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Meteorology: The Cold Front 1961 US Navy Training Film MN-9487d; Weather Forecasting
🕑 Added 2020-07-03 20:04:37 +0000 UTCmore at http://quickfound.net/
Originally a public domain film from the National Archives or Library of Congress Prelinger Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).
https://en.wikipedia.org/wiki/Cold_front
Wikipedia license: http://creativecommons.org/licenses/by-sa/3.0/
A cold front is the leading edge of a cooler mass of air, replacing at ground level a warmer mass of air, which lies within a fairly sharp surface trough of low pressure. It forms in the wake of an extratropical cyclone, at the leading edge of its cold air advection pattern, which is also known as the cyclone's dry conveyor belt circulation. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is less, a broad shield of rain can move in behind the front, which increases the temperature difference across the boundary. Cold fronts are stronger in the fall and spring transition seasons and weakest during the summer...
A cold front occurs when a mass of comparatively colder air moves into where warmer air is present. The drier, colder air forms a steeply sloping boundary under the warmer, moister air at the surface and lifts that air. This often causes cloud formations with strong vertical development, which may manifest as a line of showers and thunderstorms when enough moisture is present. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts move faster than warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary.
In the northern hemisphere, a cold front usually causes a shift of wind from southwest to northwest clockwise, also known as veering, and in the southern hemisphere a shift from northwest to southwest (counterclockwise, backing). Atmospheric pressure steadily decreases with the approach of a cold front; with frontal passage, the pressure rises sharply and then stabilizes...
Clouds
If the cold front is highly unstable, cumulonimbus clouds producing thunderstorms commonly form along the front. Anvil cirrus clouds may spread a considerable distance downwind from the thunderstorms. The other cloud types associated with a cold front depend on atmospheric conditions such as air mass stability and wind shear. As the front approaches, middle-étage gives way to altostratus and low-level stratocumulus with intermittent light precipitation if the warm airmass being displaced by the cold front is mostly stable. With significant airmass instability, vertically developed cumulus or cumulonimbus with showers and thunderstorms will form along the front.
After the passage of the cold front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called cloud streets may persist if the air mass behind the front remains humid.[9] Small and unchanging amounts of cumulus or cirrus clouds in an otherwise clear sky are usually indications of continuing fair weather as long as the barometric pressure remains comparatively high...
