus and altostratus. Once the clouds have thickened to 2,500 metres (8,200 ft) from the earth’s surface, rain can begin to fall from the heavy nimbostratus cloud.
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.
If the air mass is relatively stable, rainfall will increase until the front reaches the location, at which time the clouds can extend all the way to the earth’s surface as fog. Once the front passes, the location experiences some warming and clearing. If the air mass is unstable, thunderstorms may precede and follow the front and temperature changes will be larger.
In the northern hemisphere, a warm front causes a shift of wind blowing from southeast to southwest, and in the southern hemisphere a shift from winds blowing from northeast to northwest. Common characteristics associated with warm fronts include:
|Weather phenomenon||Prior to the Passing of the Front||While the Front is Passing||After the Passing of the Front|
|Temperature||Cool||Warming suddenly||Warmer, then leveling off|
|Atmospheric pressure||Decreasing steadily||Leveling off||Slight rise followed by a decrease|
|Precipitation||Usually none, but in summer or warm temperatures, cumulus congestus may continue to exist under cirrostratus and altostratus creating light to moderate showers.||Persistent rain, usually moderate with some lighter periods and some heavier bursts.||Light drizzle, gradually ceasing.|
|Clouds||Cirrus, cirrostratus, altostratus, nimbostratus, then stratus (pilots use the acronym CCANS) and sometimes in cold temperatures stratus or fog below the cirrus, cirrostratus and altostratus; and occasionally cumulonimbus along with or instead of the nimbostratus in summer. Other clouds can also often be seen including cirrocumulus amongst the approaching cirrus, the altostratus becoming broken in places into altocumulus (particularly if the front is weak) and the very common occurrences of stratocumulus under the cirrostratus and altostratus and stratus fractus under the nimbostratus. Often in warm temperatures rain bearing cumulus congestus clouds can continue to exist under the cirrostratus, and more rarely altostratus clouds if convection is sufficient.||Nimbostratus, sometimes cumulonimbus||Clearing with scattered stratus and stratocumulus. If the warm front is part of a depression, there is often a sheet of altostratus (often broken in places to altocumulus) above this which thickens when the cold front approaches.|
|Visibility||Poor||Poor, but improving||Fair in haze|
|Dew point||Steady rise||Steady||Rise, then steady|
The warm sector is the area of warmer air behind a warm front, usually between the warm and cold fronts in a depression. Temperatures are often warmer than they are before the warm front or after the cold front. Cloud types can be mixed, but usually consist of stratocumulus, which can range to being broken to covering the entire sky depending on distance from the centre of low pressure.
Temperature rises and the dew point remains steady. Wind direction and speed remains steady.
On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of the front. On colored weather maps, warm fronts are illustrated with a solid red line.