A surface weather analysis for the United States on October 21, 2006. The United States of America —commonly referred to as the Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.

A surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations. A weather map is a tool used to display information quickly showing the analysis of various meteorological quantities at various levels of the atmosphere The weather is a set of all the phenomena occurring in a given Atmosphere at a given Time. ^[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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Cloud cover (also known as cloudiness, cloudage or cloud amount) refers to the fraction of the Sky obscured by Clouds when observed 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 A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena.

The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems. ^[2] After the advent of the telegraph, simultaneous surface weather observations became possible for the first time, and beginning in the late 1840s, the Smithsonian Institution became the first organization to draw real-time surface analyses. Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide The Smithsonian Institution (smɪθsoʊnɪən is an educational and research institute and associated Museum complex administered and funded by the Government of Use of surface analyses began first in the United States, spreading worldwide during the 1870s. The United States of America —commonly referred to as the Use of the Norwegian cyclone model for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during World War II. The older of the models of Extratropical cyclone development is known as the Norwegian Cyclone Model, developed during and shortly after World War I within the World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including

Surface weather analyses have special symbols which show frontal systems, cloud cover, precipitation, or other important information. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric For example, an H may represent high pressure, implying good and fair weather. A high pressure area (also called a high or high-pressure is a region where the Atmospheric pressure is greater than surrounding areas An L on the other hand may represent low pressure, which frequently accompanies precipitation. A low pressure area, or " low " is a region where the Atmospheric pressure is lower in relation to the surrounding area 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.

History of surface analysis

See also: History of surface weather analysis

Surface analysis of Great Blizzard of 1888 on March 12, 1888 at 10 pm

The use of weather charts in a modern sense began in the middle portion of the 19th century in order to devise a theory on storm systems. The history of surface weather analysis concerns the timetable of developments related to surface weather analysis The Great Blizzard of 1888 ( March 11 – March 14 1888) was one of the most severe Blizzards in United States recorded history Events 538 - Witiges, king of the Ostrogoths ends his siege of Rome and retreats to Ravenna, leaving Year 1888 ( MDCCCLXXXVIII) was a Leap year starting on Sunday (click on link for calendar of the Gregorian calendar (or a ^[3] The development of a telegraph network by 1845 made it possible to gather weather information from multiple distant locations quickly enough to preserve its value for real-time applications. The Smithsonian Institution developed its network of observers over much of the central and eastern United States between the 1840s and 1860s once Joseph Henry took the helm. ^[4] The U.S. Army Signal Corps inherited this network between 1870 and 1874 by an act of Congress, and expanded it to the west coast soon afterwards.

At first, all the data on the map was not taken from these analyses because of a lack of time standardization. The first attempts at time standardization took hold in Great Britain by 1855. The entire United States did not finally come under the influence of time zones until 1905, when Detroit finally established standard time. ^[5] Internationally, other countries followed the lead of the United States in taking simultaneous weather observations, starting in 1873. ^[6] Other countries then began preparing surface analyses. The use of frontal zones on weather maps did not appear until the introduction of the Norwegian cyclone model in the late 1910s, despite Loomis' earlier attempt at a similar notion in 1841. The older of the models of Extratropical cyclone development is known as the Norwegian Cyclone Model, developed during and shortly after World War I within the ^[7] Since the leading edge of air mass changes bore resemblance to the military fronts of World War I, the term "front" came into use to represent these lines. World War I (abbreviated WWI; also known as the First World War, the Great War, and the War to End All ^[8]

