Different air masses which affect North America, as well as other continents, tend to be separated by frontal boundaries A weather front is a boundary separating two masses of air of different densities, and is the principal cause of significant weather. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift. Image File history File links Airmassesorigin. ...
Image File history File links Airmassesorigin. ...
In meteorology, an air mass is a large volume of air having fairly uniform characteristics of temperature, atmospheric pressure, and water vapor content. ...
In meteorology, an air mass is a large volume of air having fairly uniform characteristics of temperature, atmospheric pressure, and water vapor content. ...
For other uses, see Density (disambiguation). ...
For the geological process, see Weathering or Erosion. ...
A surface weather analysis for the United States on October 21, 2006. ...
For other uses, see Temperature (disambiguation). ...
Humidity is the amount of water vapor in air. ...
A shelf cloud associated with a heavy or severe thunderstorm over Enschede, The Netherlands. ...
NOAA scientists observe severe weather using a mobile doppler radar and a helicopter (in the distance) Severe weather phenomena are weather conditions that are hazardous. ...
A squall or squall line is a line of thunderstorms with a common leading convection line, or mesocyclone, which tends to create a powerful gust front. ...
A dry line, dew point line, or Marfa front[1] is a boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains of North America. ...
Illustration of a warm front A warm front is defined as the leading edge of a mass of warm air. ...
For other uses, see Fog (disambiguation). ...
Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean. In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
A geographical pole is either of two fixed points on the surface of a spinning body or planet, at 90 degrees from the equator, based on the axis around which a body spins. ...
For other uses, see Mountain (disambiguation). ...
In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
Surface weather analysis
 A guide to the symbols for weather fronts that may be found on a weather map: #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 surface weather analysis is a special type of weather map which provides a view of weather elements over a geographical area at a specified time based on information from ground based weather stations.[1] Weather maps are created by plotting or tracing the values of relevant quantities such as sea level pressure, temperature, and 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 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. In addition, areas of precipitation help determine the frontal type and location.[1] Image File history File links NWS_weather_fronts. ...
Image File history File links NWS_weather_fronts. ...
A surface weather analysis for the United States on October 21, 2006. ...
A large low-pressure system swirls off the southwestern coast of Iceland, illustrating the maxim that nature abhors a vacuum. ...
It has been suggested that this article or section be merged into Anticyclone. ...
These symbols, showing various weather fronts, might be found on a weather map. ...
For the geological process, see Weathering or Erosion. ...
Atmospheric pressure is the pressure at any given point in the Earths atmosphere. ...
For other uses, see Temperature (disambiguation). ...
Cloud cover (also known as cloudiness, cloudage or cloud amount) refers to the fraction of the sky obscured by clouds when observed from a particular location. ...
Cartography or mapmaking (in Greek chartis = map and graphein = write) is the study and practice of making maps or globes. ...
The synoptic scale in meteorology (also known as large scale or cyclonic scale) is a horizontal length scale of the order of 1000 kilometres (about 620 miles) or more [1]. This corresponds to a horizontal scale typical of mid-latitude depressions. ...
Warm front -
 Illustration of a warm front Warm fronts 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. A warm front moves more slowly than the cold front which usually follows because 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.[2] Illustration of a warm front A warm front is defined as the leading edge of a mass of warm air. ...
Illustration of a warm front A warm front is defined as the leading edge of a mass of warm air. ...
For other uses, see Stratus. ...
For other uses, see Fog (disambiguation). ...
Occluded front -
Main article: occluded front
Occluded cyclone frontal depiction An occluded front is formed when a cold front overtakes a warm front.[3] The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point.[4] 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.[2] In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
Image File history File links Occludedcyclone. ...
Image File history File links Occludedcyclone. ...
In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
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.[2] Occluded fronts usually form around mature low-pressure areas.[2]
Stationary front and shearline -
A stationary front is a non-moving boundary between two air masses, neither of which is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, usually moving in waves.[5] There is normally a broad temperature gradient behind the boundary with more widely spaced isotherm packing. In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
In meteorology, a weather front is a boundary between two air masses with differing characteristics (e. ...
An isotherm is a line of equal or constant temperature on a graph, plot, or map; an isopleth of temperature. ...
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 they can transform 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.[6] A shear line is depicted as a line of red dots and dashes.[2]
Dry line -
 Satellite view of the convection around a dry line. The Temperature in Fahrenheit at around 86 over the whole clear area but the dew point drop from the 60's, East of the brown line, to the 50', to its West. A similar phenomenon to a frontal zone is the dry line, which is the boundary between air masses with significant moisture differences. When westerly winds aloft increase on the north side of surface highs, areas of lowered pressure will form downwind of north-south oriented mountain chains, leading to the formation of a lee trough. Near the surface during daylight hours, warm moist air is more dense than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the dry air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.[7] A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in moisture. When moisture pools along the boundary during the warm season, it can be the focus of diurnal thunderstorms.[8] A dry line, dew point line, or Marfa front[1] is a boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains of North America. ...
The dew point (or dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. ...
A dry line, dew point line, or Marfa front[1] is a boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains of North America. ...
Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ...
For other uses, see Mountain (disambiguation). ...
NOAA scientists observe severe weather using a mobile doppler radar and a helicopter (in the distance) Severe weather phenomena are weather conditions that are hazardous. ...
Dew on a spider web Moldy bread Moisture generally refers to the presence of water, often in trace amounts. ...
A rolling thundercloud over Enschede, The Netherlands. ...
