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#1 cold front; #2 warm front; #3 stationary front; #4 occluded front; #5 surface trough; #6 squall/shear line; #7 dry line; #8 tropical wave ]] A weather front is a boundary between two masses of air of different Densities , and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in Temperature and may also differ in Humidity . Cold fronts feature narrow bands of Thunderstorm s and Severe Weather , and may be preceded by Squall Line s and Dry Line s. Warm Front s are preceded by stratiform Precipitation and Fog . The weather quickly clears after a moving front passes. Cold and Occluded Front s move from west to east, while warm fronts move poleward. Cold fronts and cold occlusions move faster than warm fronts and warm occlusions, due to the greater density of air in their wake. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes Stationary , it can degenerate into a line which separates regions of differing wind speed, known as a shearline. In the Tropics , Tropical Wave s move from east to west across the Atlantic and eastern Pacific Basin s. They create most of the tropical cyclones seen in the Atlantic basin, and a significant proportion of those spawned in the eastern Pacific ocean. SURFACE WEATHER ANALYSIS A surface weather analysis is a special type of , Temperature , Cloud Cover onto a Geographical Map to help find Synoptic Scale features such as weather fronts. Surface weather analyses have special symbols which show frontal systems, cloud cover, Precipitation , or other important information. For example, an ''H'' may represent High Pressure , implying good and fair weather. An ''L'' on the other hand may represent Low Pressure , which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. Areas of precipitation help determine the frontal type and location. COLD FRONT See Also: Cold front 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 can move up to twice as fast and produce sharper changes in weather than warm fronts, since cold air is denser than warm air it rapidly replaces the warm air preceding the boundary. Cold fronts are typically accompanied by a narrow band of showers and thunderstorms. 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, and it is placed at the leading edge of the cooler air mass. WARM FRONT See Also: Warm front Warm Front s are at the leading edge of the temperature rise, which is located on the equator-ward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. Warm fronts move more slowly than the cold front which usually follows due to the fact that cold air is more dense, and harder 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 half circles pointing in the direction of travel. OCCLUDED FRONT See Also: Occluded front An . It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold. In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. Occluded fronts usually form around mature low-pressure areas. MOVEMENT Fronts are generally guided by winds aloft, but they normally move at lesser speeds. Cold fronts and occluded fronts travel from a west to east direction, while warm fronts move more poleward with time. In the Northern Hemisphere a warm front moves from southwest to northeast. In the Southern Hemisphere the warm front moves from northwest to southeast. Movement is largely due to the pressure gradient force (due to horizontal differences in atmospheric pressure) and the Coriolis Effect , caused by Earth 's spinning about its axis. Frontal zones can be slowed down by geographic features like mountains and large bodies of warm water.David Roth. Hydrometeorological Prediction Center. Unified Surface Analysis Manual. Retrieved on 2006-10-22. STATIONARY FRONT AND SHEARLINE See Also: Stationary front A . There is normally a broad temperature gradient behind the boundary with more widely spaced Isotherm packing. A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement. When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shear lines.Glossary of Meteorology. Shear Line. Retrieved on 2006-10-22. If the shear line becomes active with thunderstorms, it may support formation of a Tropical Storm or a regeneration of the feature back into a stationary front. A shear line is depicted as a line of red dots and dashes. PRECIPITATION PRODUCED See Also: Convective precipitation Orographic precipitation Fronts are the principal cause of significant weather. '' Convective Precipitation '' (showers, thundershowers and related unstable weather) is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme due to Wind Shear and the presence of a jet max, "roll clouds" and Tornado es may occur. Glossary of Meteorology. Convection. Retrieved on 2006-10-22. In the warm . DRY LINE A similar phenomenon to a frontal zone is the . A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in moisture. If moisture pools along with boundary during the warm season, it can be the focus of diurnal thunderstorms.Glossary of Meteorology. Lee Trough. Retrieved on 2006-10-22. The dry line may occur anywhere on the globe in regions intermediate between desert areas and warm seas. The southern plains west of the Mississippi in the U.S. are a particularly favored location. The dry line normally sloshes eastward during the day, and westward at night. A dry line is depicted on NWS surface analyses as a brown line with scallops facing into the moist sector. Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion.University of Illinois. Dry Line: A Moisture Boundary. Retrieved on 2006-10-22. SQUALL LINE Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but can outrun cold fronts in a pattern where the upper level jet splits into two streams, with the resultant Mesoscale Convective System (MCS) forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise.Office of the Federal Coordinator for Meteorology. [http://www.ofcm.gov/slso/pdf/slsochp2.pdf Chapter 2: Definitions.] Retrieved on 2006-10-22. Even weaker and less organized areas of thunderstorms lead to locally cooler air and higher pressures, and outflow boundaries exist ahead of this type of activity, which can act as foci for additional thunderstorm activity later in the day.Michael Branick. National Weather Service Office, Norman, Oklahoma. A Comprehensive Glossary of Weather. Retrieved on 2006-10-22. These features are often depicted in the warm season across the United States on surface analyses, and lie within surface troughs. If outflow boundaries or squall lines form over arid regions, a Haboob may result.Western Region Climate Center. H. Retrieved on 2006-10-22. Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or SQUALL LINE, while outflow boundaries are depicted as troughs with a label of OUTFLOW BNDRY. TROPICAL WAVES Atlantic tropical waves develop from disturbances which drift off the continent of Africa onto the Atlantic Ocean . They are generated or enhanced by the African Easterly Jet . The clockwise circulation of the large transoceanic High-pressure Cell or Anticyclone centered near the Azores islands moves easterly waves away from the coastal areas of Africa towards North America . Tropical waves cause approximately 60% of Atlantic Tropical Cyclone s and 85% of intense Atlantic Hurricane s ( Category 3 and greater).1 Tropical cyclones sometimes degenerate back into a tropical wave. This normally occurs if upper-level Wind Shear is too strong. The storm can redevelop if the upper level shear abates. If a tropical wave is moving quickly, it can have strong winds of over tropical storm force, but is not considered a tropical storm unless it has a closed circulation. An example of this was Hurricane Claudette (2003) , where the original wave had winds of 45 mph before developing a circulation. Tropical waves are depicted with a solid orange line on the US National Weather Service Unified Surface Analysis. SEE ALSO REFERENCES BALLIN |
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