Present weather symbols used on weather maps

Despite the introduction of the Norwegian cyclone model just after World War I, the United States did not formally analyze fronts on surface analyses until late 1942, when the WBAN Analysis Center opened in downtown Washington, D.C.. In Meteorology, a cyclone refers to an area of closed circular fluid motion rotating in the same direction as the Earth. Washington DC ( formally the District of Columbia and commonly referred to as Washington, the District, or simply D ^[9] The effort to automate map plotting began in the United States in 1969,^[10] with the process complete in the 1970s. Hong Kong completed their process of automated surface plotting by 1987. Hong Kong ( officially the Hong Kong Special Administrative Region, is a territory located on China 's south coast on the Pearl River Delta, and borders ^[11] By 1999, computer systems and software had finally become sophisticated enough to allow for the ability to underlay on the same workstation satellite imagery, radar imagery, and model-derived fields such as atmospheric thickness and frontogenesis in combination with surface observations to make for the best possible surface analysis. In the United States, this development was achieved when Intergraph workstations were replaced by n-AWIPS workstations. The Advanced Weather Interactive Processing System (AWIPS is a technologically advanced Information processing, display, and Telecommunications system ^[12] By 2001, the various surface analyses done within the National Weather Service were combined into the Unified Surface Analysis, which is issued every six hours and combines the analyses of four different centers. ^[13] Recent advances in both the fields of meteorology and geographic information systems have made it possible to devise finely tailored products that take us from the traditional weather map into an entirely new realm. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary Weather information can quickly be matched to relevant geographical detail. For instance, icing conditions can be mapped onto the road network. This will likely continue to lead to changes in the way surface analyses are created and displayed over the next several years. ^[14]

Station model used on weather maps

See also: Station model

Station model plotted on surface weather analyses

When analyzing a weather map, a station model is plotted at each point of observation. A station model is a symbolic illustration showing the Weather occurring at a given reporting station. A station model is a symbolic illustration showing the Weather occurring at a given reporting station. Within the station model, the temperature, dewpoint, wind, sea level pressure, pressure tendency, and ongoing weather are plotted. ^[15] The circle in the middle represents cloud cover. If completely filled in, it is overcast. If conditions are completely clear, the circle is empty. If conditions are partly cloudy, the circle is partially filled in. ^[16] Outside the United States, temperature and dewpoint are plotted in degrees Celsius. The Celsius Temperature scale was previously known as the centigrade scale. Each full flag on the Wind Barb represents 10 knots (19 km/h) of wind, each half flag represents 5 knots (9 km/h). Wind Barbs are symbols used on Weather maps[[http //polarncep When winds reach 50 knots (93 km/h), a filled in triangle is used for each 50 knots (93 km/h) of wind. ^[17] In the United States, rainfall plotted in the corner of the station model are in English units, inches. Inches redirects here To see the Les Savy Fav album see Inches. Internationally, the standard rainfall measurement unit is the millimeter. The Millimetre ( American spelling: millimeter, symbol mm) is a unit of Length in the Metric system, equal to Once a map has a field of station models plotted, the analyzing isobars (lines of equal pressure), isallobars (lines of equal pressure change), isotherms (lines of equal temperature), and isotachs (lines of equal wind speed) can be easily accomplished. A contour line (also Level set, isopleth, isoline, isogram or isarithm) of a function of two ^[18] The abstract present weather symbols used on surface weather analyses for obstructions to visibility, precipitation, and thunderstorms were devised to take up the least room possible on weather maps.

Synoptic scale features

See also: Synoptic scale

A synoptic scale feature is one whose dimensions are large in scale, more than several hundred kilometers in length. 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 ^[19] Migratory pressure systems and frontal zones exist on this scale.

Pressure centers

Wind barb interpretation

Centers of surface high and low pressure areas are found within closed isobars on a surface weather analysis where there the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps in English-speaking countries will depict their highs as Hs and lows as Ls,^[20] while Spanish-speaking countries will depict their highs as As and lows as Bs. ^[21]

Low pressure

Low pressure systems, also known as cyclones, are located in minima in the pressure field. In Meteorology, a cyclone refers to an area of closed circular fluid motion rotating in the same direction as the Earth. Rotation is inward and counterclockwise in the northern hemisphere as opposed to inward and clockwise in the southern hemisphere due to the coriolis force. Northern Hemisphere is the half of a Planet that is North of the Equator —the word hemisphere literally means 'half ball' Southern Hemisphere is the half of a Planet that is South of the Equator —the word hemisphere literally means 'half ball' In physics the Coriolis effect is an apparent deflection of moving objects when they are viewed from a Rotating frame of reference. Weather is normally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds, increased temperatures, and upward motion in the atmosphere which leads to an increased chance of precipitation. Polar lows can form over relatively mild ocean waters when cold air sweeps in from the ice cap, leading to upward motion and convection, usually in the form of snow. Tropical cyclones and winter storms are intense varieties of low pressure. Over land, thermal lows are indicative of hot weather during the summer. ^[22]