The dry line may occur anywhere on earth in regions intermediate between desert areas and warm seas. The southern plains west of the Mississippi River in the United States are a particularly favored location. The dry line normally moves eastward during the day and westward at night. A dry line is depicted on National Weather Service (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.[9] This article is about arid terrain. ...
For the river in Canada, see Mississippi River (Ontario). ...
The National Weather Service (NWS) is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration (NOAA) of the United States government. ...
Squall line -
Main article: Squall line
A shelf cloud such as this one can be a sign that a squall is imminent 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.[10] 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.[11] A squall or squall line is a line of thunderstorms with a common leading convection line, or mesocyclone, which tends to create a powerful gust front. ...
Image File history File links Shelfcloudspc. ...
Image File history File links Shelfcloudspc. ...
A Mesoscale Convective Systen (MCS) is a complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. ...
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.[12] 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. A haboob is a type of intense dust storm characteristic of very dry regions. ...
Tropical waves -
Main article: Tropical wave 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 cyclones and 85% of intense Atlantic hurricanes (Category 3 and greater).[13][14] Tropical waves, also known as easterly waves, are elongated areas of relatively low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms. ...
Image File history File linksMetadata Size of this preview: 800 × 318 pixelsFull resolution (2374 × 944 pixel, file size: 149 KB, MIME type: image/jpeg) By NOAA, modified from http://www. ...
Image File history File linksMetadata Size of this preview: 800 × 318 pixelsFull resolution (2374 × 944 pixel, file size: 149 KB, MIME type: image/jpeg) By NOAA, modified from http://www. ...
Tropical waves, also known as easterly waves, are elongated areas of relatively low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms. ...
A world map showing the continent of Africa Africa is the worlds second-largest and second most-populous continent, after Asia. ...
The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is maximum and easterly. ...
It has been suggested that this article or section be merged into Anticyclone. ...
In meteorology, an anticyclone (that is, opposite to a cyclone) is a weather phenomenon in which there is a descending movement of the air and a high pressure area over the part of the planets surface affected by it. ...
Motto (Portuguese for Rather die free than in peace subjugated) Anthem (national) (local) Capital Ponta Delgada1 Angra do HeroÃsmo2 Horta3 Largest city Ponta Delgada Official languages Portuguese Government Autonomous region - President Carlos César Establishment - Settled 1439 - Autonomy 1976 Area - Total 2,333 km² (n/a) 911 sq mi...
North American redirects here. ...
Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ...
The Saffir-Simpson Hurricane Scale is a scale classifying most Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms, and thereby become hurricanes. ...
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 (72 km/h) before developing a circulation. Tropical waves are depicted with a solid orange line on the U.S. National Weather Service Unified Surface Analysis.[2] For the Marvel Comics character, see Windshear (comics). ...
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Precipitation produced -
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 because of wind shear and the presence of a jet max, "roll clouds" and tornadoes may occur.[15] This article is about the meteorological term. ...
For the Marvel Comics character, see Windshear (comics). ...
This article is about the weather phenomenon. ...
In the warm season, lee troughs, breezes, outflow boundaries, and trowals/occlusions can lead to convection if enough moisture is available. Orographic precipitation is precipitation created through the lifting action of air moving over terrain such as mountains and hills, which is most common behind cold fronts that move into mountainous areas. It may sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain. But precipitation along warm fronts is relatively steady, as in rain or drizzle. Fog, sometimes extensive and dense, often occurs in pre-warm-frontal areas.[16] But not all fronts produce precipitation or even clouds: moisture must be present in the air mass which is being lifted.[17] This article or section is in need of attention from an expert on the subject. ...
Movement Fronts are generally guided by winds aloft, but they normally move at lesser speeds. Cold fronts and occluded fronts in the Northern Hemisphere usually travel from the northwest to southeast, 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 reverse is true; the cold front usually moves from the southwest to the northeast, and the warm front moves from northwest to southeast. Movement is largely caused by the pressure gradient force (horizontal differences in atmospheric pressure) and the Coriolis effect, which is 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.[2] In the inertial frame of reference (upper part of the picture), the black object moves in a straight line. ...
This article is about Earth as a planet. ...
See also Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. ...
A fictitious synoptic chart of an extratropical cyclone affecting the UK & Ireland. ...
Norwegian cyclone and Shapiro-Keyser model differences in frontal structure There are two models of cyclone development and lifecycles in common use - the Norwegian model and the Shapiro-Keyser Model. ...
A surface weather analysis for the United States on October 21, 2006. ...
References Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...
is the 295th day of the year (296th in leap years) in the Gregorian calendar. ...
Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...
is the 295th day of the year (296th in leap years) in the Gregorian calendar. ...
Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...
is the 295th day of the year (296th in leap years) in the Gregorian calendar. ...
is the 339th day of the year (340th in leap years) in the Gregorian calendar. ...
The Atlantic Oceanographic and Meteorological Laboratory (AOML) is a laboratory in National Oceanic and Atmospheric Administrations Office of Oceanic and Atmospheric Research (OAR). ...
The National Oceanic and Atmospheric Administration (NOAA) is a scientific agency of the United States Department of Commerce focused on the conditions of the oceans and the atmosphere. ...
Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...
is the 206th day of the year (207th in leap years) in the Gregorian calendar. ...
Dr. Lixion Avila Dr. Lixion Avila is a weather forecaster with the National Hurricane Center. ...
Monthly Weather Review is a publication of the American Meteorological Society. ...
Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...
is the 206th day of the year (207th in leap years) in the Gregorian calendar. ...
Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ...
is the 119th day of the year (120th in leap years) in the Gregorian calendar. ...
Bibliography - Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago Press. Chicago: 1999
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