Precipitation

Weather Forecasting: "Air Masses and Fronts: Warm Front" 1962 USAF Meteorology

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• published: 05 Dec 2011
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more at http://scitech.quickfound.net/weather_news_and_links.html "DISCUSSES HAZARDS OF THE WARM FRONT AND SHOWS HOW IT FORMS. COVERS NIMBO STRATUS AND STRATUS CLOUD DECKS, SUMMER AND WINTER PATTERNS, AREAS OF INSTABILITY AND STRUCTURAL ICE." US Air Force training film TF1-5388D NEW VERSION with improved video & sound: https://www.youtube.com/watch?v=6-MiOdSnwRA Public domain film from the National Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied. The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and equalization. http://en.wikipedia.org/wiki/Warm_front 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 easier to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Approach of a warm front as seen from the ground. They are not usually as well defined as this. 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. Development Air masses are large bodies of air with similar properties of temperature and humidity that form over source regions, and the warm air masses behind warm fronts are not only warmer but higher in humidity than the colder air preceding them. Because of a warm air mass's higher temperature and thus lesser density, mixing between the two air masses is unlikely. Being lighter, 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 130 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapor condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold 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. Characteristics If the air mass is relatively stable, and the warm front is a katafront, 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, and the warm front is an anafront, 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... Warm sector 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. Depiction 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.