High pressure

High pressure systems, also known as anticyclones, rotate outward and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. 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 Under surface highs, sinking motion leads to skies that are clearer, winds that are lighter, and there is a reduced chance of precipitation. ^[23] There is normally a greater range between high and low temperature due to the drier air mass present. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high. ^[24]

Fronts

Main article: Weather fronts

Occluded cyclone example. A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. The triple point is the intersection of the cold, warm, and occluded fronts. A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena.

Fronts in meteorology are the leading edges of air masses with different density (e. For airmass in Astronomy, see Airmass. In Meteorology, an air mass is a large volume of Air that g. , air temperature and/or humidity). Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. When a front passes over an area, it is marked by changes in temperature, moisture, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Cold fronts are closely associated with low pressure systems, normally lying at the leading edge of high pressure systems and, in the case of the polar front, at approximately the equatorward edge of the high-level polar jet. A cold front defined as the leading edge of a cooler and drier mass of air Fronts are guided by winds aloft, but they normally move at lesser speeds. In the northern hemisphere, they usually travel from some west to east direction (even though they can move in a more north-south direction as well). Movement is due to the pressure gradient force (horizontal differences in atmospheric pressure) and the Coriolis effect, caused by the earth spinning about its axis. In physics the Coriolis effect is an apparent deflection of moving objects when they are viewed from a Rotating frame of reference. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Frontal zones can be contorted by geographic features like mountains and large bodies of water. ^[13]

Cold front

Main article: Cold front

A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis shows up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. A cold front defined as the leading edge of a cooler and drier mass of air A trough is an elongated region of relatively low Atmospheric pressure, often associated with fronts Unlike fronts there is not a universal symbol for Cold fronts can move up to twice as fast as warm fronts and produce sharper changes in weather, since cold air is denser than warm air and rapidly replaces the warm air preceding the boundary. The weather is a set of all the phenomena occurring in a given Atmosphere at a given Time. 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. ^[13]

Warm front

Main article: Warm front

Illustration clouds overriding a warm front

Warm fronts are at the leading edge of the temperature drop off, which is located on the equatorward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. A warm front is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead A warm front is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead A warm front is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead Warm fronts move more slowly than the cold front that usually follows because cold air is more dense, and harder to displace 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. Stratus means layer or blanket in Latin A Stratus cloud (St is a Cloud belonging to a class characterized by horizontal layering with a uniform base Fog can also occur preceding a warm frontal passage. Fog is a cloud that is in contact with the ground Stratus clouds are usually the only clouds that touch the ground 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. ^[13]

Occluded front

Main article: Occluded front

A guide to the symbols for weather fronts that may be found on a weather map:
1. Occluded front symbolsvg|left|thumb|200px|Occluded front symbol]]An occluded front is formed during the process of Cyclogenesis when a Cold front overtakes a A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. cold front
2. warm front
3. stationary front
4. occluded front
5. surface trough
6. squall line
7. dry line
8. tropical wave

An occluded front is formed during the process of cyclogenesis when a cold front overtakes a warm front. Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area ^[25] The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point in meteorology. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary ^[26] 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 not as cool as 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. ^[13] Occluded fronts usually form around mature low pressure areas.

Stationary fronts and shearlines

Main article: Stationary front

A stationary front is a non-moving boundary between two different air masses, neither of which is strong enough to replace the other. A stationary front is a boundary between two different Air masses neither of which is strong enough to replace the other They tend to remain in the same area for long periods of time, usually moving in waves. ^[27] 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 will 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 over a short distance, they become known as shear lines. ^[28] 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 tropical cyclone is a storm system characterized by a low pressure center and numerous Thunderstorms that produce strong winds and Flooding A shear line is depicted as a line of red dots and dashes. ^[13]

Mesoscale features

See also: Mesoscale Convective System

Mesoscale features are smaller than synoptic scale systems like fronts, but larger than storm-scale systems like thunderstorms. A mesoscale convective system ( MCS) is a complex of Thunderstorms that becomes organized on a scale larger than the individual thunderstorms and normally persists Mesoscale Meteorology is the study of Weather systems smaller than Synoptic scale systems but larger than microscale and Storm-scale cumulus 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 Storm-scale is a scale of sizes of Weather systems on the order of individual Thunderstorms. Horizontal dimensions generally range from around 50 miles to several hundred miles. A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States ^[29]

Dry line

The dry line is the boundary between dry and moist air masses east of mountain ranges with similar orientation to the Rockies, depicted at the leading edge of the dew point, or moisture, gradient. A dry line, (also called dew point line, or Marfa front) is an important factor in Severe weather frequency in the Great Plains of The dew point (sometimes spelled dewpoint) is the Temperature to which a given parcel of Air must be cooled at constant Barometric pressure, Near the surface, 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. ^[30] At higher altitudes, the warm moist air is less dense than the cooler, drier 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.

During daylight hours, drier air from aloft drifts down to the surface, causing an apparent movement of the dryline eastward. At night, the boundary reverts back to the west as there is no longer any sunshine to help mix the lower atmosphere. ^[31] If enough moisture converges upon the dryline, it can be the focus of afternoon and evening thunderstorms. ^[32] A dry line is depicted on United States surface analyses as a brown line with scallops, or bumps, facing into the moist sector. Dry lines are one of the few surface fronts where the special shapes along the drawn boundary do not necessarily reflect the boundary's direction of motion. ^[33]

Outflow boundaries and squall lines

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 they can outrun cold fronts. A squall is a sudden sharp increase in wind speed which is usually associated with active weather such as rain showers thunderstorms or heavy snow This outrunning occurs in a pattern where the upper level jet splits into two streams. The resultant mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern in the area of best low level inflow. A mesoscale convective system ( MCS) is a complex of Thunderstorms that becomes organized on a scale larger than the individual thunderstorms and normally persists The convection then moves east and equatorward 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. ^[34] Even weaker and less organized areas of thunderstorms will lead to locally cooler air and higher pressures, and outflow boundaries exist ahead of this type of activity, whd "SQLN" or "SQUALL LINE", while outflow boundaries are depicted as troughs with a label of "OUTFLOW BOUNDARY" or "OUTFLOW BNDRY".

Sea and land breeze fronts

Idealized circulation pattern associated with a sea breeze

Sea breeze fronts occur mainly on sunny days when the landmass warms up above the water temperature. A sea-breeze (or onshore breeze) is a Wind from the sea that develops over land near coasts A sea-breeze (or onshore breeze) is a Wind from the sea that develops over land near coasts Similar boundaries form downwind on lakes and rivers during the day, as well as offshore landmasses at night. Since the specific heat of water is so high, there is little diurnal change in bodies of water, even on the sunniest days. Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity The water temperature varies less than 1 °C (1 to 2 °F). The Celsius Temperature scale was previously known as the centigrade scale. Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 By contrast, the land, with a lower specific heat, can vary several degrees in a matter of hours. ^[35]

During the afternoon, air pressure decreases over the land as temperature rises. The relatively cooler air over the sea rushes in to fill the gap. The result is a relatively cool onshore wind. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) This process usually reverses at night where the water temperature is higher relative to the landmass, leading to an offshore land breeze. However, if water temperatures are colder than the land at night, the sea breeze may continue, only somewhat abated. This is typically the case along the California coast, for example.

If enough moisture exists, thunderstorms can form along sea breeze fronts which then can send out outflow boundaries. This causes chaotic wind/pressure regimes if the steering flow is light. Like all other surface features, sea breeze fronts lie inside troughs of low pressure.

See also

• American Practical Navigator
• Cyclogenesis
• Extratropical cyclone
• Synoptic scale
• Weather fronts
• Weather map

References

External links

• "The Mid-Latitude Cyclone"
• Norwegian Cyclone Model — NWS
• Unified Surface Analysis Manual — NWS
• Unified Surface Analysis — NWS
• Glossary of Meteorology
• Cold Front Page
• Diana : A Free Meteorological Visualisation Tool